Andrew Bailey

Ilfracombe, United Kingdom

After a working lifetime spent in visual communication of one sort of another, Andrew is now happily retired and living in peace and...

More Photograpnic Terms

These are slightly more esoteric than my last journal entry but just as important for a complete understanding of photography, For example, a knowledge of \colour temperature will show you why it is necessary to adjust the white balance on your digital camera before taking any particular shot or, if you are using film, the need to choose the right film for the job.

Black Body Radiator
The term given to a theoretically perfect, non-reflective source of energy used in determining the spectral composition of light. When a substance – usually platinum – is slowly heated from absolute zero or zero degrees Kelvin, it will appear to glow – first deep red, then blue and finally white. Discounting any light reflected from the surface, the radiant energy which is distributed by the substance is directly proportional to the temperature to which it is heated.
A light source of equivalent illumination to a known black body radiator temperature can, therefore, be stated in terms of the temperature at which the radiator emits light rays of the same colour or wavelength.

Circles of Confusion.
As has already been stated, there is no such thing as a perfect lens. It follows, therefore, that there can be no such thing as a perfectly sharp or pinpoint image. In theoretical optical science, an image is either sharp (in focus) or it is not. Science is one thing but the human brain/naked eye combination is another for the latter is necessarily subjective. Even somebody with so-called perfect vision is quite unable to distinguish between a dot and a circle of light if that circle is of less than a certain diameter. For all practical civilian purposes, that diameter is set at not more than 1/100 of an inch. In the armed forces, a more stringent test is applied where the acceptable diameter is set at not more than 1/250 of an inch. Both measurements will produce a sharp, or more correctly, an apparently sharp image. The need for an increase in the standard of sharpness, so far as military application is concerned, becomes evident when consideration for accuracy is taken into account in photographic interpretation for tactical purposes. It is the figure of 1/100 of an inch which fixes the standard for acceptable sharpness in a photographic print when it is viewed from a distance of 10 inches. In the case of an enlarged print, the negative is likely to be a lot smaller and will conform to a standard of sharpness set by a much smaller circle of confusion. Enlargement will also effect viewing distance and, in each case, the circles of confusion will have a different mathematical value. Provided they remain within the figure given and at proportionate viewing distances, apparent sharpness will not alter.

Colour. Isaac Newton demonstrated that white light comprises different constituent colours which we now call the visible spectrum. This visible band of light extends from red to blue. Infra red and ultra violet are both just outside the visible range but both are of great significance to the practising photographer.
Compare the human eye to a camera. They both have a lens which enables them to bring an image into sharp focus and they both have an iris diaphragm to control light level. The image entering the eye is inverted as it is in the camera and is focused on the retina which is full of light receptors of two main sorts. They are called rods and cones because of their shape. It is these receptors which interpret the image and send a message to the brain. It is this message which we experience as sight.
Scientists cannot agree, precisely, as to how we see colour but there is a theory which seems to suit the available facts. The receptors form three (3) distinct systems, each of which corresponds to about one third of the spectrum. One set will react to green/yellow, another to blue/violet and the third to red/orange. It is thought that these divisions overlap so that, for example, yellow light will stimulate a section of both the green and red receptor systems. The brain will then interpret the subsequent message as yellow because, in optical terms, that colour is composed of equal proportions of red and green stimulation. The optical primaries are, for all practical purposes, red, green and blue. Combinations of any two of those three will produce secondary colours which become the complement to the unused third primary. Thus, blue is the complement to yellow (red plus green) as, in equal proportions, they will produce white.

Colour Temperature
The first thing to note is that, in the ordinary state of things, temperature and hotness do not necessarily mean the same thing. As an example, hold a piece of metal in one hand and a piece of wood in the other. Whilst each is likely to record the same temperature, what is noticeable is that the metal feels colder than the wood. This is because of the difference in molecular structure between wood and metal. The tighter that structure is, i.e., the closer together the molecules, the colder an unheated material will feel. What you are feeling, therefore, is not a temperature difference but a difference in hotness between metal and wood.
Colour temperature, on the other hand, is concerned directly with hotness but only insofar as it is a measure of the distribution of energy over the spectral range, i.e., the colour quality of a given light source with a continuous spectrum. The colour temperature of an object is the match of a colour emitted by a black body radiator (q.v.), heated from absolute zero to an appropriate physical temperature on the absolute scale. Measurement on the absolute scale is in degrees Kelvin (K) and absolute zero is the equivalent of minus 273 degrees Celcius. Absolute zero is the point on the temperature scale at which all matter is at rest.
In all incandescent light sources, i.e., Flash and tungsten bulbs, the colour temperature, in effect and for all practical purposes, is the same as the physical temperature of the filament. It is not possible to state a precise colour temperature for discharge lamps as these do not have a continuous spectrum. Quoted temperatures are, in these cases, only approximations.
The most important thing to remember is that the more yellow the light source, the lower the colour temperature – red, for example, has a far lower colour temperature than blue and blue is lower than white. An understanding of colour temperature is vital for successful colour photography as the film used must match the light source if true colour rendition is to be achieved. Even though film has recently been introduced which is intended to cater for all light sources, its effectiveness remains to be tested under professional conditions. In the majority of cases, it is better and probably safer to use film which has been balanced during manufacture for certain lighting conditions. Film is usually balanced thus – blue/white for daylight; red/orange for tungsten or other artificial light. If a film balanced for daylight is used under artificial conditions, the result will show a red/orange cast. Similarly, a tungsten balanced film used in daylight will show a blue cast. In each case, the error is nor redeemable and the packaging will always show the correct rating. Films which have been balanced for use in daylight will show a rating of between 5000K and 6000K whilst those balanced for artificial conditions will show a rating of between 2800K and 3200K.

Journal Comments

  • Kris-T