Neshyba 



N = il- sec-1 (1) 



g€ 



where i^j = dark durrent in amperes, 



g = gain, and 



€• = electronic charge in coulombs. 

 Typical values yield N = 2. 08 x 10* electrons /sec. 



For the S-11 phosphor, quantum efficiency at 470 millimicrons is 



12%. Therefore, the mean rate of photon capture by the photocathode 



equivalent to the mean rate of photocathode emission above is given 



by 



4 5 



2. 08 X 10 _ 1. 73 X 10 photons ,2) 



. 1 2 second 



The area of the 6199 photocathode is 7. 75 cm . Therefore, 

 the mean rate of photon capture per cm is given by 



IT^ ^'^0 = 2. 23 X 10^ Pj^otons 



7. 75 cm^ second 



(3) 



By translating the photon density rate into a power density, one then 

 arrives at the noise power density equivalent to the dark level noise 

 in the receiver. This quantity, called anode dark current equiva- 

 lent noise power density, ENPD, exists at the illuminated surface 

 of the photocathode and is given by 



ENPD = ^ ^ P watts /cm^, (4) 



tXxlO-'^ 



where t = one second, 



h = Planck' s constant = 6. 624 x 10"^"^, 



c = speed of light = 3 x lO-'-'* microns /sec, 



p = number of photons/cm /second, and 



\ = wavelength, in microns. 



Typical values thus yield 



ENPD = 9. 4 x 10-15^atts/cm^ 



= 9. 4 X 10"" microwatts /cm 



At a minimum usable signal-to-noise ratio of 1, the noise 

 power density given in equation (4) also becomes the minimum de- 

 tectable signal at the photocathode, in the absence of noise radia- 

 tion background. 



Propagation Factors and the Range Equation 



The computation of maximum operating range for the lidar 

 follows the methods used in microwave radar studies (Povejsil, et 

 al. , I96I). Perhaps the one unusual factor is that the narrow beam- 

 width of the lidar means that all targets are beam-filling tar- 

 gets; thus, the signal returned to the receiver obeys the inverse 



432 



