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Nereid
2006-Sep-10, 02:19 AM
I've started this thread in order to discuss the experience that we individual Homo sapiens critters have wrt 'surface brightness', particularly at the low light levels we experience when we observe extended (astronomical) sources through the eyepieces of telescopes.

I am especially keen to not keep making OT posts into another Q&A thread, Surface Brightening (http://www.bautforum.com/showthread.php?t=46471), which my posts seem to be in danger of dragging OT.

Here is my last post in that thread, and George's response to it:
First, what the brain 'sees' is what the brain thinks is 'real' - and it can get incredibly stroppy if you try to convince it otherwise.


An example, way different from 'surface brightness', so there will be (hopefully) no spillover of the brain's vehement insistence: there is an Australian tree frog that looks green, and to all of us, it is, in fact, green. However, to an objective, inanimate spectrophotometer, it isn't 'green' at all, it's blue+yellow. I.e. it reflects sunlight strongly in the 'blue' and 'yellow' wavebands in the optical spectrum and essentially nothing at all in the 'green' waveband.I don't understand this. If the color we see is different than the color rendered by an instrument, then the instrument is wrong since color is defined by what we see. [Of course, this assumes the background colors and light source are not tricking us.]
So, to 'surface brightness'. The eye has two kinds of photoreceptors - rods, which have a relatively low threshhold, and cones (three* kinds), which are responsible (along with circuitry in the brain) for colour, and have a relatively high threshhold (so, at low light levels, as perceived by the eye, there is no colour, only shades of grey).The eye can see across 12 orders of light levels! Green is the peak integrated color response and is the last color to fade away with decreasing light conditions. Many emergency vehicles are painted green for this reason. [Albeit, it seems to me they have gone back to red.]

This sensitivity to green should help when observing ionized oxygen clouds. I have forgotten the BA's Astronomy article which suggests there could be regions where they have enough "surface brightness" for our color cones to perceive and render them green. I wonder if anyone has seen these? I recently visited with someone who, with a 60", noticed red in a region.

[Part of the motive to acheive greater surface brightness is to allow full color visual observations. I still remember the day I first watched a color tv show. ]
Thus, to enhance the perception of surface brightness, for humans, at low light levels, an effective strategy is to increase the total number of 'rod activations' ... so the faintest parts of the extended source do trigger a rate of rod neuron firing that the brain perceives as 'grey' (and not 'black'), and the brighter parts will thus be perceived as 'paler grey', or even 'tinted' (if at least one set of cones fire at above the threshhold rate).

How might this be done? One way is to image the extended source onto a smaller number of rods and cones - same total number of photons, but many fewer rods (of course, the source will look 'smaller'); another is to increase the total number of photons entering the eye.I'd like both - more photons on fewer rods and cones.

If we could reach the cone threshold, we might gain the greater accuity of the fovea region. It is the most densley packed area, thus capable of greater resolutions. It has no rods and has no blue cones, but the eye still sees blue by its constant movements, I think. Stars exceed this threshold, of course. I want to say Tycho Brahe used an average reading from several observers to help him achieve, in his best observations, 0.5 arc minutes.
*Some women have four - the 'red' cone comes in two varieties, and as the gene for coding cones is on the X chromosome, women can have both.

Birds have far more accute colour vision than humans - they have four quite different kinds of cones, as well as special filters, to enhance colour discrimination ... and that's just in the eye!Yes. Birds, and some other animals, are tetrachromatic. From what little I've seen, none of their cone response curves overlap. They must really have some great vision.

George
2006-Sep-10, 04:14 AM
Good idea. There are many variables which affects our vision of the heavens.

1) Acclamation time. [What is the proper name?] Time for the pupil to dilate and improve our ability to see fainter objects.

2) Why doesn't red light bother our dilation?

3) When does greater magnification improve the apparent brightness of all objects?

4) Will a larger image appear brighter simply due to the number of rods activated?

5) What is the importance of exit pupil and entrance pupil?

6) How does a telecompressor, and other optical accessories, help?

7) What is the best way to observe a region considering issues such as our blind spot?

8) How does our age affect our viewing?

hhEb09'1
2006-Sep-10, 04:45 AM
1) Acclamation time. :clap: :clap: :clap: :clap: :clap: :clap: :clap:
[What is the proper name?] Acclimation :)
What is the best way to observe a region considering issues such as our blind spot?Our blind spot is usually so far away from our center of vision, I'm not sure that it should make much difference at all.
8) How does our age affect our viewing?I couldn't read this one.

George
2006-Sep-10, 05:21 AM
Acclimation
Perhaps there's a Texas term I'm forgetting. :)


Our blind spot is usually so far away from our center of vision, I'm not sure that it should make much difference at all.
Looking directly at a very dim object is harder to see than if you offset your vision. Is that not due to the blind spot, or is it a fovea alignment thing?


I couldn't read this one.
Let me customize it...

8b) How does your age affect our viewing?
:)

hhEb09'1
2006-Sep-10, 05:30 AM
Looking directly at a very dim object is harder to see than if you offset your vision. Is that not due to the blind spot, or is it a fovea alignment thing?Averted vision. No, it's not due to the blind spot. It's due to the rods, sensitive to dim light, not being concentrated in the fovea.

The blind spot is where the fibers of the optic nerve emerge from the eyeball. It doesn't have rods, or cones. You can't "see" there. (A great demo is to put a small red dot on a whiteboard in front of a classroom, and have students stand a small distance away with one eye covered, and have them watch the tip of a small wand while paying attention to the dot in their peripheral vision. If you're familiar with the process, it only takes a few seconds to find their blind spot. The red dot winks in and out of view, disappearing and re-appearing. The mind "fills in" the blind spot with the surrounding background.)

PS: I use that demo as an illustration of how their mind can "lie" to them, and how science can expose those lies and let us truly see what's going on.

George
2006-Sep-10, 06:59 PM
Ah yes, averted vision is the term. So it was the fovea, thanks.

I have not done the blind spot demo. Looks like it is one all would like and remember.

hhEb09'1
2006-Sep-10, 07:44 PM
I have not done the blind spot demo. Looks like it is one all would like and remember.You can do it sitting at the computer. Pick an isolated small letter and close your right eye. Here, look at the H, but be aware of the o. If you can still see the o, move your point of attention up/down left/right of the H until the o disappears.


.............o.................................... ..............................H.............

Ken G
2006-Sep-11, 11:46 PM
You can get rid of the H with the left eye closed too, not surprisingly. It's a nice demo of the blind spot.

hhEb09'1
2006-Sep-12, 12:14 AM
What do you see in place of the letters? :)

Nereid
2006-Sep-12, 02:38 AM
The outer layer of its skin has a colloidal suspension, in the cells, which scatters incident light by the Tyndall effect (a.k.a. Rayleigh scattering). If this were all that there was, the frog would "look blue" (and a spectrophotometer's verdict would be "light reflected from the frog has a single peak {insert 1%, 99%, and peak wavelengths here}").

Beneath this layer is one comprised of a pigment - it reflects incident "yellow" light (and absorbs the rest ... in the optical range) - and a spectrophotometer's verdict would be "light reflected from the frog has a single peak {insert 1%, 99%, and peak wavelengths here}".

Combining the two, a spectrophotometer's verdict would be "light reflected from the frog has a double peak {insert detailed description here}".

Humans, however, report that the tree frog "looks green" - the red, green, and blue cones fire at rates which the brain interprets as "green".

In terms of the difference between colour perception and 'reality', what the tree frog's skin does is similar to a (now outdated?) high school science teacher's demo - fill a test tube with a solution of copper sulphate (or other 'blue'); fill another test tube with a solution of sodium chromate (or other 'yellow'); when you look through both, what colour do you see?

George
2006-Sep-12, 03:14 AM
You can do it sitting at the computer. Pick an isolated small letter and close your right eye. Here, look at the H, but be aware of the o. If you can still see the o, move your point of attention up/down left/right of the H until the o disappears.


.............o.................................... ..............................H.............
Thanks, that worked. A nice simple demo.


You can get rid of the H with the left eye closed too, not surprisingly. It's a nice demo of the blind spot.
Yes, that worked, too.

I went ahead and tried one other combination and obtained a dramatic result. ;)

George
2006-Sep-12, 03:32 AM
The outer layer of its skin has a colloidal suspension, in the cells, which scatters incident light by the Tyndall effect (a.k.a. Rayleigh scattering). If this were all that there was, the frog would "look blue" (and a spectrophotometer's verdict would be "light reflected from the frog has a single peak {insert 1%, 99%, and peak wavelengths here}").

Beneath this layer is one comprised of a pigment - it reflects incident "yellow" light (and absorbs the rest ... in the optical range) - and a spectrophotometer's verdict would be "light reflected from the frog has a single peak {insert 1%, 99%, and peak wavelengths here}".

Combining the two, a spectrophotometer's verdict would be "light reflected from the frog has a double peak {insert detailed description here}".

Humans, however, report that the tree frog "looks green" - the red, green, and blue cones fire at rates which the brain interprets as "green".

You are describing a metamer, I think. A given color, seen by the eye/brain (retinex), can come in different packages of wavelenths. A green frog could be green either from a green peak in the spectral irradiance or a combination of a blue and yellow peak. The net color would still be green using the human retinex sytem as the standard, which it is.

Nevertheless, your point is important because knowing the difference can be critical to knowing what is happening causing the yellow and blue peaks to occur.

hhEb09'1
2006-Sep-12, 04:48 AM
Th
I went ahead and tried one other combination and obtained a dramatic result. ;)You closed both eyes, and both letters disappeared? Ken G, maybe he'll let us be co-authors on this paper when he writes it up :)

PS: George, are you familiar with Edwin Land's Retinex theory (http://en.wikipedia.org/wiki/Color_constancy)? Maybe it has something to do with heliochromology :)

George
2006-Sep-12, 12:47 PM
You closed both eyes, and both letters disappeared? Ken G, maybe he'll let us be co-authors on this paper when he writes it up :)
No thanks, too dangerous - I was blinded! But, fortunately I recovered.


PS: George, are you familiar with Edwin Land's Retinex theory (http://en.wikipedia.org/wiki/Color_constancy)? Yes, founder of Polaroid. The term retinex is the only one I've run across that describes both the reception and color rendering in one nice word.


Maybe it has something to do with heliochromology :) Everything has something to do with this pinnacle of physics and physiology. ;)

hhEb09'1
2006-Sep-12, 01:35 PM
Yes, founder of Polaroid. The term retinex is the only one I've run across that describes both the reception and color rendering in one nice word. I should have remembered that (http://www.bautforum.com/showthread.php?p=446417#post446417).

George
2006-Sep-12, 02:43 PM
I should have remembered that (http://www.bautforum.com/showthread.php?p=446417#post446417).
I only vaguely remember that. Don't be too hard on a multi-mirrored metric mile reincarnated rookie. :)

Nereid
2006-Sep-14, 02:18 AM
I introduced the green tree frog only to illustrate that what the brain 'sees' and what 'reality' is may be quite different, and that, in terms of subjective experience, the brain's interpretation trumps reality every time.

If I can get back to the object of George's desire?

How about we start with the unaided eye?

Some folk are lucky enough* to live where the LMC and SMC ride high in the sky, and where the sky on moonless, cloudless, nights is really dark.

To these folk (may we all, someday, be so lucky), the LMC and SMC are diffuse patches of light - they have a 'surface brightness'.

We know, objectively, that at the top of the atmosphere, objective detectors would record an essentially unvarying surface brightness, for these two objects.

Down on the ground, it ain't necessarily so.

Like clockwork, both the LMC and SMC's surface brightness seems to change - on certain days every (lunar) month, their surface brightness is gone.

So sayeth the brain.

How can we relate the objective reality to what the brain insists is happening?

*For those unlucky enough to never see the LMC or SMC, you may choose your favourite patch of the Milky Way (http://antwrp.gsfc.nasa.gov/apod/ap020923.html).

George
2006-Sep-14, 12:50 PM
Roger Clark has a web site (http://www.clarkvision.com/visastro/omva1/) which gets deep in to the contrast issue of the eye.

Interstingly, the loss of the eye's ability in seeing contrast can be an early sign of a problem such as diabetes as found here (http://www.vectorvision.com/html/educationDiabeticEyeDisease.html).

I suppose there are many analogies for visual contrast. Wouldn't all of our senses have this issue? For instance, my ability to hear individuals in a crowded room is embarassingly bad. If the sense of smell is fading from our evolutionary path, then I'm ahead of the pack. :)