Human Visual Acuity


 First of all, I am flabbergasted that people claim they can see car lights on Highway 67 from more than 8 miles away, and in some instances 10-40!


How far away can you really see car headlights?

On February 3, 2001 I flew from Chicago to Baltimore. I asked the pilot to estimate how high up he could actually just start to see the lights from individual cars, and he said 'about 25,000 feet'. This is 4.7 miles. From my own estimates looking out the window, a car light at this altitude had a visual magnitude of around +3.0. Although humans can see stars to +6.0, this limit did not apply to viewing cars or street lights from out the plastic window, with cabin lights and the usual distortions looking through such a window. It is unlikely that the Lights are as faint as +6 because they would not be noticed. The photographs I have seen seem to suggest brightnesses nearly equal to, or greater than Venus near opposition (-3.0). Driving on the highway from Baltimore to Washington, I can still just see car tail lights at 1 mile, and begin to discern that they are double. You can also clearly see that they are red. From the air, car headlights can just be discerned to be double at something like an altitude of 10,000 feet. Also, it is quite apparent that unless you are within +/- 45 to +/- 90 degrees of head-on, the lights are not seen as points at all. I will continue to look into the visual properties of car headlights, but for now it seems quite clear that from the Viewing Area, you probably cannot see headlights farther than 5 miles, and for them to be bright enough to be impressive but still not resolved into two spots, you need to be between 2-3 miles from them. This is different than calculated above, because the 1 arcminute limit assumes that your acuity is the same day and night, In fact, the sources say that your acuity is about 7 times worse at night so 15 miles becomes about 2 miles for resolving the double headlights.

If the moving things people keep claiming are cars, really are cars, there is already something odd going on optically in this area...possibly due to atmospheric conditions????



1) Because most people and accounts of these lights are based on a multitude of naked-eye sightings, with binocular and telescopic studies vastly outnumbered, lets have a look into the human visual system and just how well it can discern motion, direction and shape changes at low light levels. Because the issue of 'night vision' is so important to the military, there is a substantial body of work available, even on the web, that covers the human eye in great detail. I will only use sources that are published on academic '.edu' pages as the basis for this discussion.

2)We have all used 'eye charts' and had our vision tested by optometrists. Practically all of us can recite what their visual acuity figure is. Normal visual acuity is called '20/20' and this means that you eye can just resolve two lines that are separated by exactly 1 arcminute. There are 60 arcminutes in a degree. The moon has a diameter of about 32 arcminutes. Here is a chart that shows how, under .... conditions of lighting, the 'Snellen' scale related to the eye's angular resolution (MAR). (source= file eye8.htm)

Table 1. Relationship between Snellen notation, minimum angle of resolution
and the logarithmic minimum angle of resolution.

Snellen Notation

Metric Imperial











































































My eyes are at least 20/50 so that means that distant lights look like snowballs without my glasses. My acuity is about 2.5 arcminutes according to the above table...but it often feels much worse especially when I try to make out details on the moon! When I first get my glasses, my corrected vision is exactly 20/20, but as everyone knows, over time the eye can weaken and so within a few years you can drift to 20/30 or .... (see if there are any studies that show this).

The human visual systems visual acuity curve falls off as a function of angle. This can be demonstrated by looking at this page of text. If you stare at one word and try to resolve other words elsewhere on the page you see that the other words are blurry. This effect is demonstrated in the following figure. (

Eyewitness Testimony and legal problems with it.

If you assemble a group of people at a Viewing Area, or look at a number of reports of the same visual phenomenon, you will get differing reports about what was seen, and how well it was seen, simply because no one has 'perfect' vision. This isn't too hard to accept. Driving on the freeway, my wife and I often disagree how far away from a sign we are first able to see its letters clearly. I had a new perscription filled recently. My wife still uses a slightly older perscription. It also means that, when multiple accounts are reported of faint lights seen under nighttime conditions, you can be sure that no two people will report the same thing in terms of brightness, size or distance. The reports will appear wildly contradictory, and this heightens the sense of mystery as two people standing side by side can not even agree to a common description of what they are seeing, no matter how convinced they are by the evidence of their own eyes. In law enforcement, it is often true that 'eye witnesses' do not agree to what they have seen, and are often deemed unreliable even though Juries credit them with great acuity. Here is an interesting report from the State Appellate Defender Office's 'Criminal Defense Newsletter ( Sept 1996, Vol 19. No. 12...eye13.htm)


Expert Testimony on Eyewitness Reliability In most documented cases of the conviction of innocent persons, mistaken eyewitness identification is the culprit. Regardless, many continue to believe that eyewitness identifications and testimony are generally reliable and persuasive forms of evidence, and that any inaccuracies are readily detectable by the layperson. However, recent scientific studies show that eyewitness accuracy is affected by numerous factors, including identification procedures commonly used by police. An expert witness can educate the trier of fact on the reliability (or lack thereof) of eyewitness identifications, the effects of various police procedures, and ways to improve the accuracy of these procedures. It has thus become very important for defense counsel and others to obtain a working knowledge of the results of the recent studies, to understand the relationship between the admissibility of eyewitness identifications and the experts' findings, and to know when and how to present expert testimony on eyewitness identifications.

Admissibility of Eyewitness Identification Testimony In United States v Wade, 388 US 218 (1967), the United States Supreme Court acknowledged the inherent unreliability of many eyewitness identification procedures and examined the relationship between these procedures and eyewitness identification testimony. Because police procedure was known to affect the accuracy of the identification, crossexamination of the eyewitness was often insufficient to assure reliability. Thus, the Wade Court held that the accused has a Sixth Amendment right to counsel at a post-indictment corporeal lineup. Where counsel was not present during the procedure, testimony concerning the procedure is per se inadmissible at trial. Gilbert v California, 388 US 263 (1967). Nonetheless, the accused may not have a right to counsel at pre-indictment photographic identification procedures unless there are "unusual circumstances" and the accused can be readily produced by the police. People v Jackson, 391 Mich 323 (1974); People v Kurylczyk, 443 Mich 289 (1993) cert den 510 US 1058 (1994). Thus, in many cases, defense counsel's role in assessing the reliability of identification procedures may be limited. Moreover, unnecessarily suggestive identification procedures, whether pre or post-indictment, may constitute a denial of due process rights. Stovall v Denno, 388 US 293 (1967). The rule of Stovall was extended to photographic identification procedures in Simmons v United States, 390 US 377 (1968). In fact, photographic identification procedures were found to be particularly dangerous since an initial mistaken photo identification may "color" any subsequent corporeal identification. Where the procedures are unnecessarily suggestive or conducive to mistaken identification, a hearing must be held to determine if an independent basis for the identification exists. Several factors determine whether the eyewitness identification has an independent basis assuring its reliability. People v Kachar, 400 Mich 78 (1977). These factors are:

   1.the witness's prior relationship with the accused;
   2.the witness's opportunity to observe the offender and the offense; 
   3.the length of time between the offense and the identification; 
   4.the accuracy of the witness's description of the offender prior to the identification procedure in light of the
     defendant's actual appearance;
   5.any prior identification or failure to identify the defendant; 
   6.any identification of a person other than the defendant as the culprit prior to the suggestive procedure; 
   7.the nature of the offense and characteristics of the witness; 
   8.idiosyncratic features of the defendant. 

Although many studies of eyewitness accuracy indicate that several of these factors are important, the studies isolate many others that are not among the Kachar factors. Also, judges and jurors may rely on "common-sense" factors to assess reliability and credibility. For example, although the witness's certainty has been found to be a weak indicator of accuracy, judges and jurors may rely heavily on it when assessing the reliability and credibility of the identification. See also Manson v Braithwaite, 432 US 98 (1977) (eyewitness confidence constitutionally permissible factor in assessing independent basis). In People v Franklin Anderson, 389 Mich 155 (1973), the Court reviewed the basis of the holdings in the "Wade cases" and considered their application to cases involving photographic identification procedures. The Court relied on studies showing that eyewitness confidence had little relation to accuracy, the stress of events could severely distort memory, and identification procedures encouraged "positive identification of things merely similar." Id., at 205. The Franklin Anderson Court took judicial notice of four procedural and psychological factors involved in eyewitness identifications:

   1.the natural and usually necessary reliance on eyewitness identification of defendants by the police and prosecution; 
   2.the scientifically and judicially recognized fact that there are serious limitations on the reliability of eyewitness identification of defendants; 
   3.the scientifically and judicially recognized fact that frequently employed police and prosecution procedures often(and frequently unintentionally) mislead eyewitnesses into misidentification of the defendant; 
   4.the historical and legal fact that a significant number of innocent people have been convicted of crimes they did not commit and the real criminal was left at large." Id., at 172. 

However, despite judicial recognition almost 25 years ago of the vagaries of eyewitness identification evidence, courts have only recently shown a willingness to allow expert testimony on eyewitness identification.

In 1983, a state supreme court first found reversible error in the exclusion of expert testimony on eyewitness identification. State v Chapple, 135 Ariz 281; 660 P2d 1208 (1983). Cf. People v Hill, 84 Mich App 90 (1978). California and other states soon followed suit; however, decisions were and continue to be case-specific. The eyewitness identification had to be a key element in the prosecution's case and not substantially corroborated by other evidence lending it independent reliability. The defendant was required to present a qualified expert to testify on the specific psychological factors that could have affected the accuracy of the witness's identification but would not be known or understood by the jurors. See People v McDonald, 37 Cal 3d 351, 208 Cal Rptr 236 (1984); United States v Smith, 736 F2d 1103 (CA 6) cert den 469 US 868 (1984). Some decisions required the defendant to present an alibi defense before expert testimony would be considered. State v Moon, 45 Wash App 692, 726 P2d 1263 (1986). But cf., People v David Allen Carson, where failure to appoint an expert was not error because the indigent defendant was able to present an alibi defense [___ Mich App ___ (#159501, 6-4-96) opinion vacated and conflicts panel convened on another ground, ___ Mich App ___ (6-14-96)].

In general, courts have advanced three grounds for exclusion of expert testimony: doubts about the scientific validity of psychological experiments; doubts about the effect of the testimony on the jury (invasion of the jury's province, juror confusion, prejudicial effect); and continued confidence in cross-examination and jury instructions to protect the defendant from the inherent weaknesses of eyewitness identification testimony. Recent studies of eyewitness identification and related police procedures suggest that these grounds for exclusion may no longer be valid in every case. The procedural and evidentiary requirements for admission of expert testimony will be examined later in this article. A summary of the findings of recent psychological studies will assist counsel to successfully argue for the admission of such testimony.

Summary of Eyewitness Identification Research The number of studies on the factors influencing eyewitness identification has grown quickly during the last two decades.

1 A close examination of individual studies reveals that their conclusions are based upon sound scientific methodology.

2 Moreover, researchers reject the common argument that "laboratory conditions" don't mirror the circumstances of real crimes and thus are inherently inaccurate. Many point out that lab conditions produce higher witness performance than what would occur during an actual crime. The actual identity of the "culprit" in the experiments is known, and identification responses of subjects can be evaluated for accuracy. In general, the studies reveal that several factors not included in Kachar influence the accuracy of eyewitness identifications and suggest that alternative police procedures could improve their accuracy.

3 Witness Factors One group of findings focuses on factors influencing the witness's ability to perceive and recall the offender's face.

4 "Stable characteristics" such as sex, race, intelligence, and certain personality characteristics are not useful predictors of identification accuracy. Age, however, was found to be significant: younger and older witnesses were generally less accurate than other adults. On the other hand, studies of "malleable characteristics" such as the witness's state of mind or intoxication produced mixed results. Expectation of a subsequent lineup while viewing the crime had little effect on accuracy, and similar results were reached in tests involving eyewitnesses with training in identification, such as bank tellers. Although alcohol intoxication is potentially an important factor, few studies have examined the effects of varying levels of intoxication. Several aspects of the witness's memory can be relied on as indicators of accuracy and reliability.

Greater detail in the description of an offender's face increases the accuracy of identifications only slightly. Similarly, a witness's memory of peripheral details is inversely related to the accuracy of offender identification: where the witness was able to testify in great detail about the circumstances surrounding the offense, more positive identifications were made, but these identifications proved less accurate. Perhaps most importantly, the witness's confidence in his or her ability to identify the offender during a police procedure is unrelated to the accuracy of the identification. Franklin Anderson, supra, 389 Mich at 174-175, 217-219. This may be particularly damaging since studies also suggest that the perception of the witness's confidence in his or her identification is relied upon by jurors and judges to assess the witness's credibility.


Offender and Offense Factors In addition to "witness factors," factors involving the offender and the offense may also affect the accuracy of the identification. "Distinctive targets" -- highly attractive or highly unattractive offenders -- are more often correctly identified and less often mistakenly identified. Conversely, a change in physical appearance of the offender and the use of "disguises" greatly affect the accuracy of later identifications. In particular, changes in hairstyle or the use of a hat to conceal the hairline was found to greatly hamper the ability of eyewitnesses to make accurate identifications later. Factors surrounding the offense itself proved to be the most important predictors of accuracy. Not surprisingly, the longer the eyewitness is exposed to the offender, the more accurate the description.

5 "Weapon focus" also proved to be a real phenomenon affecting eyewitness accuracy: eyewitnesses were significantly more accurate where no weapon was used during an offense. Levels of accuracy increased slightly where a weapon was only implied. "Crime seriousness" also plays a role: in studies involving the theft of various objects, results showed that, in general, the more expensive the item stolen, the more accurate the witness's description of the alleged thief. Bystanders were found to be slightly more accurate than victims.

6 Interestingly, studies have found that race and gender play a role in the accuracy of facial identification. 7 Cross-race identifications were found to be less accurate than same-race identifications. Cross-gender identifications exhibited the same disability, but the levels of inaccuracy were less significant.


Post-Offense Factors Post-offense factors also affect accuracy. Although the studies to date are somewhat inconclusive, longer delays between the offense and the identification procedure seem to produce fewer correct identifications and more false identifications. Most importantly, the studies have clearly shown the effects of commonly used police identification procedures. Researchers also suggest that several procedures not commonly used by police might produce more reliable identifications. For example, where witnesses first view a lineup without a suspect present and do not make any identification, subsequent lineups with the suspect present produce more accurate results. Researchers also urge the use of single-suspect lineups (when possible) with known-innocent distracters. The devastating effects of a mistaken identification choice are minimized by this procedure. The research on the effects of police procedure on eyewitness accuracy constitutes a broad sub-category of eyewitness identification studies. 8 However, several procedures and concepts warrant specific attention: "instruction bias," "sequential" and "simultaneous presentations," and "functional size."

"Instruction bias," the most obvious example of which involves the police telling the eyewitness that the suspect is present in the lineup or photo array, has a profound effect on false identification rates. Any suggestion by the police that a suspect is present in the lineup increases the number of positive identifications, and where police do not "instruct" the eyewitness one way or the other, the witness will fail to identify any suspect more frequently. Even where no instruction bias is present, the structure of the lineup procedure may affect the accuracy of identifications. Studies suggest that the cognitive process used by an eyewitness in selecting a person from a lineup involves a "relative judgment." This simply means that the eyewitness will choose the person who most resembles the eyewitness's memory of the offender relative to the other members of the lineup. Thus, in lineups where the actual culprit is absent, error rates will increase. To combat the effects of the relative judgment process, researchers have suggested use of "sequential presentation" procedures. In sequential identification procedures, the witness is presented with single photographs or given the opportunity to view each possible offender separately. In contrast, simultaneous lineups involve the presentation of all possibilities at once. The findings show that a simultaneous presentation -- which is used most often by police -- doesn't produce a higher number of correct identifications where the "offender" is present in the lineup, but it does produce more mistaken identifications where the "offender" is absent. Sequential procedures, on the other hand, produce fewer false identifications where the culprit is absent and do not reduce the number of accurate identifications where the culprit is present.

"Functional size" refers to the number of members of a lineup procedure who resemble the suspect closely enough to be a viable choice for the eyewitness. Functional size can be contrasted with "nominal size," which indicates the total number of members of a lineup. For example, if there are six members of a lineup or photo array, five of whom are black and one (the suspect) is white, the functional size is one, the nominal size six. The ability of the witness to identify the culprit is not significantly reduced by increases in functional size.

9 Defense Counsel's Options Although an expert witness could certainly help to explain the factors affecting eyewitness accuracy in a particular case, defense counsel typically must rely on the "protection" afforded by jury voir dire and instruction, and eyewitness cross-examination. However, counsel may not be afforded the opportunity to identify "favorable" jurors during voir dire, and studies also suggest that jurors' general attitudes toward eyewitnesses do not predict reactions to specific testimony with any degree of accuracy. Jurors tend to over-estimate the accuracy of eyewitness identifications: they undervalue the effects of viewing conditions and over-value the witness's memory of peripheral details and confidence in the identification choice. Regardless, defense counsel should request that the court read to the jury CJI2d 7.8, which alerts jurors to the inherent problems with eyewitness testimony. 10 Cross-examination of the eyewitness is limited by counsel's access to information about the viewing conditions at the crime scene, witness factors, and knowledge of the actual procedures used during subsequent police procedures. In addition, as noted above, the right to counsel during such police procedures may not apply, and when it does, the damage caused by the suggestive procedure may be imperceptible or simply too strong for defense counsel to overcome. When confronted with these problems, defense counsel must first gain a familiarity with the factors that influence eyewitness accuracy established in the recent studies. In particular, it may be useful for counsel to review the findings on the effects of weapon focus, changes in facial features and the use of "disguises," cross-race recognition, and, most importantly, the suggestiveness of commonly used identification procedures. The use of expert testimony on these factors can be very helpful.

Although the data on the effectiveness of expert testimony is necessarily limited because of the current state of the law, findings suggest that jurors are not confused or prejudiced by such testimony. It educates jurors, and this works for both the prosecution and defense. Moreover, unopposed expert testimony produces the greatest degree of juror sensitivity to the factors outlined above and the least amount of skepticism of the testimony.

11 Using Expert Testimony on Eyewitness Reliability The goal is to challenge jurors' mistaken confidence in the reliability of eyewitness testimony. To increase the likelihood that the expert testimony will be admitted, it is important to begin searching for the expert witness early so that he or she will have ample opportunity to become familiar with the facts of the case. The ability of the expert witness to testify to the connection between general principles established by the research and the operative psychological factors in the case at bar is crucial. Securing the Expert Witness The witness must be a qualified psychologist with sufficient "knowledge, skill, experience, training, or education" to meet the requirements of MRE 702. The witness must demonstrate that he or she is familiar enough with the facts of the case to aid the jury in understanding a material issue in the case. People v Boyd, 65 Mich App 11 (1975). The psychology department of a major university would be a good place to began searching for a qualified expert. Also, even a cursory examination of the scientific literature will provide counsel with several possible sources. The witness must know the methodology used in experiments on eyewitness identification. Prior experience testifying is also helpful. Expert-witness fees will vary according to the witness's credentials and experience. Indigent defendants may have a due-process right to a court-appointed expert if the testimony is crucial to the case. MCL 775.15; MRE 706. The defendant must show that he cannot proceed safely to trial without appointment of the expert, that a denial would result in an unfair trial. 12 This will generally not be the case where the defendant presents alibi witnesses. More importantly, the subject matter of the testimony must be proper. As noted above, the general principles in question must be applicable to the specific facts of the case in which the expert is to testify. Furthermore, in Michigan, the general principles underlying the proffered testimony must meet the "general acceptance" standard of Frye v United States, 54 USApp DC 46; 293 F 1013 (1921). This means that defense counsel must initially overcome the "bad science" hurdle. The issue is not whether all experts in the field agree, but whether the method of inquiry in the field is generally accepted as sound scientific methodology. This should no longer be the problem that it once was because the scientific basis of eyewitness-reliability experiments is generally accepted within the field. Defense counsel's familiarity with the studies will certainly help meet this requirement. 13 In addition to showing that the methodology underlying the proposed testimony is sound, defense counsel must show that the probative value of the evidence is not substantially outweighed by its likely prejudicial effect or the likelihood of juror confusion or waste of time. MRE 403. The inquiry will involve a balancing of two factors:

   1.the ability of the expert's testimony to aid the trier of fact to accurately determine a disputed issue or the likelihood of misleading or confusing the jurors, and 
   2.the "fit" between the subject matter of the proffered testimony and the particular factors involved in the case that may have impaired the accuracy of the eyewitness's identification of the accused. 

United States v Downing, 753 F2d 1224, 1227 (CA 3, 1985). People v Smith, 425 Mich 98 (1986). In showing that the probative value will not be outweighed by prejudice or juror confusion, defense counsel will have to overcome two commonly held assumptions: (1) jurors exaggerate the importance of expert testimony, and (2) the expert will testify to common-sense information and cross-examination of the eyewitness would produce essentially the same result as the expert testimony. To undermine these assumptions of prejudice and confusion, defense counsel may wish to argue that findings on several factors are far from unanimous: thus, the "common-sense" argument may not be persuasive in a given case. Most importantly, counsel must connect the expert's qualifications and the subject matter of the proffered testimony to specific factual issues in the case. Counsel should not simply argue factors affecting the eyewitness's immediate ability to perceive the offender. In addition, ask whether:

   1.the eyewitness is very young or old; 
   2.the delay between the offense and a subsequent identification procedure was lengthy; 
   3.the lineup or photo array was conducted using one of the fallible procedures identified above (instruction bias,simultaneous presentation, low functional size); 
   4.the identification is cross-racial or cross-gender; 
   5.the eyewitness displayed confidence in the identification choice or extensive recall of peripheral detail; 
   6.the suspect wore a hat or other "disguise;" 
   7.the suspect displayed a weapon. 

Then offer to the court the expert's work on the relevant factors and the findings of that and other researchers' work. When examining the expert, if possible given the facts of the case, stick to questions concerning the factors about which there is little debate. Again, "malleable" witness characteristics, offense characteristics, and the effects of police identification procedures are all areas upon which broad consensus within the psychology community exists. Avoid questions which invite the expert to comment directly on the reliability of the eyewitness in the case. The goal is simply to invite the trier of fact to question the assumption that eyewitness testimony is per se reliable and persuasive. If expert testimony on eyewitness identifications is allowed at trial, the jury will almost certainly make a better informed evaluation of the identification. If such testimony is more frequently admitted during criminal trials, it seems likely that fewer innocent persons will be convicted. by Tobin Miller, Research Assistant & principal author, and Fred Bell, Assistant Defender Both Mr. Miller and Mr. Bell work in the Lansing office of the State Appellate Defender Office.


   1.For a recent and complete summary of research findings, see Cutler & Penrod, Mistaken Identification: The Eyewitness, Psychology, and the Law (New York: Cambridge UP, 1995), pp 55-269. 2.Many of the studies are funded by such organizations as the National Science Foundation, the National Institutes  of Mental Health, and the National Institute of Justice. Studies are also subject to peer review. Thus, despite suspicion that psychologists do not engage in scientific research, it is clear that the methodology of the studies should withstand scrutiny under both the Frye and Daubert standards. 3.Also, the Kachar factors are broadly phrased whereas the experts' studies often precisely define the factor affecting eyewitness accuracy.  4.Shapiro & Penrod, Meta-Analysis of Facial Identification Studies, 100 Psychological Bulletin 139 (1986). This article summarizes 128 studies involving 16,590 subjects and is thus the most comprehensive review of eyewitness studies to date. Unfortunately, however, no studies on the effects of witness intoxication or "weapons focus" are included. 
5.It should be noted that, in some studies, the improvement in accuracy "levels off" as duration increases beyond a certain time. 6.Research on the effects of witness stress and arousal, and on the effects of violence, is necessarily limited because of ethical restraints on the researcher. Some studies utilizing videotaped exposure to violent crime do indicate a high correlation between violence, identification accuracy, and witness memory. As to witness arousal, the
"Yerkes-Dodson Law" holds that attentiveness levels correspond to levels of arousal in this way: a person just waking up has low arousal and low attentiveness; an athlete preparing for performance has moderate levels of arousal and an ideal level of attentiveness; a person in danger or under duress has high levels of arousal and low attentiveness.  7.Shapiro & Penrod, supra, at 145. See also, Anthony, Cooper & Mullen, Cross-Racial Facial Identification: A
Social Cognitive Integration, 18 Personality and Social Psychology Bulletin 296 (1992). 8.See Wells, What Do We Know about Eyewitness Identification? 48 American Psychologist 553 (1993) for information on the effects of police procedures on eyewitness accuracy. Wells, a well-known researcher of the topic, clearly defines key terms and provides a complete bibliography.  9.Lindsay & Wells, What Price Justice? Exploring the Relationship of Lineup Fairness to Identification Accuracy. 4 Law & Human Behavior 303 (1980). 10.This instruction reflects the general conclusions of Franklin Anderson, supra. 11.See Cutler, Dexter & Penrod, Expert Testimony and Jury Decision Making: An Empirical Analysis, 7 Behavioral
Sciences and Law 215 (1989). 12.See Ake v Oklahoma, 470 US 68 (1985) (capital case), and People v David Allen Carson, supra. 13 See Sheldon & MacLeod, From Normative to Positive Data: Expert Psychological Evidence Re-Examined, 1991
Crim L R 811 (1991). 


Acuity and visual defects:

3)Standing at the Viewing Area, a person with 20/20 vision and an acuity of 1 arc minute will be able to see at 10 miles, the shift in the position of a light by 15 feet. Alternatively, at 10 miles, if two lights are separated by 10 feet you should just be able to see them as distinct lights, though perhaps just a bit blurred together. At 15 miles, you could just see two lights 7 feet apart. If the eye behaved the same under dark viewing and light viewing conditions, you could see the two headlights on a car, separated by 6 feet, at a distance of 16 miles. Each light is substantially smaller than their separation so they would just look like unresolved points of light with no structure. However, as we all know, when we look at points of light, they often look like tiny snowballs streaked or spiked with lines. This is caused by defects in the lens of our eye which refract the light in a very unsmooth way as it passes through the lens to the retina. Also, material in the aqueous fluid of the eye also can cause 'floaters' and other visual anomalies. Recall what you saw at your opthamologists office! So, when you can't resolve what you see, your eye still sees something going on as the light is refracted and diffused. You never see a pure point of light. Just look at the stars at night...or preferably look at them at Marfa and compare what they look like in shape, with what you are seeing.


Phantom Movement.

4)Sometimes when you look at spots on the floor that you can't really resolve, they actually look like they are changing their shape as you intently stare at them. They can also seem to move jerkily. This jerky motion is the movement of your eye called 'sacaddes'. It happens all the time even when you don't realize it. It is the way that the brain moves the visual field across the retina constantly so that the rods and cones do not 'tune out' the image because of acclimation or 'habituation'. Saccades happen ... per second....... They don't cause things to really jerk around....

  " During steady fixation, the eyes are in constant motion. Under good condition, retinal image traverses a distance of about 3 minutes of arc in one second. In one tenth of a second, the eye traverses 25 seconds of arc, which is about the angular subtense of a cone. Therefore, visual acuity must be determined in 0.1 seconds or less. Experiments to stabilise eye movement have shown no improvement in visual acuity. After the initial exposure to the test object, the image begins to fade. (eye8.htm)

 The other main class of eye movement comes about because the very centre of our retina, the fovea, is specialised for high-quality, full-colour vision; to see an object really clearly, that is where its image has to be. This means that to examine different objects we must deliberately shift the gaze to bring the eye on to the target. Since the resultant motion disrupts vision, we have evolved to make these movements as fast, and therefore as short in duration, as they can possibly be; they are called saccades. In addition, since we have two eyes, they need to be co-ordinated so that images of an object fall on exactly the same parts of the two retinae. For distant objects, this means that the two eyes must always move equally: the eyes are 'yoked' or conjunct. But as an object moves closer, the eyes must unyoke themselves, and converge to line up with the target: these are called disjunct or vergence movements. Finally, even when fixating a stationary object, the eyes are not still, but are making continual small movements. These micro- or fixation movements are composed of three components: slow drift, rapid, small-amplitude tremor, and micro-saccades that bring the gaze back when the drift has moved it too far from the target. How saccades are generated A saccade is rather a remarkable performance. In Man, the eyes move together at up to some 900 degrees per second, bringing the gaze smartly on to the the new target within as little as 25 msec. Yet the mechanical properties of the eye and its muscles are rather sluggish, responding to a step change in innervation with a slow movement that may take half a second or more to complete. The speed is achieved by means of a sophisticated control system in the brainstem, that sends a cleverly-coded pattern of excitation to the muscles.(


Low Light Level Problems.

5)Another feature of the eye is that it doesn't work the same under low light levels ( night time) as it does in daylight, or when it is illuminated in a strange way ( with a single bright point of light in a dark room). Here's what a report has to say about it:

"Since the earliest studies of space perception, it has been known that judgments of distance, depth, and size deteriorate with reduced illumination. Such findings have often been cited as evidence for the inadequacy of oculomotor cues for distance. A recurrent finding is the observation that, when distance information is diminished, as in darkness, perception appears to be biased toward an intermediate distance. That is, most subjects tend to underestimate the distance of far targets and to overestimate the distance of near targets. They also tend to misperceive size, depth, and motion in a manner consistent with errors of distance perception. These perceptual errors have been studied most extensively by Gogel (1969, 1978), who attributes them to the operation of the "equidistance tendency" and the "specific distance tendency.' According to this view, as information for distance is reduced, these autochthonic tendencies cause stimuli to tend to appear in the same depth plane (the equidistance tendency) and at an egocentric distance of about 2 m. Following traditional theories of the oculomotor system, he proposed that these tendencies work in opposition to oculomotor information for depth and distance. According to Post and Leibowitz (1982), this tendency to lose stable fixation is compensated by activation of the pursuit eye movement system. While the supplementary pursuit activity helps to maintain stable fixation, it also causes the fixated object to appear to be moving in the same direction as the pursuit effort. Due to the dark vergence bias, the VOR tends to be too large for objects beyond the resting distance, and the supplementary pursuit effort causes illusory movement in the same direction as the head movement. Conversely, for objects nearer than the dark vergence distance, the VOR tends to be too small, and the supplementary pursuit effort causes illusory motion in the direction opposite of the head movements. For objects lying in the same distance plane as the dark vergence posture, the VOR is appropriate, and therefore, supplementary pursuit activity is not necessary and there is no illusory motion during head movements. ""(


From David Crawford, Dark Sky Association:(eye15.htm)

Scotopic vision is rod vision. The rods are very sensitive, and allow us to see at very low lighting levels (below approximately 0.01 cd/m2). The rods have no color vision, so there is no color perception at such very low lighting levels, no matter what the color of the light source or of the object being viewed. At higher illumination levels (above about 3 cd/m2), we are in the photopic range, where cones have taken over most of the visual task. At the lowest lighting levels, one sees motion but little or no detail. The eye is very sensitive, but visual acuity is missing. With more illumination, say moonlight (a maximum of 0.1 lux), there is still no color but acuity is now fairly good. At about 1 lux, color is becoming apparent as the cones are now beginning to be used. Fovea vision is taking over, and acuity is good. With decreasing luminance levels, the spectral sensitivity of the eye changes and it becomes more blue sensitive. This goes in concert with the fading of colors until, in low levels, we perceive brightness only. (


6)The retina is an amazing sensor. It can detect individual photons of light hitting a single retinal 'rod' cell, (8. Hecht, S.: The Quantum Relations of Vision, J. Opt. Soc. Amer., 32: 42, 1942. ) which means, because light is a statistical process, that the brightness of very faint things (like stars) will flicker ever so slightly as individual photons arrive at different times....... If you stand in a dark room with only a faint light, you can see this for yourself.

From William Verplank ( eye16.htm) The change in sensitivity which occurs as the eye remains in the dark proceeds very rapidly for the first two to three minutes, and then at a slower rate for from five to six minutes. During this first period of adaptation thecones have become dark-adapted. A second period of adaption then begins, at first rapidly then at a decelerated pace.This second phase, which accounts for the major parts of the increase in sensitivity, is attributable to the adaptation of the rods, and is associated with concentration of rhodopsin within those cells. The threshold obtained when dark adaptation is complete is the absolute terminal threshold. Figure 101 presents a dark-adaptation curve showing the progress of dark adaptation in normal and abnormal retinas. Part of this change in sensitivity can be attributed to dilatation of the pupil occurring concurrently with the earlier phases of dark adaptation, but this is only a small part. It is defective dark

adaptation and low sensitivity of the rods which is ordinarily associated with nyctalopia1 or night blindness; relatively few studies have been made of the adaptation of the cones. Seeing at night, in levels of illumination below that of the full moon, depends upon the intactness of the function of the rods, which the absolute terminal threshold of the fully dark-adapted eye best measures. Night vision shows several characteristics at variance with those of day vision: (1) Under low levels of illumination, the eye is color blind; rod vision provides no physiologic basis for the discrimination of colors. (2) There is a shift in the spectral sensitivity of the retina, from a peak in the yellow (555 mm) for high intensities, to one in the blue-green (505 mm) for low intensities. (3) Visual acuity is poor, being reduced to a fraction of its daylight value. (4) A central scotoma, which corresponds to the rod-free fovea, appears in the center of the visual field. (5) Moving targets are more readily observed, and conversely, the eye detects targets more readily if it is not allowed to rest stationary. (6) During the course of dark-adaptation the visual field is unstable, and there occur transitory and striking increases and decreases in its clarity and subjective brightness. Various illusions and entoptic phenomena may be noted. In all, the characteristics of night vision are so different from those of day vision that a special technic of observation is required, and special training and practice are necessary for those who must see well under low levels of illumination. Some persons report consistent difficulties in seeing at night, even when they are fully dark-adapted. They cannot detect objects readily visible to others and show both confusion and slow recovery after brief exposure to relatively bright light sources. Maneuvering in dimly illuminated spaces and driving or flying at night present serious problems to these individuals. The presence of such a history, whether the disturbance in sight is of recent appearance or long-standing, is usually taken as prima facie evidence of night blindness. Many, if not most, of those individuals who report difficulty in seeing at night prove to be psychoneurotic. Many who have unusually insensitive retinas, on the other hand, do not report special difficulties in seeing at night, either because they assume that others have the same difficulties, or because they fail to note them in out well-illuminated urban culture, which offers few situations in which intact rod function is required. To establish the presence of nyctalopia, it is essential to use an instrument of established validity for the measurement of retinal sensitivity. From the studies which have been made of selected groups (e.g. school children, service men), it is known that the normal population will include a small percentage of persons of low visual sensitivity whose performance will be as poor as or poorer than that of many individuals whose nyctalopia is associated with disease or degenerative processes. About 2 per cent of the Navy men were disqualified for night duties as "night blind" on this basis. Those so disqualified seldom if ever showed symptoms other than a relatively high absolute terminal threshold, and their reduced sensitivity must be taken as the consequence of the normal variability in the density in the retinal rods and the efficiency of the process whereby rhodopsin, the visual purple, is regenerated.


From Herschel Leibovitz: "Both practical experience and findings in the scientific literature show that individuals vary greatly in their visual capabilities at night, and these differences are not fully predicted by standard vision tests. It is likely that a significant portion of these unpredicted differences results from changes in oculomotor behavior described in the preceding sections. This research suggests, for example, that night myopia results largely from the dark focus bias of accommodation and that optical corrections based on the dark focus can ameliorate the effects of such anomalous refractive errors. Other studies suggest that the dark vergence bias is related to problems of visual localization that occur under low illumination. These findings indicate that assessment of the dark focus and dark vergence may provide valuable information for predicting and enhancing nighttime visual performance. For nearly 200 years, night myopia has been recognized as a source of reduced visual detection and resolution capabilities under low illumination (Knoll, 1952; Levene, 1965). Despite repeated efforts, the problem remained intractable primarily because of individual differences in the magnitude of night myopia. While some subjects exhibited as much as4 D of night myopia, others had no night myopia. It was clear that the same night correction could not benefit everyone, but it was not clear why individuals differed in their susceptibility to night myopia. Evidence for individual differences in the resting state of accommodation suggested a simple solution to this problem. "(

 Afterimage Afterimage is the continuation of the visual sensation after the stimulus has ceased. It is caused by a continuation of the photochemical process resulting in nerve impulses, which cause continued object perception for a short period of time. Afterimage effect is proportional to the luminance of the object. Afterimage can be a negative or complementary process. The original color of the object is replaced by its complementary color. Afterimage makes a series of still pictures appear to move, a phenomenon from which motion pictures are derived.

 Time Sensitivity The time sensitivity of the visual system is phenomenal. The human eye can detect a pulse of light that acts for only 1/1000 second. It can detect a second pulse if the time interval between the two pulses is 1/10 s. This latter phenomenon applies only for cone vision, since the cones are much more time sensitive than the rods.

 Stimulus Sensitivity Rods are a thousand times more sensitive to intensity than the cones are at low levels of illumination. Under scotopic vision, it is said that the rods can detect the flicker of a candle 14 miles away. (


Color perception:

From a military night vision page: ( ...eye16.htm)

Photopic Vision Photopic vision is experienced during daylight hours or when a high level of artificial light exists. Under these conditions, sight is achieved primarily by the cones, especially those concentrated in the fovea. Due to the high light condition, rod cells are bleached out and become less effective. Sharp image interpretation (fine resolution of detail) and color vision are characteristic of photopic vision. Under these conditions, objects are detected with peripheral vision but are viewed primarily with central (foveal) vision. Mesopic Vision Mesopic vision is experienced at dawn and dusk and during periods of mid-level light. Vision is achieved by a combination of the rods and cones. Visual acuity steadily decreases; the available light decreases. A reduction in color vision occurs as the light level decreases; the cones become less effective. Due to gradual loss of cone sensitivity, greater emphasis is placed on off-center vision and scanning to detect objects.

Scotopic Vision Scotopic vision is experienced in low-level light conditions. Cone cells become ineffective causing poor resolution of detail. Visual acuity decreases to 20/200 or less. Color perception is totally lost. A central blind spot occurs due to the loss of cone sensitivity. Objects must be viewed using off-center viewing and scanning. The natural reflex of looking directly at an object must be reoriented by night vision training. The use scotopic vision demands searching movements of the eyes to locate an object and small eye movements to keep the object in sight. Characteristically, in this type of vision a dim image may fade away if your eyes are held stationary for more than a few seconds.

Night Myopia At night, the spectrum of available light changes; blue wavelengths of light are dominant. Therefore, a person who is slightly nearsighted (myopic) will find it hard to see at night; blurred vision could occur. Special lenses can be prescribed to correct myopia.

Color One way night vision differs from day vision is in color vision. As light levels decrease, the eyes shift from photopic vision (cones) to scotopic vision (rods). With this shift, the eyes become less sensitive to the red end of the spectrum and more sensitive to the blue part of the spectrum. Color perception is not possible with the rods. Colors of nonlighted objects cannot be determined at night under very low light conditions. You can distinguish between light and dark colors at night only in terms of the brightness of reflected light. If, however, the brightness of a color is above the threshold for cone vision, the color can be seen.

Detail Perception of fine detail is impossible at night. Low light conditions greatly reduce visual acuity. At 0.1 footcandle (level of full moonlight), acuity is one-seventh as good as it is in average daylight. Therefore, objects must be rather large or nearby to be seen at night. Identification at night must depend on the perception of generalized contours and outlines and not on small distinguishing features. Another important distinction between night vision and day vision is the difference in the sensitivity of various parts of the retina under these two conditions. The central part of the eye is not sensitive to starlight levels. During darkness or with low-level light, central vision becomes less effective, and a relative blind spot (5 to 10 degrees wide) develops. This is due to the concentration of cones in the area immediately surrounding the fovea of the retina. Since the central fields of vision for each eye are laid over each other for binocular (two-eyed) vision, a night blind spot occurs during periods of low-level illumination. If an object is viewed directly, it may not be detected because of this blind spot (Figure D-2). Because of the central blind spot, as distance increases, larger and larger objects will not be seen. To see things clearly at night, use off-center vision and scanning techniques. Viewing an object using central vision during daylight poses no limitation. If you use the same technique at night, you may not see the object. This is due to the night blind spot that exists during periods of low light. To makeup for this limitation use off-center vision. This technique requires you to view an object by looking 10 degrees above, below, or to either side of, rather than directly at an object. This lets your peripheral vision maintain contact with an object. Even when off-center viewing is practiced, the image of an object viewed longer than 2 to 3 seconds tends to bleach out and become a solid tone. As a result, the object is no longer visible. This produces a potentially unsafe operating condition. To overcome this limitation, be aware of the phenomenon. Avoid looking at an object longer than 2 or 3 seconds.

Illusions: As visual information decreases, the probability of spatial disorientation increases. Reduced visual references also create illusions that can cause spatial disorientation.


Autokinesis When a person stares at a still light in the dark, the light seems to move. This occurrence can be rapidly demonstrated by staring at a lighted cigarette in a dark room. Apparent movement will start after 8 to 10 seconds. Although the cause is not known, it seems to be related to the loss of surrounding references that normally serve to stabilize your visual perceptions. This illusion can be eliminated or reduced by visual scanning, increasing the number of lights, or varying the brightness of the light. The most important of the three solutions is visual scanning.

Relative Motion A person sitting in a car at a railroad crossing waiting for a train to pass often experiences the illusion of relative motion. Even though the car is not moving, the person feels that it is moving. The only way to correct this illusion is to understand that such illusions do occur and to not react to them on the vehicle's controls. Using proper scanning techniques can help prevent this illusion.

Reversible Perspective Illusion A vehicle may seem to be moving away when it is in fact approaching you. This illusion is often experienced when a vehicle is driving parallel to your course. To determine its direction, watch its lights. If the brightness of the lights increases, the vehicle is approaching you. If the lights dim, the vehicle is retreating.

Structural Illusions Heat waves, rain, snow, sleet, or other factors that block vision cause structural illusions. For example, a straight line may appear to be curved when seen through a desert heat wave.

Size-Distance Illusion This illusion results from staring at a point of light that approaches and then retreats from the observer. Instead of seeing the light advancing or receding, the lights may seem to expand and contract at a fixed distance. Without additional distance clues, accurate range estimation is extremely difficult. Using proper scanning techniques can help prevent this illusion.


Here's what a Rocky Mountain Helicopter Pilot manual says about visual illusions:(

So let's talk visual illusions and how to reduce them or eliminate them from you every day flying.

1. AUTOKINESIS: (probably one of the easiest for you to remember.)
When you stare at a static light in the dark, the light will appear to move. This phenomenon can be readily demonstrated by staring at a lighted cigarette in a dark room, or a small pocket flashlight illuminated through a hole in a piece of cardboard in a dark room. Apparent movement will begin after about 10 seconds. Where in the EMS environment would we be most likely to encounter this? Have you ever shot an approach to a black hole marked by a single bean bag light, or one head light, or one lone fireman with an in-hand flashlight illuminating the touchdown zone? Sounds all too familiar. Although the cause is not known, it appears to be related to the loss of surrounding references which normally serve to stabilize your visual perceptions. This illusion can be eliminated or reduced by VISUAL SCANNING! (This is one of the most common causes of night "hard landings" and flying into the ground.) If you can, ask for more than one light, or increase the intensity of the light being used. ANY time visual references are absent, you are subject to experience autokinetic illusions. An awareness is essential to ensure safe operations at night. This is equally important to your crews who are helping to clear your tail rotor and ensure your landing area is safe. I routinely ask the crews to intentionally (during cross-country flights) try and induce autokinesis. It is a lesson well learned!
This can be a real hazard for those involved in mountain flying. In the Denver/Colorado Springs area, their terrain can rise as much as 10,000 feet in 6 minutes of flying time! A common occurrence is to mistake ground lights for stars. When this happens, the pilot unknowingly positions the aircraft in an unusual altitude to keep the lights above them. For example, some pilots have misinterpreted the lights along a seashore for the horizon and have maneuvered their aircraft dangerously close to the sea while under the impression of flying straight level. This is where your need to practice your instrument flying comes in handy. ANY TIME you have an opportunity to perform a long cross-country with flight following or practice instrument flying, take advantage of the radar service. Every flight in this mode is worth its weight in gold when it comes to night visual illusions and IMC conditions.
I call this the "going through the car wash and hitting your brakes three times to keep it from running out of brushes!" In EMS, we routinely land in large grassy fields, with the grass usually being in excess of 10 to 12 inches. At night, when this is blowing from the rotor wash and being illuminated by your light source, it can give you the feeling you are moving, and being the astute pilot that you are, you make corrections to eliminate the movement, ONLY to create unnecessary movement in the first place. Even though you are not moving, you have the sensation that you are. The only way you can correct for this illusion is to understand that such illusions do occur and that you should not react to them on the controls. Using proper scanning techniques can help and prevent this illusion. Also, pick another focal point within the LZ, e.g., a tree, or a fire truck, and conduct your landing operation as if you were doing hoist or long line, or a good camera shot for Channel 13 news. The bottom line is to keep from moving unnecessarily and having eminent contact with an object.
This illusion becomes critical in high density flying areas, e.g., the greater New York area, Boston, San Francisco, Chicago, etc. An aircraft may appear to be retreating when it is in fact approaching our position. This illusion is often experienced when an aircraft is flying parallel to your course. To determine its direction of flight, watch its lights and have your crew members participate, too. If the intensity of the lights increases, the aircraft is approaching your position. If the lights become dim, the aircraft is retreating. Keep your head on a swivel at all times while in traffic in a major airport.
The illusions of false horizons is experienced when something other than the actual horizon is identified as being horizontal to the horizon. This is a phenomenon common in local flying areas that go from virtually a "sea level" flat atmosphere to mountainous terrain in a relatively short period of time. Lights in the mountains may appear to be stars or vice versa. At least most of us have a built-in tendency to climb when flying into the "dark" mountains. This can also happen when you are flying between two cloud banks as in VFR on/over the top. You may position your aircraft in relation to the lower cloud layer because it seems to be horizontal to the horizon. The lower cloud layer may actually be at an angle to the horizon.
Approaching a line of mountains or clouds alters your planes of reference. You may feel you need to climb even though your altitude is quite adequate. Additionally, when flying parallel to a line of clouds, you have a tendency to tilt away from the clouds. Use your good scanning techniques; rely on your instruments.
If you are a frequent reader of the magazine "Approach" put out by the Navy, you know this is one of their major causes of incident. Flying over the desert, snow, or water causes an illusion of having more altitude than you actually have. This is because of a lack of visual references. To overcome this problem, it may be necessary to drop and object (such as a chemical stick or flare) on the ground before landing. While hovering over water, something as simple as dropping a plastic cup or flotation device in the water will give you a point of reference. Flight when visibility is restricted by haze, smoke, or fog produces the same illusion of height perception.
If you sit in a room with fluorescent lamps as the only source of light for an extended period of time, you might notice an unpleasant and uncomfortable experience. More and more offices are suggesting individual desks have additional incandescent light available for their consumers. Why? A light flickering at a rate of 4 to 20 cycles per second can produce unpleasant and dangerous reactions. Such conditions such as nausea, vomiting, vertigo, and on rare occasions, convulsions, and unconsciousness may occur. Fatigue, frustration and boredom tend to intensify these reactions. We need to be head up on these long cross-country flights where the only illumination we have to be exposed is to that flickering belly light of the high intensity strobe. Your rotating beacon reflecting against a cloudy sky can produce the same effect.
As EMS pilots, we fall prey to this illusion more than any other. In the cockpit we get very task saturated with radio traffic, crew demands, outside air traffic control, "squawks," dangerous landing/take-off environment, incremental weather, persona issues, etc. It is very easy to ignore orientation cues and fix our attention on one tasking. This is especially dangerous at night. We are so anxious to "get that airborne and enroute call off," we tend to forget the aviating of flying. Eight out of ten wire strikes occur on take-off! Amazing! Aircraft ground closures rates are difficult to determine because of the reduction or absence of normal daylight peripheral movement. Increased scanning is the only thing to prevent this illusion. As you have heard many times before, fly the aircraft FIRST!
Structural illusions are caused by heat waves, rain, snow, sleet, or other factors which obscure vision. For example, a straight line may appear to be curved when seen through a desert heat wave, or a wing tip light may appear to double or move when viewed during a rain shower.
This illusion results from staring at a point of light which approaches and then retreats from the observer. Instead of seeing the light advancing or receding, you may perceive that the lights are expanding and contracting at a fixed distance. Without additional distance cues, accurate range estimations are extremely difficult. Using proper scanning techniques can help this illusion. SCAN...SCAN...SCAN...!





Copyright (C) 2001 Dr. Sten Odenwald