Metamerism effect - The right light at the workplace

What is the metamerism effect?

Metamerism refers to the effect of differently composed light spectra with the same colour valence, i.e. which trigger the same colour impression. Conversely, changes in the assessment conditions or a change in the "normal observer" can also lead to different colour perceptions. The relevant assessment conditions include the light colour of the illumination and the selected type of light with the corresponding wavelength spectrum.

Different colour impressions of such conditionally identical body colours under different lighting are described quantitatively with the help of the metamerism index, using daylight or an artificial light source whose light is similar to daylight as the normal.

Green or brown: What colour is this cap? Tiktok users and the game with metamerism

Here in this video, the metamerism effect shows up very strongly and causes astonishment on the net. Another typical case in the fashion sector is the effect known as "evening colour". This is the effect when the salesperson shows the colour of two fabrics in daylight instead of in the light of the shop aisle, in front of the shop door.

This has to do with the colour temperature of the respective light source, which can emit a more yellowish light (like a light bulb) or a white light (daylight or standard light), for example. For a reliable, safe assessment of colours, a standardised, white light (norm light) is therefore always necessary in order to avoid metamerism effects.

Metamerism in violin making

In violin making workshops, light is one of the most important but mostly neglected factors for professional work. Appraisals, analyses, retouching, expert opinions, repairs or sales: they all depend on good lighting conditions. It is not enough that it is bright. The right light spectrum must be available to make the "real" violin varnish or the "real" wood colour visible.

It also happens that retouched parts of an instrument no longer look the same in the car park in front of the workshop or in the concert hall, that the retouching becomes visible, although the selected varnish and pigments led to the same colour perception in the artificial light of the workshop.

The varnish of an instrument can only remit those light waves with which it is illuminated. That is why its colour depends on the spectrum of the illuminating light.

Instrument appraisal

When examining instruments, the correct light spectrum is of great importance. Otherwise, the colour of the lacquer cannot be assessed. This can lead to misjudgements thanks to the metamerism effect.

Sales

Especially in sales, the right light helps the customer to perceive the full splendour of the lacquer and to better understand the instrument. The right light source certainly has a positive effect on sales.

Retouching

Retouching done under poor lighting conditions can stand out strongly in daylight or in a concert hall. On the other hand, it is difficult to mix the right retouching colour with the wrong light source.

How can I detect metamerism at my workplace?

We have developed a simple Test Card for violin making that allows you to quickly and easily detect the metamerism effect in order to improve the lighting conditions if necessary.

How does the Standard light - Workspace Test Card from SCHILBACH work?

The Standard light - Workspace Test Card from SCHILBACH with the Light Indicator Strip D50 is used to check the conformity of a standard lighting system to the standard illuminant D50 with 5000K. The two green printing inks of the card form a metameric colour pair, which conditions equal colours. Under standard illuminant D50 with 5000 Kelvin, the colour patches appear identical. Under any other type of light, the colour patches appear different.

This type of metamerism is called illumination metamerism, which plays a decisive role in violin making in both the assessment of varnish and retouching. The reason is the different light spectrum of the respective types of light. In this way, you can easily recognise non-compliant lighting conditions at your workplace or presentation room and are not fooled when colour matching, retouching or assessing an instrument.

The right light for your work space

Metamerism in biology - The human perception of light

The principle of human colour vision does not allow a distinction between signal sums of the same value but of different composition. The colour cyan, for example, can be represented as a monochrome colour with approx. 490nm, or as an additive mixture of the colours blue (450nm) and green (540nm).

Different spectral compositions of the colour stimulus can lead to the same colour perception and is called metamerism.

In short: The composition (wavelength spectrum) of the colour stimulus cannot be deduced from the colour impression.

The standard observer - everyone sees differently

Every observer has his or her own personal spectral value functions. As a result, different observers perceive more or less pronounced differences in metameric colours, even in the same light. Only identical remission spectra (1) lead to the same colour perception under all conditions and for all observers. This is referred to as "unconditional equality".

The colour equality of samples with different R-spectra is more or less dependent on the random light conditions and on the observers. It is a very "deceptive" sameness that changes into dissimilarity when the lighting conditions or the observers change.

(1) Remission spectra

In optics and reproduction technology, one speaks of remission when a body absorbs a part of the light spectrum (opacity) and transmits (transparent media) or reflects (opaque media) a part of the spectrum.

For example, if you take a yellow body and irradiate it with daylight that encompasses the entire light spectrum, it emits or reflects the red and green light components, but absorbs the blue component. The body appears yellow due to the addition of the reflected red and green light components.

Another example is colour filters. These filters are usually made of coloured glass and transmit only certain colour components, i.e. they are only transparent to these spectral components, and absorb the rest of the spectrum. For a cyan-coloured filter, this means that it only transmits the green and blue colour components of (white) daylight and filters out the red component. Violin makers use this in the optical analysis of stringed instruments to identify individual substances in the varnish, to classify authenticity and coherence, to determine original varnishes or to clearly distinguish and record differences in retouching (aniline vs. pigment retouching).

Colour filters for analysis

Using metamerism in the optical analysis of stringed instruments

When changing the "type of light", the phenomenon of metamerism can occur: coloured objects match under one light source, but the match is lost when the light source changes. However, we can also make use of this effect. By illuminating an instrument with different wavelengths, we can distinguish between individual substances. The originality and coherence of, for example, the ribs and back of a violin.

Under daylight or regular light sources, the instrument looks uniform. From time to time, this uniform perception can also be observed in the most commonly used spectrum of 365nm (LUMATEC SUPERLITE M05) or 320 - 400nm (LUMATEC SUPERLITE S04). This spurs a hasty assumption of sameness or originality. However, to be sure, it is essential to look at other wavelengths. For violin making, 405nm (LUMATEC SUPERLITE M05) and 415nm (LUMATEC SUPERLITE S04) have proven to be effective in creating a strong metamerism. Both wavelengths are close to each other and yet far enough apart to create the desired effect.

LUMATEC lamps for the optical analysis of stringed instruments

Which types of metamerism do existe?

• Illumination metamerism: The most common case of metamerism in practice is illumination metamerism. This refers to the correspondence of two colours under one type of reference light and deviation under other types of light. The cause of the metamerism here is the different light spectra of the respective types of light.

• Observer geometry metamerism: In this type of metamerism, the colours match under one observer geometry, such as the 10° normal observer, but not under another observer geometry. This is caused by the uneven distribution of rods and cones in the eye.
• Observer metamerism: This type of metamerism occurs between individual observers. It is caused by the fact that the perception of different observers is never exactly the same.
• Geometry metamerism: In this case, metamerism occurs when the angle of vision changes, i.e. it is caused by surface effects. This type of metamerism is called the silking effect.
• Device metamerism: This variant occurs between different colour measuring devices and is produced by differences in the installed components.

Spectroscopy

With a spectrophotometer you can measure how much light is reflected at each point of the visible spectrum. The resulting reflectance data is the "fingerprint" of that colour and material. From this data, you can create a reflectance curve.

Metamerism pairs

Metamerism pairs are hues that appear to be identical under a particular lighting condition, but actually have different fingerprints.

The diagram on the right shows the reflection curves of two red tones. Both curves absorb blues strongly, both absorb greens in moderation, and both reflect reds quite strongly. See how the curves intertwine? If the curves of two physical samples cross at least three times, they are a metameric pair. When objects are a metameric pair, metamerism is obvious, and although they sometimes appear to be the same colour, they do not match under all lighting conditions.