Selenium meter

A selenium meter is a light-measuring instrument based on the photoelectric properties of selenium. The most common use of such light meters is measuring the exposure value for photography. The electric part of such a meter is an electromagnetic measuring instrument which is connected to the anode and cathode of a selenium photo cell that produces more or less electric power when exposed to more or less light. The optical part of such a meter is a window in front of the photo cell's light-sensitive side. The window's surface is usually structured like a honeycomb made of convex lenses. This type of window helps to bundle the light coming from the direction in which the photo cell is pointed. The mechanical part of a selenium meter is an analog calculator which accepts exposure value and film speed as input parameters for showing the possible aperture and shutter-speed combinations for correct exposure.

The simplest type of match-needle selenium meter shows a clockhand on the meter's scale. This can be moved by turning one slice of the analog calculator. When the clockhand matches the instrument's needle the EV-value is set right on the calculator.

The picture on the right shows the device of the Zenit - E model, mounted on a left side of the camera. The inner (flat) circle rotates with the help of the knob and sets the film sensitivity (visible in both DIN and ASA units through tiny windows). The outer circle is coupled to the moving O - shaped marker in the scale on the left and must be rotated till the marker covers the needle (visible in the left corner). The recommended exposures can be read on a side scale against the possible apertures (top scale). The device is not directly coupled to actual aperture and exposure controls

More sophisticated cameras have the match-needle instrument coupled directly to aperture and shutter speed setting rings on the lens tube instead to a separate analog calculator. This is the most convenient way of match-needle instrument usage, especially when the meter's scale is mirrored into the viewfinder.

Various models of Zeiss Ikon Contaflex SLR and Contessa camera has built in couple match needle selenium meter. Changing the aperture and/or shutter speed and move the red marker hand with a "O" shape loop, such that the "o" aligned with the selenium meter needle, the shutter speed and aperture is thus selected. Minox B and Tessina has manually coupled meter.

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Wikipedia