Intense sunlight and its effects on contrast in shadows.


by JadedDragoon
Tags: contrast, effects, eyesight, intense, optics, reflectivity, silhouette effect, sunlight
JadedDragoon
JadedDragoon is offline
#1
Sep21-13, 09:25 PM
P: 2
First, let me just say that, while I tend to be knowledgeable on a wide variety of topics, I am absolutely atrocious with details such as names, times, and specific values.

A friend of mine has decided that I'm imagining an effect that I have experienced on countless occasions and which I have seen documented on countless more. The effect in question specifically relates to the effect of intense reflected light on the human eye's ability to discern details in shadow while excluding the possibility of more than one primary source of light.

For example, assume the following details to be true:
  • you are outdoors
  • it is a cloudless day with intense sunlight
  • the time is no later than 2pm or earlier than 10am
  • you are not at a latitude which would appreciably reduce the intensity of sunlight at mid-day as compared to the equator
  • there are no primary sources of light other than the sun
  • you are standing with the sun "behind you" (if it is morning you are facing west, if it is after noon you are facing east)
  • the surfaces reflecting the light of the sun are not typically known for their reflectivity (grass, concrete, asphalt, dirt, bark, leaves, etc)
  • a man with olive skin is standing in front of you with a hood over his head
  • his hood is pulled low so that it casts a shadow over his face
  • his hood is not low enough that his face is physically blocked from your sight
  • you have just exited a building with no windows and whose only source of light was a fire in a fireplace, your eyes have had no time to adjust.

The question is: given these details, would the average human being be able to make out the fine details of the hypothetical man's face well enough to recognize the individual later without the hood?

I have argued quite adamantly that they would not be able to. Instead the intense sunlight reflected by every surface except his face would result in a silhouette effect for the hooded area making it impossible to make out the fine details of his face even with the ambient light that would inevitably reach it.

I have personally been in the situation I described above and many more producing similar effects. I have also seen this effect portrayed in anime and live action fiction both with hoods and also just shadows cast on a person by a building... so I know others have experienced this as well (they must have to feel it necessary to portray it in their works of fiction). I have also seen this effect documented as part of separate research when I was working on trying to accurately reproduce real world effects of sunlight in a virtual 3D environment. Because of these things I am very very hesitant to even consider that I might be wrong about this... however, I have a standing policy of always making myself open to the possibility of such no matter how preposterous it seems.

I have chosen to place the question here in biology because my friend has insisted quite vigorously that the effect I described is flat impossible due to the workings of the human eye... though he has not really expanded on why.

He also acknowledges this effect is possible if the primary light source is directly behind the person but insists that it is impossible with reflected light... though personally I can not see where the fact that the light has been reflected is relevant. Only the over all amount of the light striking the retina seems relevant to me in that context.

He has also rejected my comparison to the effect of a camera with a shutter speed set too low (causing over exposure to the areas of the scene with the most light and exaggerating the dark areas) though it seems to me that this is a one-to-one comparison for the purpose of the question outlined above. He specifically claims that, should the scene change, a human eye would update the image fresh with no "streaking" where a camera would blur (an effect used for time-lapse photography). This to him seems to be the main basis of why it would be different... though again I can see no relevance to the question above.

Unfortunately, I can not think of any of the terminology involved in this phenomenon and thus have no way to research it and find the aforementioned documentation again. This has left me in the lurch so to speak between being unable to prove myself right and also unable to prove myself wrong.

Any one care to help untangle this mess or perhaps toss some terminology my way so I can research it myself?
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mfb
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#2
Sep22-13, 08:12 AM
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All objects in the scene (concrete, grass, whatever) scatter some fraction of the light - the face will receive a relevant fraction of the light other parts (not in the shadow) get. I would expect something like 10% - certainly enough to see details of the face. In your scenario, however, I would expect to be dazzled by the bright light everywhere - in that case it does not matter if the face is in the shadow or not, the eyes will need some time to adapt to the bright light.
zoobyshoe
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#3
Sep22-13, 11:10 PM
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Quote Quote by JadedDragoon View Post
He has also rejected my comparison to the effect of a camera with a shutter speed set too low (causing over exposure to the areas of the scene with the most light and exaggerating the dark areas) though it seems to me that this is a one-to-one comparison for the purpose of the question outlined above. He specifically claims that, should the scene change, a human eye would update the image fresh with no "streaking" where a camera would blur (an effect used for time-lapse photography). This to him seems to be the main basis of why it would be different... though again I can see no relevance to the question above.
There is no analogy to shutter speed in human vision. The only analogy to photography here would be to aperture. Your pupils would be opened up (dilated) to a large "aperture" from being in the darkened room. The larger your pupils, the more light they let in. As soon as you go out into the brighter light, they would start constricting.

I'm not sure about your scenario. In my case, when I'm suddenly dazzled by going outdoors, I try to keep my gaze focused on shadows simply because it's too painful too look anywhere else at first. I don't have any recollection of not being able to see details in those shadows, but I also don't recall the opposite. The thing that comes to mind is just that looking in the shadows is the only way to avoid the pain of the bright patches.

mfb
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#4
Sep23-13, 07:23 AM
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Intense sunlight and its effects on contrast in shadows.


Quote Quote by zoobyshoe View Post
The thing that comes to mind is just that looking in the shadows is the only way to avoid the pain of the bright patches.
Closing the eyes is another good option, but then you can't identify the person in front of you of course.
JadedDragoon
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#5
Sep24-13, 12:51 PM
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Quote Quote by mfb View Post
All objects in the scene (concrete, grass, whatever) scatter some fraction of the light - the face will receive a relevant fraction of the light other parts (not in the shadow) get. I would expect something like 10% - certainly enough to see details of the face.
Would that not then make silhouette effects completely impossible? Even though we know that they are no only possible but common place?

I know that some light is reflected... but the issue in my experience is that so the difference between what little is reflected by the shadowed portion of the scene and the entire rest of the scene is so great that the shadowed portion registers as almost solid black. This is, in fact, the definition of a silhouette as I understand it. Not that no light is reflected but that the difference is so great the dark portion seems black.

Quote Quote by zoobyshoe View Post
There is no analogy to shutter speed in human vision. The only analogy to photography here would be to aperture. Your pupils would be opened up (dilated) to a large "aperture" from being in the darkened room. The larger your pupils, the more light they let in. As soon as you go out into the brighter light, they would start constricting.
But the shutter speed on a camera serves exactly the same purpose. That is, it's purpose is to control the amount of light allowed into the camera. To me it seems this is a 1-to-1 comparison... as in a comparison between "controlling the amount of light that strikes the unit's sensors" to "controlling the amount of light that strikes the unit's sensors". The sole difference between the two... that in the human eye light continues to pour in after the retina has received enough light to form an image (allowing it to create a new image) doesn't seem relevant the scenario in question.

Also you are quite right about being dazzled by the bright light... but I find that while dazzled and looking into the shadows I am unable to make out details in the shadows at all unless there is a way to make the shadows occupy most of my vision (such as by using my hands to block out the intense glare of the reflected light).

---

The key to me seems to be that so long as the majority of the scene is composed of intense reflected light the shadows would end up no more than a silhouette with only the most vague details discernible. If you angle your view so that the scene is composed mostly of shadow you can then see much more clearly into the shadows. This is what I've experienced in the past and it was the increase in apparent contrast in shadow resulting from me looking away from the harsh reflected light that made me aware of this effect.

However, it seems pretty clear that I am in the minority in my view at this point. So I dunno.
mfb
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#6
Sep24-13, 02:07 PM
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Quote Quote by JadedDragoon View Post
Would that not then make silhouette effects completely impossible? Even though we know that they are no only possible but common place?
If the sun (or some other source of light) is in front of you, the difference between "dark" and "bright" can be much more pronounced.
FainAvis
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#7
Sep27-13, 06:16 PM
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See dynamic range in Wikipedia. Moonlight to sunlight is a dynamic range of a billion to one. But can you go from sunlight to moonlight (or a dark room of similar light intensity) and immediately see details? No, because it takes a while to become dark adapted.
The photographer using film has to think in terms of the zone system. Full white on the page represents maximum white intensity. Black the darkest shadow. And between them, nominally at 18% grey, is the half tone. Clearly then, not a linear relationship between max white and max black. But some people or purposes prefer or require the half tone shifted to 13% or 40% say, judged by the artist's eye. So we speak of the gamma. That is the (instantaneous) slope of the graph between light intensity and its representation on the paper. It has to be on some kind of log scale because of the extreme variance between the gamma at high light intensities and that at low light intensities.
At the moment I am sitting in front of two computer screens, both showing black text on nominally white background. (Both backgrounds html color #ffffff). If I write on one for a time the second screen begins to look green. Work on the second and the first screen begins to look some tint of magenta, and that persists over at least ten minutes.
Consider also what happens when the dark adapted eye is suddenly exposed to a paparazzi camera flash. You cannot see anything for a while.
So in your scenario, containing objects in deep shadow, and simultaneously others in bright light, there will be a time lag while the eye becomes adapted either to the bright object of interest or the deep shadow object of interest.
Again - The eye takes time to adapt. So you cannot see both extremes at once.
jim mcnamara
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#8
Sep28-13, 05:56 PM
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Human eyes have a "set" of light sensing neuronal endpoints that work well in
low light, another "set" that works well with higher light levels. They are
rods and cones, and provide scotopic and photopic vision, respectively.

The lower level sensors, rods, provide poorer resolution (20/200 for persons
with good uncorrected visio) and are everywhere in the eye, except in the
fovea. They do not function well in high light levels.

Cones are located only in the fovea, and provide much greater resolution (20-20)
for normal eyes. Cones function poorly at lower light levels.

Note: this means both rods and cones can be active (mesopic vision) at the same
time. Like a dimly lit room - or viewing objects in shade and full sunlight.
However both are functioning suboptimally.

Both rods and cones contain light-sensitive photopigments. Rods have rhodopsin.
The three different photopigments, opsins, are slightly different from
rhodopsin. They provide color vision.

Upon exposure to light, photopigments undergo a chemical reaction that converts
light energy -> electrical activity -> neuronal activity. Nerve fibers connect to
the brain.

One chemical reaction is light adaptation. Full light exposure means photopigments
are decomposed. Like accidentally looking to the setting sun.
Intense light will decompose the photoreceptor pigments rapidly
and completely, thus eliminating retinal sensitivity to dim light or bright light. Regeneration of
the photopigments occurs during dark adaptation. This is recovery from bright light "overload".

A completely dark-adapted eye (rhodopsin regeneration is complete),
has restored retinal sensitivity to its maximal level.

Rods and cones differ markedly, however, in their rate of dark adaptation after a blast of brilliant light.

Cones attain maximum sensitivity in 5-7 minutes, while rods require 30-45 minutes or longer of
absolute darkness for full sensitivity after exposure to very bright light.

Answer: So, during transitional periods when none of the sensors is working
optimally, conditions like you describe, the ability for photopic (cones) vision
to function well is not established. And rods do not work at all.

Or: this is why attacking armies prefer the sun at their back. The old 'attack
at dawn' idea. Screw up the other guys vision as much as possible.

The "other guy" is making stuff up. Going either way (brilliant light -> dim or dim->brilliant) puts our eyes into a short period of suboptimal vision. Pilots get training about this.

See this for a detailed description.

http://www.aoa.org/optometrists/tool...d-night-vision


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