تأثير طبيعة ونوع السطح الملون في مقدار الأشعة المنعكسة .. كتاب الإضاءة والفلم

The effect of the nature and type of colored surface on the amount of reflected rays:

Assuming that the origins of the colors and the degree of their saturation are spent, and the substance of the colored surfaces, then it is certain that the amount of reflected rays differs from each surface. The shiny silk cloth dyed in a red color reflects more of the red rays than what is reflected in the matte wool cloth or velvet cloth dyed with the same dye.

In support of this, we show below a table summarizing the results of experiments similar to what was previously mentioned on page 163, with the use of a Veston exposure meter to measure the rays reflected from each of cardboard paper and amaranth cloth dyed both in one red formula.

Thus, we conclude from this table that the type and nature of the colored surface has an effect on the amount of reflected rays, and accordingly an effect on the exposure estimate.
And if we wanted to know the effect of the difference between the origin of the hue color and the nature of the colored surface together on the amount of rays reflected from two different surfaces, we would find that there is a big difference between them, as is evident from the following table:

Undoubtedly, after studying the results shown in the three previous tables, we will wonder what we must do in order to estimate the correct exposure if the subject being photographed combines surfaces of multiple colors and the amount of rays reflected on them varies. We have seen in the first table on page 163 that yellow reflects the amount of of light equals approximately 300%. The rays that red reflects. We also saw in the last table shown on page 164 that yellow cardboard reflects an amount of rays equal to that of yellow cardboard. 800% of the rays reflected by the red velvet fabric.

Assuming that we have been exaggerated in our experiments and that we were in the process of photographing a colored vase of bright china placed on a table of red velvet, we will find that the difference between the amount of rays reflected from each of them has become a very large difference. And if the correct exposure is estimated for the bright yellow surface, then the exposure will be very deficient for the velvet bedspread, as it appears in the picture very red and does not appear in any clear details. On the contrary, if the correct exposure is estimated for the red velvet bedspread, the exposure will be much more. As for the polished yellow vase, which results in its color appearing very faded in the image, as the color saturation decreases when the exposure is more than the correct amount, in addition to the loss of detail in high light areas.

What is the way to avoid the previous problem, especially if we know that the tolerance of the emulsion latitude colored pen and its tolerance for error in exposure is much less than that of black and white film?

In response to that, we say: It is possible to follow either of the two methods: - (a) We work to reduce the contrast of illumination by increasing the amount of rays falling on the subject of red color and to an extent sufficient to bring the ratio of the rays reflected from the two colors closer, except that from the other side - This solution will result in an increase in the value of the red color, that is, its brightness increases as a result of the increase in the amount of rays reflected from it.

(b) Reconciliation between the suitable aperture of the diaphragm to expose any of the two colors correctly. Assuming that 11f is suitable for red and 22f is suitable for yellow, let us take a medium aperture in its width, such as 16f. However, if this solution is permissible in the cases in which the amount of the reflected rays are close to the different subjects, then it is not acceptable in the other cases, and from here we see that if the photographer wants the color values ​​to be transmitted faithfully in the color image, then he must work on selecting subjects that do not have contrast Large i.e. the amount of rays reflected from each of them converge according to their color and the nature of their surface.

The effect of the light level on the degree of color contrast:

What is meant by the level of illumination is the amount of light available during photography, so its level will be low if the amount of incident light decreases, and its level will be high if it increases. If it was coated with a dark brown color. Conversely, the luminance of the colors increases as the light level rises, so the colors appear bright, and the contrast between them increases. This phenomenon is not strange to us, as we all feel how bright and contrasting colors appear in a room illuminated by direct sunlight, then how these colors darken when the strength of the light source decreases. In semi-darkness, and now you wonder what would happen if we photographed a colored subject (such as an oil painting in a museum, for example, at a low light level) such as natural daylight inside the museum room without using any other auxiliary light sources, or if we photographed it at a high light level? In response, we say that the degree of contrast between colors and their brightness will decrease in the first case, and they will not appear bright even if the exposure time is increased to compensate for the lack of light. In the second case, the colors appear bright and contrast with each other. And it is worth noting that the previous results remain correct even if the exposure foams in proportion to the extent of the decrease in the level of illumination.

From the foregoing, we draw the following conclusions:

(a) Estimating the correct exposure in the case of color photography depends, and therefore the fidelity of color transmission, depends on the amount of rays reflected from all the subjects being photographed, as these rays are the ones that actually affect the film.

(B) When we estimate the exposure, we are very concerned about the different origins of the colors as much as we are concerned with the difference in the amount of rays reflected from each color.

(c) In order for the exposure to be correct, the amount of rays reflected from each part of the subject being photographed must approximate, regardless of whether the general level of illumination is a high level that suits subjects with a high-key level of illumination, or The light level was too low to suit the low key subjects.

Is there a way to estimate the degree of variance in numerical units?

There are known scientific methods for measuring the degree of contrast in the image and estimating it in numerical units instead of mentioning these fluid expressions (medium contrast), strong contrast, or weak contrast. These methods were known after the progress of science and sensorytometry, and I will mention them in connection with the study of this science in the last part of this book.