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As we all know, that the same Colour patch, appears different under different lighting /viewing conditions. Different illuminants such as incandescent light and daylight have their own special characteristics and the color’s appearance is greatly affected by these characteristics.

Different sources of lighting may make same Colour appear different under different lighting conditions.

However measurement of spectral data eliminates this phenomenon because spectral data are device and illuminant independent.

Spectral data measures the reflected light from an object before a viewer or a device interprets it. Different light source appear differently when they are reflected from an object because normally any object may contain a combination of different colors or wavelengths of a visible spectrum.

However an object always absorbs and reflects same percentage of each wavelength regardless of amount and SPECTRAL DATA is a measurement of this percentage.

It is a very common occurrence and I am sure many of us have experienced this phenomenon first hand when a garment viewed under fluorescent lighting in a department store may appear different in hue when viewed under bright daylight or under home’s incandescent lighting.

This phenomenon is called METAMERISM when two or more colors that is actually not same (Having different wavelengths) but appears same under certain lighting conditions. Colors that match under some light but under other lighting conditions are called METAMERS.

A spectral power distribution describes the proportion of total light given off (emitted, transmitted, or reflected) by a color sample at each visible wavelength; it defines the complete information about the light coming from the sample. However, the human eye contains only three-color receptors (three types of cone cells), which means that all colors are reduced to three sensory quantities, called the tristimulus values.

Metamerism occurs because each type of cone responds to the cumulative energy from a broad range of wavelengths, so that different combinations of light across all wavelengths can produce an equivalent receptor response and the same tristimulus values or color sensation. In color science, the set of sensory spectral sensitivity curves is numerically represented by color matching functions.

Metameric matches are quite common, especially in near neutral (grayed or whitish colors) or dark colors. As colors become brighter or more saturated, the range of possible metameric matches (different combinations of light wavelengths) becomes smaller, especially in colors from surface reflectance spectra.

Metameric matches made between two light sources provide the trichromatic basis of colorimetry. For any given light stimulus, regardless of the form of its spectral emittance curve, there always exists a unique mixture of three "primary" lights that when added together, or added to the stimulus, will be an exact metameric match.

The basis for nearly all commercially available color image reproduction processes such as photography, television, printing, and digital imaging, is the ability to make metameric color matches.

Making metameric matches using reflective materials is more complex. The appearance of surface colors is defined by the product of the spectral reflectance curve of the material and the spectral emittance curve of the light source shining on it. As a result, the color of surfaces depends on the light source used to illuminate them.