Violets are blue

Reflection of photographer in dichroic filter letting through violet light

The colour violet is intriguing as it’s one we don’t see every day. It’s outside the colour range of television screens and computer monitors, and it can’t be faithfully reproduced by standard colour printing processes. The reason for this is complex but I’ll attempt to explain it in simple terms in one page.

In the 1670s Isaac Newton explained why white light from the sun can be split into a spectrum of colours using a prism. Although light is a continuous spectrum, Newton named seven colours of the rainbow as it fitted nicely with the seven notes in a western musical scale and the seven known planets at the time. This is why we learn the colours of the rainbow as red, orange, yellow, green, blue, indigo and violet but in fact there are as many as you choose to name.

Most humans have three sets of colour receptors each sensitive to a band of colours with peaks in sensitivity corresponding roughly to red, green and blue light. When we perceive yellow light in the rainbow both our red and green receptors are stimulated as the pure yellow falls between them on the spectrum. To a human, red and green light combined or the pure yellow of the spectrum appear to be the same.

By combining red and green with blue light it’s possible to create the perception of most everyday colours. This process is known as the RGB colour model. The full range of possible colours depends on the exact hues of red, green and blue that are chosen. Historically it was hard to produce the really deep red and vivid blue-violet light needed at sufficient intensity to produce a wider range of colours. As an example the sRGB colour standard developed by HP and Microsoft encompasses less than 50% of the colours visible to most humans.

The difference between violet and purple causes some confusion as they are conceptually very different but the names are often used synonymously. In physical terms colour can be considered a linear spectrum but our brains perceive colour more like a wheel. Between red and violet is a wedge of purples combined by mixing these two extremes. Purples are extra-spectral colours which means you won’t find them in the rainbow. They are, if you like, pigments of your imagination.

Violet, the pure spectral colour on the inside edge of the rainbow as opposed to the purple in your brain, is beyond the blue of an RGB monitor so it is impossible for me to recreate it for you on this page. True violet occurs rarely in nature but when you do see it and try to photograph it with a digital camera you normally end up with blue. This is because most cameras are based on the same RGB model.

I used a specialist dichroic filter to produce violet light, not the purple light you can see from a screen or the effects of ultraviolet light you see in clubs, just an intense, pure version of the visible violet you see in the rainbow. It’s a truly beautiful colour. If you want to experience violet and don’t want to wait for the next rainy day remember there are many ways to make your own rainbow.

One Response to “Violets are blue”

  1. [...] Tinley has a great post about the colour violet on his blog, following up his talk [...]