46 Light and the Eye \1 : 4 



measure electrically the response of only one receptor. Such experiments 

 indicate either one or two photons are necessary to initiate an electrical 

 response in the nerve fiber. Most investigators today use the number 

 one; that is, one photon absorbed in the receptor, one response. (Note 

 that this is very different from the statement, one photon reaching the 

 receptor, one response.) 



This quantum threshold seems surprisingly small since one photon has 

 so little energy. It is instructive to compute the size of a photon of 

 visible light. Applying Equation 5 to the energy £ of a photon of 

 green light, wavelength about 5,000 A, one finds that 



E = 4 x 10 " 12 ergs 



In terms of a mole of photons (often called an einstein) , this becomes 



kcal 



£ = 40 



mole 



Readers familiar with chemical thermodynamics will recognize that 

 these numbers imply that a photon of green light can break only a small 

 number of molecular bonds when it is absorbed. It is indeed impressive 

 that such a small change can alter the electrical state of the photo- 

 receptor in such a fashion as to initiate a nervous pulse which results in 

 the sensation of vision. 



B. Luminosity Thresholds 



The above-mentioned sensitivity is based on the number of photons 

 absorbed. This absorption is the result of the action of certain photo- 

 sensitive pigments found in the rods and cones of the retina. The 

 relative fraction of light that reaches the rods and cones and is absorbed 

 varies markedly with wavelength. It is convenient to separate the effect 

 of wavelength from the numerous other factors altering threshold 

 intensities. To do this, a set of threshold data is taken, varying only 

 the wavelength. The entire set is multiplied by a normalizing constant 

 chosen to reduce the minimum threshold to an arbitrary value. The 

 reciprocal of the normalized threshold is known as the relative luminosity. 

 Relative luminosity curves have been measured both for dark-adapted 

 eyes and for light-adapted eyes. Vision under conditions of dark 

 adaptation is called scotopic, whereas vision with light-adapted eyes is 

 called photopic. 



In either case, one may interpret the thresholds as the intensity at 

 which a response is obtained 50 per cent of the time. For short flashes, 

 less than 10 milliseconds, the product of the intensity and exposure 

 length determines the observed threshold, whereas for long exposures, 



