GENERATION, CONTROL, AND MEASUREMENT 213 



employed to eliminate undispersed stray flux. The double monochroma- 

 tor consists of two monochromators in tandem such that the first instru- 

 ment acts as a predispersion device fo%the second. This is the most 

 refined method of obtaining a spectrum of great purity, but such instru- 

 ments are usually available only with prisms and in relatively small sizes 

 and at relatively great cost. With simple predispersion systems consist- 

 ing of filters or chromatic optical elements, the stray flux of the single 

 monochromator can be reduced by a factor of 10 or more. The interfer- 

 ence filter is one of the most effective predispersion elements, since it has 

 a high effective aperture when used in a converging beam before the 

 entrance slit. 



CONTROL OP^ IRRADIANCE 



It is frequently necessary to vary the intensity of the irradiation field 

 with relatively high precision. The three most common methods are 

 (1) by electrically varying the power input to the source, (2) by inter- 

 posing various types of neutral filters, and (3) by varying the distance 

 of the object from the source. 



VARYING INPUT 



The power input may be varied either by selecting lamp sources of 

 different power ratings, thus varying the intensity in discrete and rela- 

 tively large steps, or by varying the power input electrically. It will be 

 noted from Table 3-13 for the tungsten-filament incandescent lamp that 

 for any particular class the higher-wattage lamps operate at a higher 

 color temperature and have a higher proportion of energy in the shorter 

 wave lengths than lamps of lower wattage. This may be a significant 

 factor when wide spectral regions are employed. 



The intensity of an incandescent lamp can be varied continuously from 

 zero to the maximum rating of the lamp by varying the applied voltage. 

 This method is satisfactory where narrow regions are being isolated and 

 adequate voltage regulation is available. For wide spectral regions the 

 shape of the spectral-energy-distribution curve changes rapidly with 

 increasing voltage, as indicated by increase in color temperature. Some 

 investigators have specified intensity in terms of applied lamp voltage. 

 This yields rather meaningless data, since the irradiance and spectral 

 energy distribution are both complex functions of voltage and the con- 

 stants vary with the lamp type. 



The volt-ampere characteristic of the gaseous discharge lamp has a 

 large negative slope at low values of lamp current, and the required lamp 

 voltage increases rapidly as the current is decreased. For this reason all 

 gaseous discharge lamps become unstable at supply voltages much below 

 the normal rating for the lamp and its ballast. By introducing a varia- 

 ble resistance in series with the lamp, it is usually possible to vary the 



