EXPOSURE AND EXPOSURE DEVICES 



253 



Photoelectric Exposure Meters. — The photoelectric type of exposure meter, when 

 properly used, is the most precise type of exposure device available. It gives an indi- 

 cation of light conditions on a scale and is devoid of subjective estimates which intro- 

 duce additional human errors. It is not well suited to use where the light intensity 

 is low, largely because of the low output of 



the photoelectric cell, and it is the most expen- .^ 



sive type of exposure instrument generally 

 available. When properly used and within the 

 range of light intensities for which it gives a 

 positive indication, it is unusually satisfactory. 



The photoelectric type of instrument con- 

 sists essentially of a barrier-layer type of 

 photoelectric cell and a sensitive direct-current 

 measuring instrument. The barrier cell may 

 consist of disks of copper and copper oxide in 

 contact, or of iron and selenium. When 

 such a combination of elements is exposed 

 to light, electrons are released resulting in a 

 current flow both in the photoelectric cell and 

 the external circuit. This current will flow 

 so long as light falls upon the sensitive surface. 

 If this external circuit consists of a galvanom- 

 eter or a sensitive d.-c. meter a deflection of 

 the meter will be obtained and the meter can 

 be calibrated to indicate the amount of light 

 falling on the photoelectric cell, or the scale 



can be calibrated in exposure values. The ^'^\ ' . ® ^i,^^TTf . ^"^cP^ ^^^ J 



. , sure meter using the Weston nlm-speed 



current flowmg depends upon the type of system and calibrated in foot-candles, 

 materials used in the photoelectric cell, but 



for any given cell the current is approximately proportional to the area of the cell 

 and to the amount of light falling upon its sensitive surface. Consequently, 

 for a given cell, the meter may be calibrated in terms of some convenient illumina- 



FlG. 8. — Weston Model 650 exposure meter, calibrated in candles ]jer square foot. 



tion units. A peculiarity of the barrier type of cell is that it converts, directly and 

 without the assistance of batteries, luminous flux into an electric current. Conse- 

 quentlj^, since no batteries are required, such photoelectric exposure meters may be 

 made quite small and compact. Moreover, so far as is known, there is no deterioration 

 of the barrier-layer type of photoelectric cell, so that the life of a photoelectric exposure 



