杏吧原创

Colour coding

I have noticed there is always a series of numbers electronically printed on the back of colour prints made by photo labs from 35-millimetre negatives. What does each group of numbers represent? And will my reprinted photographs turn out the same in terms of exposure level and colour if they are printed by different processing companies and different makes of colour photo printer or even by a different person, using the same settings as those used for my original set of prints?

鈥�The characters that are printed on the reverse of a photograph are usually referred to as the backprint. Common data, which varies according to the lab, may include: an order number; the date your prints were made; the negative鈥檚 frame number; and any colour or density corrections that are made to it.

Data showing colour or density adjustments made by the operator will usually be four or more characters, indicating the addition or subtraction of the key colours that create a photographic image: yellow, magenta, cyan or density. The values usually appear in the same order 鈥� YMCD.

Modern negative-scanning systems mean that operators seldom need to modify or 鈥渃orrect鈥� the colour of prints, as they automatically arrive at the correct exposure.

First, the negative is scanned during which the light transmitted by the negative is measured and the characteristics of the individual frame, plus that of the overall film type, determine the values for exposure. During exposure the light is filtered into yellow, magenta and cyan. The balance of these three colours should produce an image that is neutral in colour. How long each of these colours is exposed controls the final density. Any corrections made by the operator are in addition to these. Colour correcting is a very subjective process and relies on skilled operators.

Some analogue printers are able to backprint the exact duration, in milliseconds, of each coloured filter used in the exposure and creation of each print. But even duplicating these values exactly when reprinting a photograph elsewhere will not ensure an identical print because other variables such as the condition of the paper, processing chemistry or intensity and colour of the printing lamp may have changed.

Reprinting negatives at a different lab, even on an identical machine, will often produce slightly different results because of the variation in set-up and also the machine operator鈥檚 personal preferences.

Colour cast preferences in photos can also vary from one country to another, as well as from lab to lab, usually offering the most flattering skin tone for the customer. Therefore, most light-skinned Europeans favour a yellow-red print colour bias, because this gives them the appearance of having a slight tan.

Mark Bound, Consumer imaging product support specialist, Agfa UK

Rainham, Kent, UK

Rubber horror

Why do rubber bands spontaneously melt? Often I find an ageing one on my desk that has turned into a sticky mess. After a few more months, the sticky mass solidifies and become brittle. Why?

鈥�Natural rubber is made of polyisoprene chains that slip past each other when the material is stretched. When raw, the substance is too sticky and soft to be of much use, so it is toughened with the addition of chemicals such as sulphur that create cross-links between the chains, making the rubber stiffer and less sticky. This process is called vulcanisation.

With time, ultraviolet light and oxygen in the air react with the rubber, creating reactive radicals that snip the polyisoprene chains into shorter segments. This returns the rubber to something like its original state 鈥� soft and sticky. Meanwhile, these radicals can also form new, short cross-links between chains. This hardens the rubber and eventually it turns brittle. Any vulcanisation agents left in the rubber contribute to the process.

Whether a rubber band goes sticky or hard depends on the relative rates of these processes, and these rates in turn depend on the rubber鈥檚 quality 鈥� such as what additives, fillers and dyes it contains 鈥� and how it is stored. Heat and light speed up the reactions (for example, a 10 掳C rise in temperature will roughly double reaction rates), and the presence of strong oxidisers such as ozone creates even more radicals. The eventual fate of your rubber band depends on the temperature in the room, and whether you have a desk by a window or near a machine such as a photocopier that creates ozone.

How much light and heat is required for these changes? The polymer chemistry of rubber is fairly messy and so this is difficult to answer precisely. Obviously, the chemical reactions run slowly if the rubber is in a fridge, more quickly if left on a sunny desktop. A rule of thumb is that reaction rates roughly double for a 10 掳C rise in temperature, but this is complicated when you take oxygen and light into consideration. The quality of the rubber is also important, such as whether it contains additives, fillers or dyes that absorb light energy or help transfer radicals. The final factors that influence the change are ozone concentration, UV light intensity and whether the band is stretched or not 鈥� stretching brings chains closer together, allowing radicals to jump from one chain to another more easily, and to create new bonds between chains 鈥� Ed

This week鈥檚 question

Barrel boom

A wooden barrel of gunpowder is a familiar sight in many historical movies, but were they really used? Wooden barrels were surely made to hold liquids. A dry barrel shrinks and the gaps between the staves open up. To store gunpowder the barrels would have to be dry, so wouldn鈥檛 a barrel leak powder?

Jack Hedger

Nottingham, UK