The technology of the rainbow: a use for every colour

The last time you saw a rainbow – which might have been some time ago given this dry, hot summer – you might not have realised how much technology it represents. I don’t mean the natural phenomenon itself, of course, but the ways in which man has exploited the seven colours that comprise the visible light band of the electromagnetic spectrum. Graphic design, engineering, chemistry and optics have all contributed to the pot of gold.

Red…

has dominated the history of lasers, with synthetic ruby forming the medium for the world's first device in 1960. Its output – generated by exposure to a high-intensity flash lamp – was in the infrared but two years later a visible, bright red beam was possible. It was famously described as a solution looking for a problem but over the decades that followed, commercial applications repeatedly appeared for a device that could produce concentrated, single-wave light. These included surveying with a perfectly straight line, scanning barcodes and optical discs and cutting skin but also steel. Lasers help us communicate through fibre optics and validate theories through spectroscopy. And they can point, harmlessly to a presentation screen or lethally by illuminating a target for an air-dropped smart bomb.

Orange…

is rare enough in our environment and particular enough when seen by the human eye to make it the internationally accepted colour indicating safety. Curiously, the colour was named after the fruit (the word’s roots are found in the Sanskrit word nāraṅga) which is appropriate given that is by far the most common occurrence in nature. This rarity and its powerful contrast in a world of blue, green and brown makes it stand out, as does its low-light visibility. Our vision is also sensitive to the hue, whilst cultural usage has assigned red to danger. The result is that orange is used as the colour of traffic cones, construction barriers, life jackets, survival suits and aircraft flight recorders, those vital black boxes.

Yellow…

advanced motion picture effects technology by enhancing the illusion that two subjects shot separately had in fact been in the same place at the same time. Filming something through a blue filter against a blue screen results in a black silhouette of the subject against a white background; optically reversing this yields two strips of film bearing complementary masks or mattes that can be sandwiched with normal footage to ‘cut’ a clean hole out of one scene into which the subject of the second can be inserted with no double exposure. Flooding the screen with narrow-spectrum sodium vapor light, which is yellow like traditional streetlights, makes more precise mattes with less ‘spill’, and Disney’s Ub Iwerks invented a camera prism that automated matte creation into the bargain.

Green…

is the colour of the image seen in most night vision goggles. This electro-optical device, resembling binoculars but invented to help soldiers see in low light, gathers photons (the basic element of light), converts them into electrons, multiplies these to improve the picture and presents the results on a small screen close to the eye. Active systems produced before, during and after World War 2 used infra-red lights – detectable by the enemy – to generate illumination that was picked up and magnified by a receiver sight; NVGs depend on passive technology invented in the 1960s that intensifies available light many thousands of times through microchannel plates and photocathodes. As the human eye is hyper-sensitive to green light, allowing users to process subtle tonal variations very efficiently with minimal battery use, this was chosen as the final presentational colour.

Blue…

floods the background of permanent reproductions made using the photo-chemical cyanotype process, which was cheap, simple and widespread in early Victorian Britain. A physical object or photographic transparency is placed in contact with a paper or fabric surface coated with ferric ammonium citrate and potassium ferricyanide and exposed to sunlight. This ‘fixes’ the lines of the object or image in white against the distinctive blue background that also develops; it was this last that prompted the name ‘blueprints’ to describe the results when employed to copy engineering designs. This low-tech solution remained the standard for such work for a century.

Indigo…

exists at a wavelength that causes reactions in bacteria which are inimical to their survival. Oxygen, perversely, is a threat to these organisms’ reproduction and indigo light triggers its production. This reduces the ability of pathogens to multiply and so brings about disinfection. Importantly, indigo is visible light and so does not carry the risks of exposure of ultraviolet light, which can damage eyesight and skin, and so does not need to be employed in reduced amounts or over short periods of time. For this reason indigo light is now used for the continuous protection of hospital and other medical spaces.    

Violet…

was, historically, indistinguishable from a close secondary colour. In ancient times its ultimate iteration, Tyrian Purple, was a dye made from the mucus of sea snails missed with sea water and reacted with sunlight. Intensely laborious to make, relying on thousands of creatures for a tiny amount of the finished product, it was accordingly expensive and affordable only by the very wealthy, priests or royalty. Indeed in Rome and associated civilisations it was eventually reserved for Emperors, with lesser figures permitted it in thin bands.