by Gerald Huth on February 22, 2011

My thoughts here elicit many queries so I will revisit the subject. The distinction between the purported existence of two separate vision systems – “color sensitive cones” and “black and white sensitive rods” –  represents perhaps the most fundamental error that has been passed down in the literature of vision science  and is even today universally taught to students of the subject.

For the reader who has been so educated I would propose that, following the realization that light wave-accepting, nano-antenna dimensionalities are central to  light interaction with the retina, this dogma is completely erroneous.

In actuality, there is only one continuum controlled by the pupil of the eye contracting and dilating the diameter of the opening from a light constricting 2 millimeters during daylight (photopic) condition to a fully open 7 millimeters for low level (or scotopic) night vision. Under scotopic (night vision) condition the pupil is fully open allowing what light that is available to fall on the larger area of the short wavelength (“blue”) sensitive peripheral retina. Rods themselves are not more sensitive to light (for which there has never been experimental evidence) but rather it is  light interacting on the totality of rod area of the peripheral retina that has historically given this impression.

As taught in this work, it is the peripheral rod-containing retina beyond 20 degreees sensitive to the short wavelength (again “blue”) end of the visual band acting as a “wide angle light meter” that controls pupillary constriction. It is not simply the intensity of light but also precise light wavelength that controls this function.

For example, the following text is abstracted from Wikipedia:

“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”.

That “cone cells are non-functional” (or “shut down”)  -   there is just no direct  experimental evidence for such an assertion!

That “scotopic vision is produced exclusively through rod cells”… how on earth? …invoking what mechanism?

Scotopic and photopic vision have historically been presented as two separate systems. This conclusion  follows from the general observation that the sensation of color disappears at low levels of illumination giving the idea that, what has been thought to be (again erroneously) the source of color – the cones – “shut down” at this point. What is left is “monochromatic” sight that must be ascribed to the response of rod receptors- thus a  ”black and white” image.

Following from the realizaton that light interacts as a wave with nano-antennas at the plane of receptor outer segments, it becomes instantly clear that there is really only one system.


“The sensation of color derives from, precisely as Edwin Land described, the eye obtaining a ratio of light intensities in the region from the central all-cone (“blue blind” according to George Wald) fovea to ~20 degrees. This region is bisected by the geometrically determined mid band  (i.e. 550 nanometers) point at 7-8 degrees- Land’s “fulcrum”. When the level of light  falls to a point below where a meaningful ratio can be obtained the sensation of color ceases but monochromatic vision remains – the “black and white” image.. The predominance of rod receptors that form the peripheral retina function as a “wide angle light meter” controlling pupillary constriction and the light level that is allowed to fall on the retinal surface.


But, if one must classify two types of vision  these are the proper definitions:

Scotopic vision: “Under low light level conditions the rod receptors of the peripheral retina are linked together (as has been historically shown) to act as a “wide angle light meter” with the exact short wavelength limit of visual response (~400 nm) controlling pupillary constriction, dilating the pupil of the eye and admitting the maximum amount of light to the retina. Under these conditions light intensities of the three primary  wavelengths that interact with the image-forming portion of the retina (from the fovea to 20 degrees) 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 explained.

Photopic vision: “Under normal daylight levels of illumination the three primary light intensities abstracted by the retina are sufficient to activate the “Land color mechanism” as defined above and the the hues of color are perceived superimposed on the visual image. The peripheral rods, as above, constrict the pupil  controlling the intensity of light entering the eye to levels that will not damage the retina.

There is only one “system”!




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