VISIBLE AND NEAR-VISIBLE RADIATION 191 



ment or owing to the expense. The possibiUties are then as indicated in 

 the three sections following. 



BLACK BODY PLUS FILTER 



One may use a black-body detector, such as a thermocouple, in com- 

 bination with filters. For this purpose, it is necessary to refer only to 

 the filter-transmission characteristics as the black body is nonselective. 

 The sensitivity is obtained by multiplying the sensitivity of the black- 

 body detector by the filter transmission for the specified wave-length. 

 Such an arrangement is particularly convenient where one wishes to 

 evaluate the energy for all wave-lengths longer than some specified wave- 

 length, the procedure being to use a short-wave cut-off filter with the 

 black-body detector. If one wishes to obtain an evaluation of energy 

 for all wave-lengths shorter than some specified value, it is possible to 

 use a differential thermocouple where the two components are opposed 

 and produce no deflection if equally illuminated. If one junction is then 

 covered by a short-wave cut-off filter and the other by a transparent 

 material having the same transmission characteristics for all wave- 

 lengths longer than the region of cut-off, the response will then correspond 

 to the difference in energy transmitted by the two filters. A similar 

 method may be employed for securing sensiti\aty to a band of wave- 

 lengths. Two short-wave cut-off filters are employed having the same 

 transmission characteristics for the long-wave-length region. The sensi- 

 tivity of the combination will then correspond to the difference between 

 the transmissions of the two filters. A detector of this type has been 

 developed by Coblentz, which has been adopted internationally as the 

 standard method of measurement of radiation lying chiefly in the range 

 from 2400 to 3200 A. 



Figure 30, upper section, shows the transmission characteristics of the 

 following materials: quartz, corex A, corex D, and a special barium flint 

 glass of the type chosen by Coblentz (26). In the middle section, the 

 corresponding difference curves have been plotted (full line) , which when 

 used with a differential thermocouple, constitute relative-sensitivity 

 curves. Thus, Q-BF is the quartz-barium-flint combination chosen 

 by Coblentz and recently adopted as the international standard of ultra- 

 violet measurement. It will be noted that this combination is practically 

 nonselective from 2500 to 3100 A, a region of great biological importance. 

 The cut-off, however, is not so sharp as might be desired. Dropping 

 fairly abruptly to 3600 A, it extends with appreciable sensitivity to 

 3900 A or even as far as 4400 A, depending on the particular filter. The 

 barium-flint filter indicated here and referred to in Table 16 differs slightly 

 from that used by Coblentz. When used with a mercury arc, no great 

 correction is necessary if one assumes that the measurement is for 3130 A 



