Colour of light
We are familiar with the idea that very hot objects emit light in differing colours. On a gas stove, the hottest part of the ﬂame shines blue and less hot areas shine orange or red.
The coloured bar below depicts the colour of light emitted by an object or ﬂame at different temperatures (measured in the scientiﬁc unit of “Kelvin”). Each colour is associated with a temperature - its “colour temperature”.
Light from the sun has a mixture of these colours but as sunlight hits the Earth’s atmosphere, the blue light tends to be scattered more than the red light. The scattering of blue light in the atmosphere makes the sky look blue, while the direct light from the sun at sunset looks red.
If we take a photograph on a sunny day, the light is a mixture of direct and scattered light, tending neither to red nor blue. This white light has a colour temperature of about 5200K. If we step into the shade, we block the direct light from the sun and our scene is lit by the bluish light from the sky. This slightly bluish light has a higher colour temperature of around 7000K. If we step indoors, into a room lit by standard tungsten lightbulbs, the reddish light from the bulbs comes from the ﬁlaments glowing at around 3200K.
|Sunrise / set||2000 K|
When we take a photograph, white objects in our scene will reﬂect the colour of the ambient light and so may take on a blue or reddish tint.
To correct for this source of colour error, most cameras allow us to set a “white balance” of: daylight, shade, cloudy, tungsten, ﬂuorescent etc. This tells the camera by how much to shift the colours in the image to remove the tint and record the white areas as true white.
Most of the time it sufﬁces to set the white balance to automatic and allow the camera to do its best. Any residual tint can be removed later when processing the pictures. However, if we want to get the colours correct in camera, especially under artiﬁcial light, it helps to set the correct white balance.
When processing the pictures in Lightroom or other software, we can correct any error in the white balance by adjusting the Temperature slider or selecting the appropriate light source in the temperature options. Notice that these sliders seem to work the “wrong” way around; if we increase the temperature, the picture shifts to “warmer” (more red) tones – because we are correcting for the colour cast produced by higher-temperature (more blue) light.
In discussing colour it’s usual to start with the traditional artists’ palette of primary colours – red, blue and yellow – and then progressively mix them to produce secondary colours, then tertiaries and the full artists’ colour wheel. However, in modern times we are perhaps equally familiar with the “additive” primary colours – red, green and blue – used in digital cameras and computer screens. Then again, in the printing industry, the primary inks are cyan, magenta and yellow. All a bit unhelpful…
However, as photographers we are not so concerned about how to create colours in paint or ink – we are more concerned about how colours in the real world interact emotionally when we capture them in a photograph. So is there a way to come to colour theory from a photographic perspective?
The natural spectrum of colours is found most dramatically in the rainbow:
As we all learnt at school, the colours are made by different wavelengths of light, from long wavelengths at the red end to shorter wavelengths at the blue / violet end. The visible spectrum is just a part of a broader spectrum, which extends to even longer wavelengths in the infra-red and shorter wavelengths in the ultra-violet.
What can we observe, looking at this spectrum? First, the colours all fade seamlessly from one colour to the next – from red to orange to yellow and so on, with no obvious boundaries. So we can think in terms of colours that are close together being in harmony – while colours that are further apart can contrast, notably the contrasting pairs of red & green and blue & yellow.
There is a separation between “warm” colours at the red end (the colour of fire) and “cool” colours at the blue end (sky, water, ice). Also, the colours seem to have differing brightness, with yellow and orange much brighter than blue and violet – and red and green in between.
Johann Wolfgang (von) Goethe (1749 – 1832), the German writer and scientist, conducted detailed studies of colours and published his Theory of Colours in 1810. He assigned brightness values to colours, indicating how much of one colour would balance another colour in a scene. His colour values were: yellow 9, orange 8, red and green 6, blue 4 and violet 3. So for example, when blue and orange occur together in a composition, the area of orange should be half that of blue (as orange is considered to be twice as bright). He also discussed the concept of colours having opposites: “Let a small piece of bright-coloured paper or silk stuff be held before a moderately lighted white surface; let the observer look steadfastly on the small coloured object, and let it be taken away after a time while his eyes remain unmoved; the spectrum of another colour will then be visible on the white plane.” (Theory of Colour) This is an easy experiment to try at home and reveals pairs of “opposite” colours, for example:
- ♣ red and green
- ♣ blue and orange
- ♣ yellow and purple
To illustrate this concept of colours having “opposites”, we need to introduce a colour wheel. A remarkable aspect of nature helps here; the violet colours at one end of the spectrum work in visual harmony with the reds at the other end. If we wrap red onto the violet end of the rainbow, there is no discontinuity at all:
Now it becomes a simple matter to draw this rainbow spectrum as a circle, starting and ending at red – a colour wheel:
We can immediately see how blue falls opposite to orange, yellow opposite purple and red opposite green – as Goethe observed. Indeed for every hue in the spectrum, we can visualise an opposite. Taking this a step further, if we make a second, slightly smaller wheel and rotate it half a turn, we can show the strong contrasts between opposing colours all along the wheel:
The observation that colours that are opposite each other in the colour wheel seem to work in balance gives rise to the expression that the colours are “complementary”. Butchers shops in the high street make use of this visual phenomenon; red meat is traditionally surrounded by green vegetables or other greenery. The green surroundings make the meat look more red, while the red meat makes the greenery look more green; each complements the other. This is a harmonious contrast:
In the picture below the orange and yellow of the sailing boat work well with the blue sea background and the areas of each colour are roughly in line with the observations of Goethe, i.e. orange is perceived as twice as bright as blue and balances when it takes up half as much area.
Colours that are adjacent on the colour wheel also work well together and are referred to as “analogous”. These may be a range of cool hues around the blues, warm hues around orange, earth hues and so on. A scene with analogous hues will likely look relaxed:
On the other hand, if colours are spaced about a third of the way around the colour wheel (known as a “triad”), there will be high contrast and energy between the colours.
With an analogous colour scheme, it can be effective to have an isolated area of colour that is either complementary or high-contrast as a “colour accent”. In the picture below, the leader’s red t-shirt puts vibrance and energy into a picture that is otherwise a calm green – yellow landscape:
I think we have seen that for photographers it is not necessary to think in terms of primary and secondary colours and so on. We are capturing the colours as we find them, not creating them from pigments. We can then move straight to the concept of a colour wheel based on the natural spectrum of the rainbow and thence to an understanding of colour theory and relationships.