البوزمتر أو جهاز قياس التعريض .. طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

Posometer or exposure meter ..
How the camera works .. Book of photography

Posometer or exposure meter -

Photographic exposure meters - electrical are new in the history of photography. Before their invention, several methods were used to calculate exposure to light, but no method could give an accuracy similar to the accuracy obtained by current measurements: but it was sufficient for emulsions and film development methods at the time.

The advent of fixed-speed films and the need for fast instant machines required a better determination of the duration of exposure to light. The first exposure meters were inefficient and difficult to use, some of them used the direct blackening of sensitive paper (actinometer) and others used the sensitivity of the human eye (in which sensitivity is relative). The results were not entirely convincing.

The first exposure meters allowed visual comparison of the light illuminating the subject with a small lamp powered by a battery. The greatest advance came when it was discovered that a simple semi-metallic body, selenium, generated an electric current when exposed to light. The intensity of the current was proportional to the intensity of the light hitting it. Thus a new generation of exposure meters was born. The pseudometer that operated on selenium was better than the previous mechanisms and did not require a battery. The selenium cell, as it was called internationally at that time, became very widespread.

Unfortunately, the current generated by selenium was weak and required a large receiver to record low levels of illumination. When the pseudometer was placed on a camera, this made the device very large, difficult to transport, and useless for low lighting. Something else had to be found: photoresistors were discovered. Unlike the selenium cell, this resistor does not produce any current, but its internal resistance increases when it is exposed to light, proportional to its strength and intensity.

The most widely used resistor is the cadmium sulphate (CdS) one. It must be fed by a constant voltage battery throughout its use in order for the measurements to be accurate. The circuit and the resistor are well-coordinated and since the Cas cell is very sensitive to light it can give accurate measurements even in poor lighting conditions. The CdS cell pseudometer was easy to fit into a photographic instrument and integrated into an automatic exposure system. The final and logical development of the pseudometer was to integrate it into the photographic instrument itself. This began with the selenium pseudometer which was simply added to the instrument case but has now been integrated into the instrument. Most 24 x 36 mm instruments are equipped with a similar system which measures the light behind the lens. This measurement can be global, i.e. covering the entire image surface, or relative or selected, so that the measurement is made on a small part of the image. The pseudometer is usually linked to the pupil and speeds and all that remains is to focus the needle on a mark in the objective to get a correct image. In machines where the light control is automatic, the pupil must choose the aperture and speed because the self-movement automatically calibrates the other member. In some simple automatic photographic machines, the pseudometer calibrates the pupil and speed according to a pre-set program. It should be noted that the Cas cell is often replaced in modern machines by a cell that works on silicon (SI) because it is more sensitive and less rigid than the CdS cell.

The pseudometer with the selenium cell

This pseudometer consists of a very simple circuit that does not require a battery (the drawing on the left). The light strikes a surface of selenium which in turn generates a small amount of electric current that is proportional to the intensity of the received light. This current operates on a galvanometer whose needle indicates a scale of values ​​that is proportional to the received light. A computer made up of graduated cylinders converts the light value into exposure values ​​(apertures and speeds) according to the film speed in (ASA).

Using the Selenium Posometer

In the presence of strong light, a shutter with small holes covers the selenium cell (A). By directing the posometer toward the subject, the galvanometer pointer indicates the amount of light on a scale.

In the case of weak light, the shutter is opened to replace the scale with a new scale with values ​​weaker than the previous values ​​(B). In the case of incident light, a Batt cone is placed over the cell (C).

Cas resistance puzometer

From a different principle, the Cas resistance is more sensitive than the selenium cell. It does not generate any electric current. It is the resistance that changes according to the intensity of the light that strikes it. A high resistance value is suitable for strong lighting. The current is supplied by a small battery (1.3 to 6 volts). The Cas puzometer must be placed under voltage during use. It is very sensitive to low light and is very strong and rigid. The galvanometer with the pointer and its frame are sometimes replaced by a pair of LEDs and the aperture ring that corresponds to the correct exposure is read on the speed pairs. The improved Cas puzometer, independent of the photographic apparatus, can be equipped with additional parts that allow it to be used on the base of a magnifier or on the aiming glass of a large-sized apparatus or on a microscope. The angle of the cell can be reduced to only one degree.