حساسية الأفلام لأشعة الطيف .. كتاب الإضاءة والفلم

Chapter Five

Spectral sensitivity of the films

Sensitive layers are characterized by important characteristics, and these are:


The spectral sensitivity. Spectral sensitivity

2- Speed ​​of sensitivity. Speed ​​of Sensitivity

3- Contrast and color gradient. Contrast & Gradation

4- Image sharpness. Included under that are:

(a) Resolving Power Resolution
(b) Lit. Accutance

e - Graininess.

We will talk about the first characteristic in this chapter, provided that we defer talking about the second, third and fourth characteristics to the following chapters. As for the fifth characteristic, we have already talked about it in the previous chapter.

Spectral sensitivity:

Are sensitive films affected by all visible and invisible spectrum rays as well? Assuming that this actually happens, is the extent of its influence equal to each of the colors of the rays, so that it is affected, for example, by blue rays to the same extent as it is affected by the visible red rays, or to the same extent as it is affected by the invisible ultraviolet rays, for example?

In response, we say: This matter depends on what has been termed spectral sensitivity, which is an expression that means the extent to which the film is affected by both visible and invisible wavelengths.

If we say that the sensitivity of the film is high with respect to a specific color of the spectrum (such as blue, for example), then this indicates that a small amount of blue rays suffices to have an acceptable effect on it. For other rays (such as red rays), it will require an increase in the energy of these red rays or an increase in exposure in order for the film to be moderately affected by these rays and its density to reach the same degree as before (assuming that other factors related to exposure are equal).

Our knowledge of the spectral sensitivity in different films is of great importance in the following cases:

(a) When estimating the exposure factors, as the exposure decreases if the sensitivity of the film increases to a certain type of visible or invisible rays, and vice versa.

(b) When we want to know how a spectral color, such as green, looks like when shooting with a specific film. Black and white sensitive films that are not intended for color imaging, translate the colors of the spectrum (such as red, green, blue, etc.) into shades of gray that range between light and dark tones, in addition to black and white.

(c) When light filters are placed in front of the photographic lens, or when photographing with artificial light sources whose spectral characteristics differ from the characteristics of daylight, because the correct exposure estimate for a film with a certain medical sensitivity depends to a large extent on the color of the rays that pass through the optical filter or on the color of the rays of the light source.

(D) When we want to know if a particular film is susceptible to a particular type of invisible radiation (infrared, for example).

When we mention, for example, that the “ortho-chromatic” sensitive film is not affected by a specific spectral color (such as red), this means that the image of red surfaces will appear translucent white in the negative image, and thus become black in the positive image.

Evolution of spectral sensitivity in films:

One of the laws of photochemistry, which is the law known as the Grotthus & Draper Law, states that matter is not affected chemically by light except by radiation that it is able to absorb, and this means that only light rays that are absorbed by silver salt are capable of causing the latent image that appears to the eye after chemical treatment By the visible solution, if the silver salt does not absorb any rays, it will not be affected by the optical effect. Depending on the difference in the ability of silver salts to absorb different colors of rays, the extent of their sensitivity to the colors of the spectrum varies. Silver chloride, for example, is sensitive to invisible ultraviolet rays, violet rays, and part of the blue rays, while the sensitivity of silver bromide extends to a measure of green rays as well, while none of the silver salts is affected by yellow rays or red rays.

It was observed that the sensitivity of silver bromide to green rays increased if it was added to it (a quantity of silver iodide in the sensitive layer. In 1873 the German H. W. Vogel discovered) that by adding some colored dyes to silver salts, its spectral sensitivity to light waves longer than green increases. This fact appeared when Vogel was conducting some of his experiments to study the solar spectrum by means of a sensitive plate in which silver bromide salt was inserted. This tablet was covered with a layer of an unknown yellow dye, and he noticed that the sensitivity of this tablet decreased in the regions of the shorter wavelength rays than the green rays, and it increased significantly in the region of the green rays. This researcher concluded that the increase in sensitivity to green rays is due to the unknown yellow dye that led to the absorption of silver bromide to green rays, or in other words: that the yellow dye had led to an increase in the sensitivity of salt to green rays. Then he studied the effect of mixing different dyes on silver bromide in order to increase the sensitivity of sensitive plates to rays longer than blue (such as green, yellow and red rays).

Coraline dye was the first dye that increased the spectral sensitivity until it extended to the yellow rays region, and then it was found that some green dyes (which have the properties of absorbing red rays) have the ability to increase the sensitivity of the sensitive plate to the red rays as well. Vogel then wrote in his research, which was published after these experiments, stressing the possibility of controlling the sensitivity of panels sensitive to different colors of rays by adding colored dyes to the silver bromide salt.

Vogel's research aroused great interest among film and sensitive plate makers, as they had no prior knowledge of the sensitivity of silver salts to the colors of the spectrum. With the progress of the study of this subject, it was proved that some dyes have the ability to increase the sensitivity of films to different colors of the spectrum, and that others have no ability to cause any change whatsoever.

It was also noted that the spectral sensitivity depends on the type of silver salt used, whether it is chloride, iodide, or silver bromide. It also depends on the medium in which the bite salt is deposited, and whether this medium is gelatin or coliodion.

In 1804, many researchers conducted this connection, and chlorophyll and eosin were used as dyes to increase the sensitivity of silver bromide to the colors of the spectrum, and the use of eosin in particular led to impressive results in the manufacture of sensitive plates, which were known at that time as lsochrome plates, and they could be produced on a large commercial level.


Then he discovered the effect of Erihyrosin as a bromida dye on the production of sensitive orthochromatic plates.

In 1904, new dyes were discovered that are the basis of the modern film industry currently circulating, including Pinacyanol Pinachrome dyes, which had the first credit in the manufacture of panchromatic films, which are films that are sensitive to all visible wavelengths of the visible spectrum and less than the length of invisible rays such as ultraviolet rays.

Until the end of the First Great War (1914, 1918), the dyes that were used to sensitize films to infrared rays had not yet been discovered, and then research progressed and this type of film could be made with complete success.