“The visual system is at an absolute terminal point in the evolutionary chain..succeeds in counting each absorbed photon” and the Retinal Fourier Plane
December 9th, 2006 | No Comments »I have put forward in the body of this work that the fact that the geometric/antenna explanation means that the retina is a diffraction surface with the consequence that it forms the focal or Fourier plane and not the assumed image plane of the optics of the eye.
From my previous writings, the focal (or Fourier) plane of a condensing lens (such as the lens of the eye) is the position behind that lens where as children we imaged the sun and caused leaves or paper to catch fire. The ‘image’ plane lies behind this point and is the position where film is located in a camera. It was so logical to assume that the eye acted as a camera – but the assumption is incorrect.
Begin your thought process with the subject about which I have recently been writing, namely, it is well known that the vision process has evolved to the state where single quanta (photons, if you wish) can be detected and ‘counted’. This is quite an extraordinary accomplishment. As I have previously noted the eye accomplishes this at elevated body temperature where in physics we require that such detectors be cooled almost to absolute zero (-273 degrees Celsius) to detect single photons. How does the eye do this?
First of all, the retina would be placed at the point where the maximum amount of light was concentrated. This is obviously the focal (or more technically ‘Fourier’) point of the lens of the eye. But you ask, the retina is a very sensitive biological structure. Wouldn’t the sun burn the retina as it does paper? OK..but we all understand that we dare not look directly at the sun or we will injure our retina. The protective mechanism built into the eye to prevent this from happening is papillary constriction that, as I have explained, is controlled by the precise short wavelength (the most damaging) that enters the eye.
Vision therefore operates as a balance between allowing the maximum amount of light to be focused on the retina (thus attaining maximum quantum limited sensitivity) with the protective pupillary constriction mechanism that limits the intensity of this light to levels that will not cause damage.
I have written that it is probably an upset of this balance that is the primary causative factor in the disease state of macular degeneration. This could be a genetic disposition for an individual’s rod receptors to be slightly smaller than normal. This would lead, according to this explanation, to the short wavelength limit of the visual band shifted to shorter UV-damaging wavelengths allowing these wavelengths to enter the eye and, over a period of time, damage the retina.
GC
12/09/06
