Daylight simulation – is it necessary for colour evaluation?
By David Crowther
Why do we need to consider daylight when evaluating colour? Quite simply because daylight renders colour accurately. Even though natural daylight changes throughout the day, together with changes in the weather, a need to simulate daylight does exist.
Research over one hundred years ago identified the need to use simulated standardised daylight for making accurate visual colour judgments.
Depending on the light source, colour perception varies. This is because the spectral quality or colour content of the light affects how we see colour. The spectral quality of natural daylight depends upon atmospheric conditions, geographic location, time of year – even time of day. For example, the colour appearance of early morning sunrise and late afternoon sunset can be as low as 2300° Kelvin (also known as horizon daylight*). At noon, the colour appearance of light is approximately 5000° Kelvin and can exceed 10,000° Kelvin (on a clear day facing a north sky).
David Crowther explains further.
Daylight is evenly balanced across the visible spectrum – it has equal amounts of red, orange, yellow, green, blue, indigo and violet light energy. This means it more accurately renders colour. Without this balance, colours are not rendered precisely causing the light source to accentuate certain colours while suppressing others. Even slight deficiencies or excesses in light energy could skew your colour perception causing you to approve or reject samples erroneously.
Metamerism has been discussed in these pages before but it is worthwhile to go over this important phenomenon once again.
Metamerism is the phenomenon of objects having their colour match under one set of conditions – real or calculated – and not match under different conditions. Two objects exhibiting metamerism are said to be a metameric pair.
What causes metamerism? It’s caused when the spectral reflectance or transmittance curves of each object differ.
When the reflectance curves of the two objects cross one another three times or more, metamerism takes place. (See illustration.) For example a mobile telephone consists of several component parts that may be comprised of different resins and colorants. When the parts are assembled into a phone, all the components should visually match under daylight and at least two other light sources giving the phone a uniform appearance. If, for example, the battery housing appears to be a different colour than the main phone housing, metamerism could be a factor.
Colourants used to create a product can differ from batch to batch, lot to lot or supplier to supplier. Colourants used in some applications may not work in others. The more colourants added to create a batch or correct a mis-tint, the greater the risk of metamerism.
How can you avoid metamerism? You cannot completely avoid metamerism in most applications; however, you can minimise its effect through early detection. Remember to evaluate samples under daylight and at least two other light sources. Review spectral curves from a spectrophotometer (not a colorimeter) to detect and evaluate the degree of metamerism. Finally, samples that may have a directional characteristic such as textured plastic must be viewed from the same angle to avoid geometric metamerism.
Other Forms of Metamerism
Observer metamerism is when samples that appear to match to one observer do not seem to match to another. Often the cause is differing colour vision between the viewers – a fact that can be established with a colour vision test.
Geometric metamerism is when object colour matches at one angle of illumination but not when the angle is changed. This situation often occurs with materials that are directional – velvet, suede, broadlooms, plastics, and metals.
To summarise, there is a lot more to daylight than meets the eye. Critical colour decisions, on which potentially millions in profit and loss are at stake in industrial markets, require careful consideration and a clear understanding of the advantages and limitations of simulated daylight technology. For the photographers amongst us this is especially important when dealing with the many and varied types of print and/or output engines combined with media types, media finishes and the ink or colourant types used.
This and many other workshops by David Crowther are first published in Digital Reproduction magazine