EXPOSURE AND EXPOSURE DEVICES 249 



Actinometers. — Of historical interest, although seldom used any more, are actinom- 

 eters which determine the "actinic value " of the light by determining the time required 

 for a piece of photographic printing-out paper to darken to a standard tint. Most 

 POP materials are insensitive to the yellow, orange, and red portions of the spectrum 

 and are therefore more suitable for use with "ordinary" negative materials than with 

 orthochromatic or panchromatic materials. 



When using an actinometer, it should be held in the shadiest area of the scene where 

 full details are desired. It should be directed toward the light, and the time for the 

 incident light (that which falls upon the object or scene) to darken the sensitive paper 

 to a standard tint should be measured. 



The Watkins and Wynne actinometers are made in watch form, and by means of 

 scales the time required for the sensitive paper to darken can be translated into expo- 

 sure values. While many actinometers have appeared on the market, the Watkins 

 and Wynne have been accepted as standards of this type. Since they measure the 

 light falling upon the scene, certain corrections are necessary for scenes of abnormal 

 reflectance. These corrections, which are furnished with the meters, vary from Ko to 

 1}4 times the indicated exposure. Actinometers have an advantage of being small 

 and inexpensive. It is sometimes difficult to determine when the photographic paper 

 is exactly the same tint as the comparison standard, but the proper tinting time can 

 usually be determined with an error of less than 100 per cent, and such error can be 

 tolerated in black-and-white photography because of the wide film latitude which is 

 available in modern films. A disadvantage is that under low light conditions an 

 appreciable time is required for the sensitive paper to reach the same tint as the 

 standard, thus requiring considerable time to obtain exposure data. 



Visual-type Exposure Meters. — Visual types of exposure meters may be divided into 

 three broad classes: (1) photometer devices by which the brightness of a scene is-deter- 

 mined from a comparison with a calibrated incandescent lamp whose brilliance may be 

 measured or estimated in some manner, (2) wedge-extinction types in which the 

 exposure is determined by varying a graduated neutral-density wedge until shadow 

 detail disappears, and (3) stepped-wedge devices whose steps are lettered or numbered, 

 the dimmest letter which can be discerned being taken as an indication of the inte- 

 grated light intensity of the scene. The wedge is usually a piece of celluloid or glass 

 coated with a gelatin dye of varying density so that it varies approximately from 

 transparency to opaqueness. When using visual-exposure meters, care and sufficient 

 time must be taken to allow the eye to accommodate itself properly in order to obtain 

 correct meter settings or readings. Unlike the actinometer, visual exposure meters 

 measure reflected light. 



Photometer devices operate essentially on the same principle as that of an optical 

 pyrometer. The brilliance of a lamp filament is matched with the brightness of the 

 scene as seen through an optical finder. To make the indications dependent only 

 upon the intensity of the lamp filament and not on its color, both scene and filament 

 are usually viewed through filters which transmit a narrow band of wavelengths. The 

 temperature of the filament may be estimated by determining the power taken by the 

 lamp from a small filament lighting battery. The lamp temperature is usually adjust- 

 able by means of a variable resistance in series with the lamp. Several years ago the 

 Bell and Howell Company manufactured such a photometer exposure device, but this 

 instrument has been superseded by photoelectric exposure meters. The Bell and 

 Howell photometer contains a small electric bulb and dry cell connected together 

 through a self-contained rheostat. The meter is directed at the object or scene, and 

 the rheostat set so that the lamp filament can be clearly seen. The rheostat is then 

 turned, slowly dimming the filament, stopping just at the point where it blends with 



