Technical Documents - Documentos Técnicos Home

Colour.

Colour is not an absolute characteristic of an object, but a human perception. The colour stimuli registered by the retina of the eye are made up of the energy distribution and the spectral properties of the visible light passing through or reflected by an object. The sensation of "colour" only comes about after a complex operation, in which the brain processes the information relating to the incoming stimuli. The photo-receptors of the retina (rods and cones) convert light into nerve impulses. Three kinds of cones with different sensitivities for different wavelengths are responsible for colour vision, i. e. responding to stimuli in the visible spectrum. The ability to see colours is, over a broad range, independent of the brightness of the visible light (range of photopic vision). In the case of radiation-sensitive systems, e. g. measuring devices, photographic films, printing materials and the human eye, we use the term "spectral sensitivity", not "colour sensitivity".

Colour density curves.

The colour density curves (characteristic curves) shown in the AGFA Technical Data Sheets describe the dye densities of the processed film and printing materials as a function of the exposure.

The yellow, magenta and cyan emulsion dyes are formed in the blue, green and red-sensitive emulsion layers (depending on the exposure) as a result of the subsequent colour development. Densitometric measurement can then be used to obtain the colour density curves for the yellow, magenta and cyan layers .

Fig. 5: The colour density curves (printing densities) of the developed emulsion dyes of a colour negative film.

The colour density curves show to what extent the material is able to reproduce the photographed object or the original being printed, based on the luminance ratios (range of subject contrast). They are also called gradation curves, since they show the increase in dye densities with the increase in exposure. In the case of negative films, the printing density is measured, while in the case of slide films and positive printing materials, the visual density is measured. Status filters are particularly suitable for densitometric measurement of the dye densities. The exposure (measured in lux-seconds) is given in the form of logarithms to the base 10. The zero on the exposure axis corresponds to one lux second (lxs).

Colour temperature.

The term colour temperature is used in physics to describe the ability of an incandescent metal tube (black body or Planckian radiator) to emit, at a given temperature, the same radiation as is emitted by a given radiator. The unit of measure for the colour temperature is the Kelvin (K). At high incandescent temperatures, e. g. 5,500 K, the short-wave radiation of the blue spectral range dominates, whereas at low incandescent temperatures (e. g. 3,200 K), it is the long-wave radiation with primarily red to yellow spectral components which dominates. The term colour temperature applies only to thermal radiators, e. g. halogen lamps, not to the frequently used luminescent radiators, e. g. fluorescent tubes, which do not have a continuous spectrum.

Colour films are sensitized for a certain type of photographic light (a certain colour temperature is fixed as the so-called achromatic point). Tungsten light films are generally adjusted to incandescent light (approx. 3.200 K), whereas daylight films are adjusted to the spectral radiation distribution of mixed sky light and sunlight (approx. 5,500 K).

Average daylight contains many more blue and many less red spectral components than tungsten lamp light; this is why the blue-sensitive emulsion layer of daylight reversal films is much less sensitive in relation to the green-sensitive emulsion layer, whereas the red-sensitive layer is far more sensitive than in tungsten light films. If the spectral sensitisation ( spectral sensitivity) of the film does not match the photographic light, the final picture will have a colour cast. (Colour films register discrepancies in the photographic light much more accurately than the human eye, which compares the image colours with one another when observing objects or even colour photographs, and in this way subconsciously balances out the colours.) To prevent a colour cast which results from not keeping to the prescribed colour temperature, the temperature scale of the light used for the exposure must be shifted with suitable blue or yellow-red correction filters.

Complementary colours .

Colours which together make up white (additive colour mixture) or black (subtractive colour mixture) are com-plementary (1) .

Core shell crystals.

Agfa-Gevaert began researching core shell silver halide crystals back in 1960 and recently developed them a stage further. The core and the shell have different halide compositions. One special form is termed a structured twin crystal. As a rule, these are silver halide crystals consisting of a core with a high silver iodide content and an exterior shell of silver bromide with a low silver iodide content. Such crystal structures, which are manufactured both in flat platelet form (like Kodak's T grains) and in compact form, intensify the developable silver blackening per unit of incident light (it is therefore also possible to talk of in-creased quantum efficiency). Special manufacturing methods reduce the formation of excessively small crystals, which would impair the sharpness due to light scattering (fine grain is not necessarily equivalent to high sharpness), and of large crystals, which would result in the grain becoming coarser ( crystal technology).

Cross cast.

Colour shifts occur either as a colour cast or as a cross cast. A colour cast results from one of the three colour density curves (on condition they have been standardized to grey (3) ) being flatter or steeper than the other two curves. A colour cast can be eliminated by suitable correction filtering. A cross cast is the result of one of the three colour density curves being intersected by one of the others, with the result that one of the three curves is higher in the area of minimum density and at the same time lower in the area of maximum density than the other two curves (it can also happen the other way round with a lower minimum density and a higher maximum density). This can mean for example that the highlights - with neutral filtering of the medium tones - can have a complementary colour cast to the shadows ( complementary colours). Unlike a colour cast, a cross cast cannot be eliminated by correction filtering, since colour correction filtering of the highlights would lead to an intensification of the complementary colour cast in the shadows.

Crystal technology.

The basis of all photography are the silver halide micro-crystals, which are often smaller than 1/ 1000 mm. They are photo-sensitive and are the smallest elements for storing the optical information.

The silver halide crystals of the very latest generation as used by Agfa - they are also called structured twin crystals - measure between 0.2 and 0.3 thousandth of a millimetre. Because of their special structure ( core shell crystals) they absorb more light than conventional crystals. Some light is inevitably scattered on the crystals and produces halos (= loss of sharpness), but the effect is much reduced with the new silver halide crystals, minimise the loss of sharpness. Flat, thin crystals can also be packed more densely. This enables the emulsion layers to be made thinner without sacrificing any of their sensitivity: the thinner the emulsion layer, the sharper the film.

Secondly many more crystals can be accomodated in the emulsion layer without increasing the layer thickness. This makes the film very much faster without having any negative influence on the granularity. To summarize, structured twin crystals bring about an improvement in sharpness and grain in relation to the film speed.

Densitometer.

Photosensitive, photo-electric meter for measuring the density (in the form of logarithmic densities) of black-and-white materials, or the colour densities of colour materials. Densitometers are used in printing and repro work and for photographic process monitoring. The key components of a densitometer are the light source, measuring filters (in high-quality densitomers use is made of status filters), a circular measuring diaphragm, a multiplier for converting the reflected or transmitted test light rays into electric signals, and an analogue or digital data display.

A distinction is made between the two different types of densitometers.

Reflection densitometers measure the proportion of incident light reflected by the surface of the material, e. g. by a photographic paper or by a colour printed on paper. Transmission densitometers measure the proportion of light from the densitometer light source which passes through transparent material, e. g. a photographic film. For an explanation of the measuring filters used in densitometers see: status filter, visual filter.

Density curve.

The density curve, also known as the characteristic curve, is the most important element in sensitometry.

The density curve is the graphic representation of the optical density D of a developed photographic material as a function of the exposure H. In black-and-white photography, the density curve is also called the characteristic or H and D curve, whereas in colour photography we speak of colour density curve(s). Colour photographic materials always have three colour density curves in line with the sensitisation of their different emulsion layers.

The density curve of a photographic material is obtained by plotting, in a system of coordinates, the optical density D measured with a densitometer (on the y-axis) against the logarithm of the exposure H = l x t (on the x-axis).

At the beginning of the density curve we have the minimum density (lowest exposure, not producing any image), then comes the threshold (beginning of the exposure which does generate an image). This is followed by a virtually straight-line section (area of normal exposure), then comes the shoulder (over-exposure) and the maximum density.

In the case of certain special photo materials (not shown in Fig. 6) there is then the area of solarization, in which further increase in the light intensity lowers the density again.

Fig. 6: Schematic representation of the density curve of a black-and-white photo-material.

The lux-second is used as the unit for measuring the exposure lg H. In the diagrams shown in the AGFA Technical Data Sheets, 1 lux-second corresponds to the value 0 on the scale of the exposure axis. The positive values +1.0, +2.0 ... represent 10 and 100 lux-seconds respectively. The negative values on the exposure axis, -1.0, -2.0 and -3.0, represent fractions of a lux-second, i. e. 0.1 lxs, 0.01 lxs and 0.001 lxs respectively.

Unlike the density curves of negative materials, those of reversal materials do not slope upwards from left to right, but downwards from right to left. The area of low (i. e. subliminal) density with negative materials represents, conversely, a high density with reversal materials, which is also called the maximum density.

Fig. 7: The colour density curves of a colour reversal film slope in the opposite direction to those of a negative film (compare with the slope in fig. 6)

<< PREVIOUS << - HOME - >> NEXT >>