Technical Manager Paul Lee tackles Metamerism in under two minutes…

In the last few months, the Cranfield road-show has been out and about in the UK, Europe and the USA and one of the demonstrations that catches the most attention is the illustration of metamerism. Even if you don’t know the term, you may well have encountered it. Most people have had the experience of viewing an item of clothing in a shop which appeared to match our eyes, hat or bag under the shops expensive lighting, only to find that the match is not so convincing when outside the store!

This phenomena, where colours appear to shift under different lights is known as metamerism and it is more complex than coloured materials simply having a warm appearance because they are viewed in a warm red light, or a cold appearance because they are viewed under harsh fluorescent light. We need to understand a little of the colour spectrum to really understand what is going on.

White light loved by artists is North Sky daylight, where the colour spectrum (Red, Orange, Yellow, Green, Blue, Indigo and Violet) are present in equal measure. North Sky day light is perhaps better understood as the average readings for the colour of the diffuse light coming in through a north facing window in the northern hemisphere at noon at various times throughout the year.

North Sky Light is the most equal and accurate way to view colour, but is seldom the light under which art work is viewed, which presents us with a challenge when matching or using colours in paints or inks. There are of course multiple ways to match colours. This is true for garments, car parts, artists inks and paints, kitchen work tops, household paints… the list is almost endless. And if one manufacturer makes a green scarf from one single green dye and another makes a hat from mixing blue and green dyes, there’s no ‘right or wrong’ and both will look broadly similar when viewed under the forgiving full gamut of North Sky Daylight.

But dyes and pigments have no in inherent colour of their own, they simply have the ability to reflect certain wavelengths and absorb others. If a red apple is viewed under a green spotlight it will appear black as it’s not receiving the proportions of white light it requires to reflect red. So if our two items are viewed under more selective conditions, such as a tungsten bulb which emits uneven light (skewed towards the hot red end of the spectrum) then this skewed light will disproportionately amplify the warmer of the two pigments in the hat, favouring the green, whilst giving insufficient blue light to let the blend of dyes reflect equally. The result will be more striking for the blended colour in the hat than the singly pigmented scarf.

Generally in the art world, single pigment paints and inks are referred to as ‘Genuine’ and mixtures are commonly known as ‘Hues’. We should not however assume that genuine colours are always good and hues are invariably inferior. There may be very good reasons why a blend or hue makes sense. The genuine or single pigment paint may not be lightfast, there may be continuity, sustainability or availability issues with the single pigment. It may be desperately expensive or have extremes of opacity or transparency. The colour chemist will make a value judgement on which route to follow to avoid metamerism in the laboratory.

Artists too can take steps to make sure that their own blends reduce the chances of the same metameric risk by making mixes using pigments that are close together on the colour spectrum rather than randomly ‘flinging colours in to the mix’! Secondly the artists can create a studio or work space with (where possible) natural light augmented by a range of light sources. Colour corrected fluorescent tubes and daylight bulbs being preferable to a studio lit only by tungsten spot lights. Some of the cheaper long life bulbs and tubes rely on a very limited number of phosphors within, so whilst in theory the light covers the full spectrum, it does so in a very ‘spikey’ way and it is not a convincing replication of North Sky day light. So a bit of money spent on good lighting is the key, as is keeping the receipt for that green hat… just in case!