Design trends in recent years have introduced a variable not often considered – the combination of leather with other materials such as fabrics or plastics leading to problems of colour change under certain lighting conditions. Market demands have tightened the acceptability limits relating to colour difference and its evaluation, but for many tanners the phenomenon is misunderstood, with the consequent claims from buyers dissatisfied with the final product.

A metameric pair can be defined as colours with different spectral compositions generating the same colour stimuli under certain conditions. Metamerism is of great importance because the higher the spectral difference, the greater the likelihood that the matched colours will be different when changing the viewing conditions. So as to explain more clearly the problem, the next section concerns the variables involved in metamerism.

To perceive the colour of an object, we need:

* a light source, with a characteristic spectral composition

* an object with its light absorption/diffusion/reflection

* an observer with his discriminatory capacity, who will perceive a certain colour sensation.

Physiologically, the retina is responsible for light perception (Figure 1). Basically the retina can be defined as a thin membrane covering the ocular globe. Cones and rods (Figure 2) are the photodetectors within the retina. Rods are useful for scotopic vision (low light levels), while the cones are for photopic vision (high light levels). There is only one type of rod, with maximal sensitivity at around 510nm, and three types of cones called r, g, and b (corresponding to r, g and b, red, green and blue), with maximal sensitivity at wavelength 580nm, 540nm and 440nm, respectively. Humans can distinguish two areas in the retina: one central area called the macula, containing the central fovea, rich in cones, and the middle and extreme area with prevailing rods. The macula is responsible for day vision or photopic, while the mean and peripherical fovea is responsible for mesopica and scotopic vision, (intermediate lighting levels and night vision).

Each cone and rod receives light, converting it into an electrical impulse, which is transmitted to the optical nerve in the brain. Figure 4 shows the cones and rods distribution inside the macula. The proportions will define a chromatic sensitivity, which will be unique for each individual (it may even be different for each eye). The defect or lack of one or different types of cones or rods defines the various defects in chromatic perception.

To study the spectral sensitivity of a certain number of observers, the CIE (Commission Internationale del Ecclairage) established, in 1931, the standard colourimetric observer representing the response of a control observer, figure 5.

Due to the trichromatic nature of human vision, the physiological variable as well as the lighting level and quality, represents a potential metameric situation, because colours with different spectral compositions may produce similar chromatic perception stimuli, or colour sensation. In this way, lighting levels, spectral characteristics of the light source, and visual sharpness of the observers are fundamental in metamerism perception.

Figure 6 shows the spectral composition of two metameric colours. In figure 7, colour samples are observed under a D65 light source, while in figure 8 the same samples are under an A light source (Tungsten incandescent). Overlapping between the corresponding spectral curves of both colours, suggests that there will be a metameric pairing under different conditions.

The next points explain each of the variables which define possible cases of metamerism.

Light sources/colour temperature

Light sources may be natural, (sunlight) or artificial (incandescent such as Tungsten, gas discharge like neon). Each of these sources has its peculiar spectral composition, figures 9 and 10. The CIE has standardised different light sources, such as A, B, C, D50, D55, D65 and D75 and others. Some of the standards established by the CIE are shown in table 1.

The importance of the light source is related not only to its colour but also to its yield. As can be seen in figure 9, the A light source has a yellowish/orange dominance without components in the blue/green area, if compared with a D light source. This property will be perceived as an increase in the yellow/orange dominance of the samples it lights, and a relative lack of blue/green shades.

Sunlight variability and surroundings influence Sunlight, our natural reference, also suffers big variations during the day, depending of the season and the climate conditions. Early in the morning, the colour temperature is around 900-1000 K (very red) increasing to a maximum at noon, and returning to the same 900-1000 K with the sunset. All the intermediate values will suffer variations depending on the climatic conditions. For example, noon sunlight on a cloudy day (total diffusion) 6,500 K. During a clear sky day, the colour temperature can be over 11,000 K. Figure 13 highlights the dominant shade of light at different correlated colour temperatures.

Another variation factor to consider is surroundings. So as not to influence the chromatic composition of daylight, the surrounding should be medium grey, with 50% reflectance, and matt. However, usually we have walls in different colours, which will contribute to a non-standard specular component. It is due to this variability that the CIE established different standards as already seen in table 1.

Types of metamerism

It has been shown that the eye does not have an equal response to each of the visible wavelength, but responds to a trichromatic stimulus. In this way, infinite spectral compositions will be able to generate the same colour sensation. This effect will define the different cases of metamerism:

Observer metamerism: Two observers will always have a different perception of the chromatic stimuli, depending on the composition and spectral distribution of the eyes’ cones. The problem of the observer metameric effect may be solved working traditionally with observers having a very acute colour difference range perception. Unfortunately as the statistics shown later point to, the percentage of observers with visual defects is higher than imagined.

Visual field metamerism: Due to the concentration of the cones in the central fovea, a variation on the visual field or viewing angle may generate different chromatic stimuli. In the case of visual field metamerism, it is enough to adopt a standard viewing method: usually the light source on the normal and at 45° to the observer.

Light source metamerism: a variation on the light source will evidence a different chromatic stimulus. In case of light source metamerism, the use of standardised light cabins (so called artificial skies) is used.

For all the cases of metamerism described, it is possible to use spectrophotometric reflectance control systems, which are able to anticipate the metameric characteristics of hypothetical dyes.

Figures 3 and 5 represented the perception range of the different types of cones from the standard observer. Figure 5 particularly corresponds to an experimental media of different observers, and lots of possible variations can be found, from a distribution similar to the control to the total deficiency of cones of each type. The main colour blindness types are namely:

* Protoanomalous: reduced discrimination between red and green. The red shade is perceived less intense than the control

* Protoanope: total discrimination absence between red and green. Red shade is perceived less intense than the control

* Deuteranomalous: reduced discrimination between red and green. No shade is perceived less intense than the control

* Deuteranope: total discrimination absence between red and green. No shade is perceived less intense than the control

* Tritanomalous: reduced discrimination between blue and yellow

* Tritanope: total discrimination absence between blue and yellow

* Cones monocromatism: no colour discrimination, with perception at normal intensity

* Rods monocromatism: no colour discrimination. Perceived intensity at scotopic levels (night vision). Cones are absent

Because colour blindness has a genetic origin dominant on the males and recessive on females, it is enough for one of the parents of a male child to have the defect to transmit it.

In the case of a female child, both parents must have the defect to transmit it. Table 2 shows the colour blindness statistics.

From these statistics we can deduce that about 8% of males have a serious colour blindness defect, while for women the percentage is reduced to about 0.4%.

To evaluate the discriminatory capability of colourists, many tests are available. One of the best known is Ishihara’s (Figure 11 and 12). This is a confusion test where, depending on the perceived number or figure, it is possible to deduce if an observer has a normal vision, or of what type of colour blindness is present.

Figure 11 shows one of Ishihara’s confusion charts and in figure 12, a simulation with a patient suffering from Deuteranope vision. Another easy to use method is the Munsell chromatic discrimination test.

The problem can be eliminated by evaluating samples with an appropriate observer in an appropriate environment, and avoiding the use of solar light due to the variability demonstrated previously. Figure 13 shows the samples evaluated with artificial light.

Here the tanner should apply the CIE suggested standard D65, and then compare the matched colour with respect to the reference sample under a second light source (for example A) keeping as a reference the grey scale for assessing the colour difference.

If the colour difference is not greater than the customer’s limits, it’s fairly certain that the matched colours will not be very different, if compared under new conditions or by other observers.

Unfortunately, human physiological limitations to trichromy mean that perception to single spectral components of the colours being matched is often lost. This factor may cause erroneous trials especially with greys, browns and beiges, where colour perception between the colours is shared by all three cone types.

By means of spectroscopy, combined with computer colour matching systems (figure 14), it is possible to choose the right dyestuff or pigment combinations to give the smallest colour differences. This is possible because instead of human trichromatic integration, the system evaluates the spectral conditions at each wavelength.

Spectrophotometry calculates the behaviour of two measured samples under different light sources. The values may be expressed in terms of colour difference.

In the case of CIELab, colour differences are defined as a distance between two co-ordinates in a three dimensional system, and the calculation is carried out by the application of Pythagoras’ theorem in 3D. Values obtained can be expressed as a metameric index quoting the reference light source.

To avoid interpretation differences and misunderstandings, it is fundamental to establish with customers and suppliers the conditions for colour evaluation. It would be ideal to apply the international standards.

This choice must be related to the environment in which the final articles will be exposed or used.

In the case of car upholstery, the colour combination of leather with textiles, plastics and metals, should not alter in daylight, at night, under artificial lights, or with the internal illumination light.

With regard to the finished articles, it is very important to consider the internal illumination of the showrooms and commercial centres. Marks & Spencer were one of the first companies to adopt a standard over thirty years ago. And remember that once established, the quality of an article will be defined by colour consistency and the uniformity of the colours, or colour change, over time.

Gustavo Defeo, Via Lombardia 43, (56029) Santa Croce sull’Arno, Pisa, Italia.