Human Visual Acuity

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.

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= http://retina.umh.es/Webvision/KallSpatial.html...see file eye8.htm)

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

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. (http://schof.colorado.edu/~kubala/Home.html)

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 (http://www.sado.org/19cdn12.htm#19cdn12a Sept 1996, Vol 19. No. 12...eye13.htm)

   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. 

   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. 

   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. 

   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. 

                                        ENDNOTES 

   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). 

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.

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.( http://www.cai.cam.ac.uk/caius/subjects/medicine/oculo.html...eye14.htm)

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. ""( http://www.ulib.org/webRoot/Books/National_Academy_Press_Books/night_vision/00000097.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. (http://www.darksky.org/ida/ida_2/info136.html)

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 (http://web.utk.edu/~wverplan/biblio10.html 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. "(http://www.ulib.org/webRoot/Books/National_Academy_Press_Books/night_vision/00000097.htm)

 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. (http://www.arce.ukans.edu/book/eye/color.htm...eye10.htm)

From a military night vision page: (http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/21-305/Appd.htm ...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:( http://www.rmhllc.com/wpapers/night3.html...eye18.htm)

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!

2. CONFUSION OF GROUND LIGHTS WITH STARS:

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.

3. RELATIVE MOTION:

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.

4. REVERSIBLE PERSPECTIVE ILLUSION:

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.

5. FALSE HORIZONS:

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.

6. ALTERED PLANES OF REFERENCE:

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.

7. HEIGHT PERCEPTION ILLUSION:

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.

8. FLICKER VERTIGO:

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.

9. FASCINATION/FIXATION:

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!

10. STRUCTURAL ILLUSIONS:

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.

11. SIZE-DISTANCE ILLUSION:

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