SCOTOPIC AND PHOTOPIC VISION REVISITED
Friday, April 17th, 2009My thoughts here seem to be the subject of many queries so I will revisit the subject. The distinction between these purported two types of vision represents perhaps the most fundamental error that has been passed down in the literature of vision and is today universally taught to students of the subject.
And….it is absolutely in error!
The following text is quoted from Wikipedia on this subject:
“Scotopic vision is the monochromatic vision of the eye in dim light. Since cone cells are nonfunctional in low light, scotopic vision is produced exclusively through rod cells. Vision in normal light with functioning rod cells is photopic vision”.
One must reason from this that cone receptors somehow “shutdown” in low light. There is absolutely no experimental evidence for such a statement! Further, one is left with the model that the scotopic and photopic systems of vision are two separate systems.
Implicit in these thoughts is the oft repeated statement that it is the “cone receptors that detect color and the rod receptors black and white”.
Again…absolutely in error !
There is really only one system.
One who has studied the main body of this work and my explanation for light interaction with the retina will understand that, outward from the all-cone fovea to approximately twenty degrees of retinal angle, the evolved admixture of cone and rod receptors is geometrically structured to detect only the three primary wavelengths. It is from this region that the sensation of “color” is derived. If the level of light entering the eye falls below a certain threshold, the ability of the retina to process the intensity ratios needed to produce the sensation of color fails ( Edwin Land - read the ext!) and vision becomes colorless (or “black and white”). Beyond twenty degrees the primarily all-rod retina, in addition to precisely defining the short wavelength limit of visual response, acts as a wide angle “light meter” controlling papillary constriction and the level of light entering the eye. It is this function of the rod-containing area that has led to the misunderstanding that rods individually detect low levels of light.
There is therefore really only one visual imaging system and it resides in the region surrounding the fovea to retinal angles of twenty degrees. The peripheral retina serves an entirely separate and totally different function
If, however, one persists in using the scotopic and photopic terms the proper definitions are:
Scotopic vision: “Under low light level conditions the rod receptors of the peripheral retina, linked together as has been experiments found, act as a single or integrated “wide angle light meter” with total area encompassed by rods being the cause of such observed sensitivity. It is further of note that, in addition to defining the exact short wavelength limit of visual response (~400 nm), this “light meter” controls pupillary constriction, that under low light level conditions, dilates the pupil of the eye admitting the maximum amount of light to the retina. Under these conditions light intensities of the three primary RGB wavelengths falling on the retina are insufficient to activate the “Land color mechanism”, i.e., there is insufficient intensity incident on either side of the geometrically determined mid-band (550 nm) reference point at 7-8 degrees of retinal eccentricity to allow a ratio to be obtained and the hues of color perceived. The historic misconception that “rods detect black and white” is explainable.
Photopic vision: “Under normal daylight levels of illumination the three primary RGB light intensities abstracted by the retina are sufficient to activate the “Land color mechanism” as defined above and the image including the hues of color is perceived. The peripheral rods, as above, constrict the pupil\controlling the intensity of light entering the eye to levels that will not damage the retina”.
Again, there is only one “system”!
GCH
4.17.09
