EXPOSURE METER (Fr., Photometre, Luci metre, Actimometre ; Ger., Expositions messer, Belichtungsmesser, Aktinometer) An instrument for ascertaining the necessary duration of exposure when taking a photograph.
The terms " exposure meter " and " actino meter " are often used interchangeably, but the latter refers to an appliance for simply testing the actinic power of light, whereas the former means an instrument that not only does this, but indicates also the exposure requisite under such conditions, with any given subject. While an exposure meter may be an actinometer, an actinometer is not an exposure meter. The term actinometer now tends to be restricted to appliances used for finding the light value when printing, as in the carbon and similar processes, where no visible image is at first obtained.
Of the many different kinds of exposure meters proposed the best are those that provide for an actual test of the light intensity ; among these may be mentioned the Watkins and the Wynne devices. The Watkins Standard Exposure Meter A has an enclosed chain pendulum for counting seconds or half-seconds, the cap or lid shown on the left forming, when removed, the weight of the pendulum. At the opposite end is an opening under which runs a coil of sensitive paper, which may be pulled out through a slot as required, in order to expose a fresh portion under the aperture. To use the meter, a new piece of paper is brought into position and quickly covered with the thumb, pointing the meter towards the source of the light that falls on the object to be photographed. The pen dulum is then started swinging, and the finger at the same time removed from the test paper. The number of seconds taken by the paper to darken to the depth of the standard tint, as painted in the circle beside the opening, is now carefully counted. The pointer P is then set against the plate speed number, the pointer D to the diaphragm number, and the pointer A to the actinometer time just obtained, when the correct exposure will be indicated by the pointer E. When the light is weak, or with specially
dark subjects, the exposure of the camera and the meter may be carried out simultaneously, a second tint being provided, which the sensitive paper takes only one-quarter the time to match. This instrument is very complete, and permits of special calculations for other than ordinary subjects, such as enlarging, copying, etc.
A simpler form of the Watkins meter B re sembles a watch. Presh paper is adjusted under the opening by rotating the back of the case, and the diaphragm number on the inner ring is set against the plate speed ; the required exposure will then be found against the actinometer time.
Wynne's " Infallible Exposure Meter " C also resembles a watch. A small disc of yellow glass (not shown in the illustration) is .cemented to the revolving dial to cover the sensitive paper until it is wanted. It is thus possible to see to adjust fresh paper under the aperture without its being prematurely exposed to light, and to have it always ready by merely sliding aside the yellow disc. The Wynne meter has the valuable feature of showing at once the necessary expo sure with all the different stops. A table is supplied with the meter assigning to the various makes of plates a speed number, which repre sents also a diaphragm number. To use the meter, the actinometer time on the inner ring, as found with the sensitive paper, is set against the plate speed on the outer circle. Against each of the diaphragm numbers on the latter will then be indicated the correct exposure. Obviously, if a stop corresponding with the plate speed number is used, the exposure will be identical with the actinometer time, and the camera and meter may be exposed simulta neously.