172 HANDBOOK OF PHOTOGRAPHY 



precise and reproducible results as measurements made with an objective detector such 

 as a phototube or thermocouple. This is especially true when the eye is subject to 

 fatigue. A very important advantage of subjective determinations of density, how- 

 ever, is the wide range of light values over which the eye is sensitive. Another advan- 

 tage is that such systems may be less expensive than objective methods, since no 

 indicator need be incorporated in the densitometer itself. 



In the case of objective densitometers, the indicating instrument may be a photo- 

 tube (either photovoltaic, photoconductive or photoemissive), a thermocouple, or 

 similar radiant-energy-detecting device. These detecting indicators may be selec- 

 tively sensitive to radiations of various wavelengths, as in the case of phototubes, or 

 may respond uniformly to radiations over a wide range of wavelengths, as in the case 

 of the thermocouple. In selecting an indicator for a densitometer, consideration must 

 be given to the wavelength-response characteristic of the detecting device, as well as 

 the frequency-energy characteristic of the luminous source, for the results obtained 

 depend considerably upon these two factors. 



Subjective-type Densitometers. — The subjective or eye-matching densitometers are 

 usually modifications of some well-known type of photometer. Because of the simple 

 optical means which it provides for matching two beams of light and because of the 

 fairly high precision attendent with its use, the Lummer-Brodhun cube employed in 

 photometry is most frequently used as the method of comparing the intensity of the 

 two light beams. In order that variations in the brightness of the lamp may be 

 eliminated, it is common practice to use a beam-splitting device which produces two 

 optical beams from the same luminous source. The intensity of both beams will vary 

 together if fluctuations occur in the intensity of the light source, but the ratio of the 

 intensities of the two beams will remain unchanged. The principal variations in the 

 design of photometers of this type occur in the means used to diminish the intensity of 

 the stronger beam. Methods most commonly used for this purpose include absorbing 

 filters or wedges, rotating sectors, polarizing plates whose axes may be varied with 

 respect to one another, variable diaphragms, and variable distance between the detec- 

 tor or comparator and the lamp in order to make use of the inverse square law. 



Objective-type Densitometers. — The objective (or physical) tj-pe of densitometer 

 may be similar in manj^ respects to the subjective type of densitometer. The two 

 forms differ in one important respect, however, in that as a means of comparing the 

 two light beams, the human eye is replaced by some sensitive element such as a photo- 

 electrically sensitive device, a thermocouple, radiometer, or similar piece of physical 

 apparatus. This sensitive element is connected directly or through appropriate 

 amplifying systems to a deflecting device, such as a galvanometer, so that objective 

 comparisons may be made between the two light beams. Two advantages result from 

 the use of an objective indicator. In the first place, the results obtained are likely to 

 be more precise and reliable, since they do not depend upon the judgment of the 

 observer. Secondly, by making an appropriate choice in the selection of the sensitive 

 element and light source, it is possible to make measurements in portions of the spec- 

 trum to which the eye is insensitive. 



Objective densitometers may be made in three types: (1) null instruments, (2) 

 deflection instruments, and (3) automatic-recording densitometers. The null type of 

 instruments are in general similar to subjective instruments in that the measuring 

 and comparison beams are balanced against one another. Their intensities are 

 brought to equality by reducing the intensity of the stronger beam until they are 

 matched, when the indicating device will read zero or some other reference value. 

 Null instruments are among the most precise types of densitometers which have been 

 constructed, since it is usually possible to balance out most, if not all, of the factors 

 producing irregularities or systematic errors. 



