مقاييس الأشعة الساقطة .. كتاب آلة التصوير

incident ray measurements

These meters are used, as is evident from their name, to measure the amount of radiation falling on the subjects that you wish to photograph. Some types, such as Spectra, Norwood, have a hemispherical light collector, meaning that it has three dimensions in order to facilitate the measurement of the total energy of the rays emitted by the light sources. Different sources, whether the placement of these sources is lateral, frontal or posterior.. The incident rays are usually measured from the same place where the object or subject is with the light collector directed towards the imaging lens (please refer to figures 177, 178, p. 326).

As with any type of exposure meter, the exposure meter must be preset according to the sensitivity of the film being used for photography. The sensitivity speed may be adjusted by means of a button that is rotated in the scale until the cursor falls on the sign indicating the speed of the film’s sensitivity, or this is done by means of special slides for this purpose, and each slide of them corresponds to a certain speed, and this last method is used in the two scales known as Norwood Spectra
Although many of these scales were originally designed to measure the amount of incident rays, they are also suitable (with a slight modification) for measuring reflected rays.

Advantages of measuring incident rays:

1- The method of measuring the incident light is characterized by the fact that it achieves uniform face tones, no matter how many shots are in different scenes, and therefore it is suitable for cinematography (where the color of the face must remain the same throughout the film, so it is not present in one shot and light in another) because The scale of the incident rays is not affected by the color of the medium surrounding people, but is mainly affected by the strength of the light source and its distance from the subject being photographed, and they are the two factors that determine the amount of incident light. (For this reason, this type of measurement is called the incident ray measurement).

2- It is possible by means of this type of scale to verify the equal amount of light falling on the . Parts of the scene being photographed. Therefore, it is very suitable in cases of photographing an interior scene in a room with the use of artificial lighting sources. It is then possible for the photographer to move around all parts of this room, directing the meter towards the source of light to verify the equal amount of rays falling on its parts, so that part of the image of the room does not appear standing, or the other appears completely light, and the human eye may be unable to perceive the difference between the amount of light falling on a part of The chamber and that aspirator are on another part of it, while this is evident in the photograph, especially if the imaging was with color films or even (black and white) films, which conveyed their ability to resist error in exposure.

For the aforementioned reason, the method of measuring incident rays is considered very useful in photographic reproduction cases, such as oil pictures or maps, especially large areas, or even in photographic images. What increases the validity of the incident radiation scales in this case is that the scale is not affected by the colors of the assets that we transmit, because the optical cell is not affected then except by the spectral characteristics of the light source only.

When is it necessary to measure the reflected light and it is not permissible to rely on measuring the incident rays:

There are circumstances in which it is desirable or necessary to measure the reflected light, or in which it may not be possible to measure the incident rays. As an example of these cases, the following:

(a) When the main subject in the foreground of the image is white, light in color, or more luminous than what lies behind it in the background of the image, or in front of it in the foreground, then the light reflected from this main subject is measured completely regardless of what surrounds it of black or standing objects. or shadow areas. The method of measuring the incident light may not lead to the desired effect (which is to focus the eye on the image of this main subject without anything else).

(b) When the main subject is very dark or black while what is behind it is bright white or very light in colour. (Fig. 195, p. 351) In the two previous cases, it is necessary that the place for measuring the reflected light be from a point close to that main subject, so that the angle of acceptance of the measurement includes only the main subject itself without what appears behind it in the back of the image or in front of it in the foreground. .

(c) There are special cases in which it is impossible for the photographer to stand in the place of the scene that will be photographed, such as the case of photographing a view from the window of the plane, for example, or the case of photographing a predatory animal inside a forest, or the case of photographing fish inside a closed glass tank that is lit from the inside (Fig. 198). The reflected light must be measured, not the incident light.

(d) When the photographer wants to show a special effect in the photo by focusing the eye on a specific part of the subject being photographed without other subjects or other parts of the same subject (Figures 192, 193, 196)

(e) When wanting to photograph against a light source to obtain the effect known as “silhouette” (Fig. 194).