218 RADIATION BIOLOGY 



source. The detector converts the incident radiant flux into a signal 

 that is measured quantitatively by an electrical, optical, mechanical, or 

 chemical measuring system. Common examples of detector-measuring 

 instrument systems are the thermopile and galvanometer, the photo- 

 emission cell and vacuum-tube voltmeter, the photographic plate and 

 densitometer, and the eye and visual photometer. 



The detector-measuring system is calibrated with a standard source. 

 The primary standard of radiation is the complete, or Planckian, radiator 

 (black body), whose spectral intensity is precisely determined by temper- 

 ature. In the working laboratory, secondary standards are used which 

 have been calibrated indirectly against a primary standard. A few 

 detectors, such as the pyrheliometers used in solar-radiation research, 

 are absolute instruments whose response can be calculated from the 

 intrinsic properties of the detector system. However, as a class, these 

 detectors are too insensitive for general laboratory use. 



Ideally the detector-measuring system is linear in response over the 

 entire range of intensities to be measured, and a plot of the response 

 against flux intensity yields a straight line. The ratio of signal or 

 response to intensity, known as " responsivity " or "sensitivity," is a 

 constant for all values of intensity for a linear detector. Many of the 

 electrical detectors deviate from a linear response by less than 1 per cent 

 over many orders of magnitude of intensity, and the associated electrical 

 measuring instruments can be made even more linear. The photographic 

 plate and the eye are notable examples of nonlinear detectors. For pre- 

 cise measurement they must be used as null detectors for indicating bal- 

 ance between two fields of equal intensity. 



DETECTORS 



The most useful detectors for quantitative measurement produce a 

 change in electromotive force (emf or voltage), current, on resistance. 

 In considering the electrical detectors it is necessary to evaluate them 

 not only in terms of their intrinsic sensitivity and signal/noise ratio, but 

 also in regard to other physical characteristics and the limitations that 

 they impose upon the measuring system. For each type of detector 

 there is a type of electrical system yielding optimum conditions of sensi- 

 tivity, linearity, and speed. 



There are five general classes of detectors for measuring ultraviolet, 

 visible, and infrared radiant energy: 



1. Thermal detectors (also known as "radiometers," or temperature 

 or heat detectors). In this class the radiant energy is degraded to heat 

 energy at a blackened receiver. The temperature rise of the receiver is 

 determined by suitable electrical or other means and becomes a measure 

 of the intensity of the incident flux. 



2. Photoelectric detector. The photoelectric cell has a photosensitive 



