Colourlight Standards Lab

4.1 Definition of Metamerism

Metameric colour stimuli are colour stimuli with the same tristimulus values but different spectral power distributions. An equivalent definition states that metameric colour stimuli are colour stimuli that have different spectral radiant power distribution but match in colour for a given observer. Metameric colour stimuli, as defined above, are often referred to as metamers, and the underlying concept is referred to as metamerism.

A phenomenon known as metamerism occurs when the colour of two objects appears to be the same under a specific light source but really has different spectral power distributions. The colour difference between them may be seen when a different type of light source is used. Metameric objects are those that, when illuminated by a given reference illuminant, reflect stimuli of different spectral power distributions (SPD) that produce the same colour under the same viewing conditions.

4.1.1 Metamerism fields

According to Wyszecki (1985), there are four types of metamerism:

1. Illuminant Metamerism
2. Observer Metamerism
3. Geometric Metamerism
4. Field-size Metamerism

5.1 Additive and subtractive colour generation

There are two modes of colour generation that are important to distinguish and analyse; one of them is additive and subtractive colours generation. In daily life, for example, an additive colour model is used for television sets or flat panel display, where three primary colours red, green, and blue are added in different quantities to the colours generated.

The objective description of the physical correlates of colour perception is the subject of colourimetry. To provide an objective description of the colour at first, coloured samples were used. Physicists developed techniques for comparing coloured lights directly to reference lights. To improve the correlation between colour measurement and visual perception, many colour specification systems and colour differences have been proposed.

Modelling the human perception of colour is the aim of colourimetry that implies the following objectives. Colourimetry focuses on matching colours and predicting small colour differences when two stimuli are not matching. Colourimetric data alone do not specify what colour can be seen.

7.1 Museum lighting

There are two main and very important issues to museum lightings: preserving artworks from damaging of light effects and provide the best visual impression to the visitors, through adequate light source. In many museums there are some principles of placing objects of artwork, according to their sensitivity to light and levels of luminance. But there is a very important issue about that, what source of light to use inside the museums to collide with the preservation of artworks and getting the best impressions to visitors at any time of the day. The development of technology, specifying here the lighting technology, enabled flexibility in some characteristics of lighting and setting such as luminance levels, correlated colour temperature and colour rendering properties.

All these characteristics have a significant impact on the visual impression of visitors. In the past decade, several studies have investigated the preferences and visual impressions of the observer about museum lighting.

Effects of illuminant metamerism on artistic paintings

It is especially important to mention that not all light is the same since different temperatures derive from different light sources, this is because under the warm light of a sunset colours will look different compared to the cool light of an overcast day. According to research and experiments there are supposed to be two steps to use correct painting the light. The first step is to choose the correct light that illuminates the painted subject, where it would be the easiest way to use certain points. This is because of the fact yellow and red are always warm, light, and bluish-white is always cool.