مقاييس التعريض للضوء .. كتاب آلة التصوير

Exposure metrics

In the past, it was customary to estimate exposure factors by means of visual exposure meters (Fig. 175), and they are almost not used now, as it was possible to produce electrical exposure meters that we are talking about as follows:

Photo electric exposure meter:

The use of these scales raised the problem of estimating the correct exposure factors from the level of intuition and guesswork to the level of the estimate closest to the correct one. The theory of these scales depends on discovering the ability of some materials to convert light energy into electrical energy that increases with the increase of the light energy, and this electrical energy affects an indicator that moves on an apparent scale division that decides for us the relative strength of the light falling on the Photo Electric Cell. It 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 the light falling on it can penetrate without much weakening of its strength. Around the sides of the layer of gold or platinum, there is a metal link connected to the electrometer (Fig. 176).

2- As for the second part, it is nothing more than a very sensitive gauge for weak electric current (micro ampere meter). The pointer of the gauge is connected to an electric coil of very thin wires wrapped around the two poles of a strong horseshoe-shaped magnet.

With the light falling on these two layers, the light energy is transformed into electrical energy between the areas of selenium and gold. And this energy reaches the electrometer through the metal connection connected to the gold layer and through the iron disk on which selenium is deposited. This electrical energy is measured by a sensitive meter. The indicator that runs on the graduated division guides us to the extent of the strength or weakness of this energy. From its location on the division, the amount of light or its value can be estimated, as it has been called Light Value (Fig. Therefore, we finally find that the exposure meter has linked each of the “light value” and “film sensitivity speed”, and based on these two factors, we can know the appropriate focal number for the shutter speed we choose or vice versa. (i.e. knowing the shutter speed that fits the focal number we choose). These last conversions are carried out in the light scale by means of tables, some of its parts are fixed and the rest are variable. And where the method of using each type of these differs, therefore, we find that it is necessary for the photographer to read the learning manuals for each scale, with practical training on the type that will be used. We see that this general idea of ​​the theory by which exposure scales operate is sufficient.

The electrical exposure scales currently used in photography fall into two main categories:

(a) Incident light meters, which receive rays falling on objects from all directions, as they receive them in their entire surface (Fig. 178).

(B) Reflected light meters (Fig. 180), which measure the rays reflected from the body within the limits of a specific angle called the light acceptance angle. This angle may be narrow or wide (Fig. 182). It receives rays falling on it from 180 degrees.