مقاييس التعريض للضوء .. كتاب أسس التصوير الضوئي

Exposure meters

Exposure meters can be divided into two classes.

A - Measures based on vision

(Fig. 88b) It is called the Visual Exposure Meter, and its use may almost require the production of the second type of electrode.

Exposure metrics:

A - Photo Electric Exposure ineter.

​​​​​B - Visual Exposure Meters.

Photo electric exposure meter:

The use of this type raised the problem of estimating the power of light from the rank of intuition and guesswork to the rank of correct estimation based on practical measures. The theory of these measurements is based on the discovery of the property of light being affected in some materials combined with each other, where the light energy turns into weak electrical energy, so if the light energy increases, the electrical energy increases, and that electrical energy can be measured by a scale that works with an indicator on an apparent division that decides for us the relative strength of the light falling on These materials are affected by light. This group is called the electric cell. This cell consists of: -

1) A disc of iron on which a layer of selenium is deposited, which is a gray-colored substance that resembles metals in its shape. A very thin layer of gold or platinum is placed on top of it so that light can pass through it without weakening the power of the incident light much. Around the sides of the layer of gold or platinum, there is a metal connection connected to the electrometer (Fig. 89).

And by the fall of light on those two layers, electrical energy is generated between the areas of selenium and gold, and that energy reaches the electric meter through the metal connection connected to the gold layer and through the iron disc on which selenium is deposited.

2- As for the second part of that electrical cell, it is nothing more than a very sensitive measure of the weak electric current. The gauge pointer is connected to a coil of very thin wires wound around the poles of a strong magnet
Horseshoe shape.

As a result of the exposure of the selenium and gold materials to light, this light energy is transformed into electrical energy that can be estimated by means of the indicator that is carried out on a graduated division, and from the location of the indicator on the division, the amount of light can be estimated and read. Then we link the amount of light falling on the electric cell and the speed of sensitivity of the films used.

After linking the two previous factors, it is possible to estimate each of the appropriate shutter speeds for this aperture. These last conversions are carried out in the light meter by means of special disks that fix some of them and move the others. The lesson in studying the method of using each of these scales is the practical training on the type that will be used.