164 HANDBOOK OF PHOTOGRAPHY 



2. Using different areas of a uniformly lighted source. 



3. Reducing the intensity of the light reaching the different portions of the sensitive 

 material by means of screens of known absorbing power. 



The last of these is the only method now in general use. 



Time-scale Exposure Devices. — Although, in practice, photographs are taken with a 

 shutter giving sensibly the same time of exposure to all parts of the negative, it is most 

 convenient and accurate to use time-scale devices in photographic sensitometry. The 

 reason lies in the greater simplicity, ease of construction, and inherent precision of 

 time-scale exposure modulators as compared with intensity-scale modulators. Such 

 time-scale devices make use of a constant source of illumination and vary the exposure 

 by altering the time during which this light impinges on the photographic material. 

 Consequently a simple light chopper will suffice for this system. Such a light chopper 

 may take the form of: (1) a rotating wheel or drum with various segments cut out 

 through which the light passes as the wheel is rotated, (2) a slotted plate falling under 

 the influence of gravity or moved at some uniform velocity, or (3) a slotted tape which 

 moves past the aperture through which the light passes to expose the photographic 

 material. The slotted rotating disk or drum is usually the most convenient form of 

 varying the exposure and can be made to give either intermittent or nonintermittent 

 exposures. Nonintermittent exposure modulators may be made by rotating the disk 

 sufficiently slowly that the required exposure is obtained with one complete revolution 

 of the disk. Intermittent exposures result if the required exposure is built up through 

 successive individual exposures obtained from several revolutions of the disk. The 

 slots may be cut to give steps of varying exposure. 



Differences between Intermittent and Continuous Exposure. — A distinction must be 

 made between these two types of exposure devices, as it has been found that they do 

 not give the same results. The reason behind this discrepancy lies in the fact that 

 photographic materials have been found not to integrate an intermittent exposure 

 correctly, except to a rough first approximation. For this reason the characteristics 

 of the photographic material observed by intermittent exposure will depend upon 

 the intermittency of exposure as well as the total time duration for which the exposure 

 is made. As might be expected the magnitude and character of the intermittency 

 effect varies with the character of the photographic material under consideration. 

 Furthermore, whereas the speed is higher and the gamma lower^ for nonintermittent 

 exposures as compared with intermittent exposures for certain intensity levels, this 

 condition may be reversed at other intensity levels at which the comparisons are 

 made. For these reasons the effects obtained with intermittent and nonintermittent 

 exposures cannot be compared. Since intermittent exposure is not encountered in 

 practice, there is good reason for preferring the nonintermittent-exposure type of 

 device in sensitometry. 



Rotating disks, falling plates, and other time-varying exposure modulators can 

 be made rather accurately, the precision being determined by the precision with which 

 the slots are cut or the precision with which the shutter devices are moved past the 

 film to give the necessary exposure. When rotating-disk exposure devices are used, 

 the speed of rotation may be determined with precision by means of rotation counters. 

 With a precisely cut disk properly driven, accurate results are easily obtainable. 



Since the characteristic curves of photographic materials are customarily plotted 

 in linear rectilinear coordinates with density D as the ordinates and logio E as the 

 abscissas, it is convenient to make the progressive exposure steps of the sensitometer 

 bear a logarithmic ratio to one another. This may be accomplished by so cutting the 

 disks that the angle of arc in successive steps bears a known ratio to one another. The 

 disk used by Hurter and Driffield was based on this principle. It had nine apertures 



' See p. 190 for definitions of speed and p. 179 for definition of gamma. 



