GENERATION, CONTROL, AND MEASUREMENT 219 



surface at which incident quanta eject electrons or produce electronic 

 displacements which appear as a change in electromotive force, current, 

 or resistance. 



3. Photographic emulsion. Electrons are displaced in the silver halide 

 crystal lattices. The development process causes reduction to spread to 

 whole crystals and thus amplifies the initial photoeffect. The resulting 

 increase in optical density of the emulsion layer then becomes a measure 

 of the incident energy. 



4. Chemical actinometer. A photochemical reaction occurs as a conse- 

 quence of altered electronic energy levels in the pigment system of the 

 actinometer. 



5. Human eye. A photochemical reaction in the retina yields products 

 that ultimately excite the neurons of the optic nerve. 



The thermal detectors are relatively nonselective as to spectral sensi- 

 tivity and are often referred to as "nonselective" detectors. They may 

 be used in any region for which the receiver is "black," and the sensi- 

 tivity is usually constant, to within a few per cent, from the ultraviolet 

 to at least the middle infrared. They are much less sensitive in the 

 ultraviolet and visible than the other classes of detectors, but at wave 

 lengths longer than about 5 /x in the infrared the thermal detectors are 

 the only detecting instruments available. Since they can be made with 

 constant and reproducible sensitivity to all spectral regions of photo- 

 chemical interest, they can be calibrated in absolute units in one region 

 and used with precision in another. 



All the other four classes of detectors basically depend upon some form 

 of interaction between the incident photons and the electrons in the 

 photosensitive surface. For each type of system there is a minimum 

 quantum energy representing a minimum frequency or maximum wave 

 length beyond which the quantum energy is insufficient for the transition. 

 Also the absorption of the active surface varies in a complex manner 

 with wave length. For these reasons the spectral sensitivity of the 

 electronic-actuated detectors varies in a complex manner, and the long- 

 wave-length limit or cutoff is usually rather sharp. These four classes of 

 detectors are frequently referred to as selective detectors because of the 

 limited nature of their spectral response. 



The selective detectors have a fast response, since electronic displace- 

 ments occurring in much less than a microsecond are involved. -How- 

 ever, the actual speed of measurement is often reduced to milliseconds or 

 longer because of time constants associated with the detector itself or 

 time constants imposed by the measuring system. Some of these detec- 

 tors can approach theoretical limits of sensitivity in certain spectral 

 regions where the absorption is high, resulting in a quantum efficiency 

 that approaches unity. The sensitivity and spectral response of selec- 

 tive detectors are usually a significant function of previous exposure, 



