What are radiation detectors and what are their properties?

Radiation Detectors and Their Properties
RADIATION DETECTORS
Before discussing the accessory instruments used with optical telescopes, let us consider briefly the most important component of these instruments: the radiation detector. The telescope is capable of col­lecting light over a very wide range of wavelengths, but it is the radiation detector that determines what the telescope sees. One radiation detector with which we are all familiar is the human eye. It possesses most of the properties of radiation detectors in general and is thus illustrative of the points we wish to make about them.
The properties of interest are the wavelength re­gions to which the detector is sensitive, the differing response of the detector over that wavelength region, and the natu re and range of detector response. Using the human eye, we can briefly illus­trate each of these properties.
PROPERTIES OF RADIATION DETECTORS 
The eye is sensitive to the narrow wavelength region between about 3500 and 7000 angstroms. However, the eye does not respond equally to all colors in the visible spec­trum. It is most sensitive to the middle of the wave­length region, the green wavelengths, and the sensi­tivity drops to zero toward either the violet (short wavelengths) or the red (long wavelengths).
The nature and the range of detector response are expressed by the ways in which the eye responds to one photon and to a tremendous flood of photons. Common experience tells us that the eye does not respond in the same way for both. There is some threshold number of photons, depending upon their wavelength, necessary to make the eye respond. In other words, there is a limit to how faint a light source we can see, and that vis­ibility limit depends upon whether we are looking at violet, green, or red light.
All of us have experienced the loss of response of the eye when we try to look at too bright a light. That is, the eye saturates-it no longer responds-and no scene is visible to us, just an intense and painful bril­liance. To be useful, the radiation detector's dynamic range between threshold and saturation of visibility should be quite large, say, a factor of 100 or 1000. Now we may ask, "What is the response of the eye to doubling the number of photons in between the low­er and upper limits of threshold and saturation?" If we double the number of photons, do we observe that the light is twice as bright? The answer in general is no. By and large, over the dynamic range of response of the eye, doubling the stimulus does not double the response; in other words, we say that the response is nonlinear. This concept of linearity is important be­cause, in seeking the amount of radiant energy emit­ted by an astronomical source, astronomers usually compare the unknown light source against one of known energy output. Thus they have to know how their radiation detector responds to increasing or de­creasing numbers of photons.
Now we look at two other radiation detectors, the photographic emulsion and the photoelectric device.