Telescope Reviews: Colors of stars or colors of equipment?

at the same time, you shouldn't presume your ignorance speaks to (or for) the comprehension of other people, nor that your ignorance requires an analysis in other terms than those customary in a particular field.

the basic principle: if you start with the concept that visual response is produced by a photon of light striking a specific molecule of visual pigment, then the photon has a much better chance of striking a photopigment molecule if there are millions of photopigments in a receptor cell than if there is only one molecule remaining. the analogy between a photopigment molecule and a silver grain in a photographic emulsion or the electron flux in a CCD sensor is physically exact, so we are looking at a general sensor principle.

"bleaching" or saturation occurs when previous photons have already struck and "converted" nearly all the photopigment molecules or silver emulsion grains, so that many, many more photons are necessary before one happens to strike that last molecule or grain. this means that the effectiveness of added illuminance in producing a contrast difference decreases as the total level of illuminance goes up. this is very familiar to photographers as the loss of contrast detail in parts of an image that are very bright, or in the total image that is very overexposed.

visually, you won't be able tell the difference between the luminance of the disk of the sun and something that is twice or half the luminance of the disk of the sun, because both of them will completely blitz every photopigment molecule in your retina. (this produces a temporary but complete incapacity to see anything in any other direction, called an afterimage.)

if you plot (graph) the change in visual response as a change in stimulus illuminance or luminance, you get something like this:

and this is not only a familiar function from psychophysics, but also from videography, film and print photography, computer display, projection film display, projection video display and nearly all other sensor or display technologies. (please excuse my dire presumption that you were familiar with at least one of them, or would prefer to launch a few google searches in your self education.)

the main point of the graph is that increasing luminance (horizontal dimension) produces very little difference in response (vertical dimension) at the very high or very low ends. the greatest response difference is produced in the middle range. on your computer monitor, this middle range contrast display is evaluated as your display gamma. your eye also has a response "gamma" as the change in luminance necessary to produce an incremental visual response. obviously, for best contrast sensitivity, you want the luminance of the target to match the luminance at the middle of your visual response range -- in other words, where the slope of the curve is steepest.

as you say, you look at the sheet of paper rather than the light itself, and by that means you dim the luminance of the stimulus so that it falls closer to the middle part of your response function or characteristic curve. but the sheet of white paper is still far brighter than anything else in your working environment: therefore it somewhat bleaches your retinal photopigment supply, and may somewhat reduce your dark adaptation. therefore is "bright" in the functional sense.

i regret that you may be unfamiliar with the concept of a log scale, but, as i said, google is always there to help.