Technical Documents - Documentos Técnicos Home

To expose the photographic paper , the printer projects light through a negative onto the photographic paper  The light is focused by a lens .

The total  exposure affecting the paper is a product of the intensity of the light and  total time that the paper is exposed to the light . 

Or , E (exposure) = I (intensity ) x t (time ) 

Exposure and filtering

The printer lamp emits light containing all colors . The printer operates subtracting ( filtering out ) light in appropriate amounts to recreate the colors and shadings of the original scene. 

For the three main dye-forming layers in the paper , the printer regulates the total exposure by controlling the appropriate amounts of red , green and blue light  The density of the negative and the magenta and yellow light balancing filters  in the printer regulate intensity . The printer regulates time with balance memory changes in the computer and the timing of cutoff filters that move in and out of the path of light . 

The subtractive colours yellow, magenta and cyan are mixed colours. They are formed when white light passes through yellow, magenta and cyan filters, or when white light is reflected on the opaque surface of yellow, magenta or cyan objects. A yellow filter absorbs the blue spectral components of white light, and only lets the green and red spectral components pass through. The green and red rays of light combine to produce the mixed colour yellow.

In the same way, a magenta filter absorbs green and allows only blue and red (= magenta) through, while a cyan filter absorbs red and allows blue and green light through.

This means that colour filters and coloured objects either reflect or allow pass through the rays of light which correspond to their inherent colour.

Correction filtering by the subtractive method is done with colour filters having the mixed colours of yellow, magenta and cyan. Only with these colours is it possible to produce all the other mixed colours. In the absence of all three filters, we get white, and in the presence of all three filters, we get grey, because the technical dyes in the filters (like the developed emulsion dyes in the colour film) do not completely absorb the light passing through them. If they did absorb the light completely, the result would be black. For this reason, only one or two different filter colours are used for subtractive colour filtering. The combination of three filter colours is not recommended, because the grey resulting from the third filter colour would lengthen the exposure time unnecessarily.

The principle of subtractive colour mixture is also applied in modern-day colour negative films and colour slide films, which have, after exposure and processing, yellow, magenta and cyan dye layers on top of one another in the photosensitive emulsion layers. Only by combining these three subtractive colours (each with different intensity) is it possible to reproduce all the colours of the visible spectral range with three dye layers on top of one another.

Although overlapping of the yellow, magenta and cyan filters (or the corresponding layers of the colour material) does not produce a complete black, because of the previously mentioned incomplete absorption of the light, "black" is nevertheless a relative impression, because even parts of the image which only allow a small amount of light through can appear black compared with other lighter areas. This fact makes it possible to use three-layer colour materials with the colours yellow, magenta and cyan.

Apart from certain printing processes, the principle of subtractive colour mixture is applied in all modern colour films and in most colour printing materials ( chromogenic process).

In the additive photographic processes, the blue, green and red primaries are produced from white light using appropriate filters (e. g. additive colour filtering in printers).

The original is exposed successively behind blue, green and red colour separation filters onto the photo-paper. The desired colour can be controlled by varying the exposure times behind the individual colour separation filters.

Prism Converging lens
Blue, green and red primary colours are made visible through
a prism (simplified representation without secondary colours), and are
then mixed additively again to give white light.

Light Intensity

The intensity of the exposing light ( the printer lamp ) is changed : 

1. By the density of a particular negative being printed. The darker the negative , the less the intensity of the light that reaches the paper to make the exposure . 

2. By positioning of light balancing filters . The balancing filters , placed in the path of the light source , regulate the intensity of red , green and blue light that are transmitted through the negatives . 

3. By regulation of the printer lamp and also through normal lamp aging . Troughs normal use of the printer lamp intensity gradually changes .  

Printer time

The printer computer is programmed to calculate the time required to produce a pleasing print . there are several ways to adjust the duration of the exposure ( how long it takes place ) . 

The photocell in the printer measures how much light is being passed from the print lamp through the negative to reach the paper . This allows the printer to calculate the time needed for proper exposure . The printer regulates exposure to each of the three main dye-forming layers of the paper with individual cat-off filters. With correct setup , the printer also regulates the durations of the exposure for particular negatives types , such as over and underexposed negatives .

<< Previous - Home - Next >>