Color matching light source

Clearly, standardized light sources are desirable for color matching, particularly in view of the phenomenon of illuminant metamerism described below. Over the years CIE has defined several standard illuminants, some of which can be closely approximated by practical sources. In 1931 there was Source A, defined as a tungsten filament incandescent lamp at a color temperature of 2854 K. Sources B and C used filtering of A to simulate noon sunlight and north sky daylight, respectively. Subsequently a series of D illuminants was estabUshed to better represent natural daylight. Of these the most important is Illuminant E). ... 

With the light curing mechanism, there is a limitation to the penetration of the light. The dentist may have to place a restoration that is 6 + mm thick, whereas the light may penetrate only 2 mm [182]. Factors that affect this penetration are the translucence of the material, the color or shade used to match the tooth, the ability to place the light source close to the material being polymerized, and the intensity of the source. Under relatively ideal conditions, the mean depth of cure is approximately 4-5 mm. Thus, the dental application requires that the material be placed in layers. Due to the oxygen inhibition of the outside surface of the resin layers, additional layers can be laminated and cured with the appearance of uniformity of the final restoration. 

Tristimulus values X, Y, and Z are computed for a primary light source with power spectrum L(X) from the color-matching functions x, y, and z as follows ... 

How does the color match trial appear relative to the color standard under different light sources Is there metamerism ... 

The Broadest Choice of Standard Light Sources in the World s Most Advanced Color Matching Booth, New Windsor, NY, Macbeth, Division of Kollmorgen Instruments Corp. (6/93), Spectra light II. 

Metamerism The condition that exists when two colored articles appear to match under one light source but not another. The three common light sources used in color matching are simulated daylight, incandescent, and cool white flourescent. 

On occasion, a colorist may be asked to match to a set of reflectance numbers, or L, A, B coordinates, generated by spectrophotometric measurement of a color standard from a remote location. In other words, he or she will be asked to match a color that they have not seen. While instrumentation technology has improved over recent years, there still may be enough variance between individual spectrophotometer to cause a color rejection. Instruments are extremely helpful quality control tools to measure color and numerically define color differences. However, their use as a device to reduce a color target to a set of numbers and then send that set of numbers to a color formulator as a target for matching is seldom successful. Physical color standards that can be viewed under various light sources are preferred. 

Let us consider color match quality. The color formulator should ask the customer if the closeness of a match to the color target is the highest priority. Since there are hundreds of colorants available for the color formulator to select from, more than one possible combination of colorants may produce an acceptable match. Is the best, nonmetameric match of most importance to the customer and application If a perfect, nonmetameric match is not possible, which is often the case when trying to match plastic colorant systems to color targets in paint, ink, ceramic systems, or plastic media, will the customer accept some metamerism Also what light source will be the customer s preference when judging a slightly metameric match ... 

The ability of a light source to correctly represent colors of objects is its color rendering index (CRI), and with the CCT it suffices to help colorists select an equivalent source. Established by the CIE this standard rates lamps as to their color matching ability. A CRI value of 100 indicates that a source truly represents all colors of the CIE standard. Sources with CRIs of 70 to 80 are considered good and those with values of 80+ are rated as excellent. 

Fluorescent lamps are rated in catalogs by two indices, their correlated color temperature (CCT) and color-rendering index (CRI). The CCT is the temperature of a black body whose chromaticity most nearly matches that of the subject light source. Because a fluorescent lamp only approximates a "black body," it is called correlated to distinguish it from actual. The CRI on the other hand is a subjective method developed by the CIE in which eight test colors are viewed under the test and a reference lamp(s) and its ability to reproduce the test colors numerically rated. Both numbers are not absolute but useful in selecting lamps for color rendering applications. 

The color of a light source is typacally characterized in terms of its color temperature. If the x,y coordinates of an illumination source do not exactly sit on the blackbody locus, the color of a light source is characterized in terms of its CCT. The CCT is the temperature of a blackbody radiator that has a colour that most closely matches the emission from a non-blackbody radiator. For high quality white light illumination the CCT should between 2500K and 6500 K. There is an accepted method (Wyszelki et al 1982) to determine lines of constant correlated color temperature in x, y space. CIE, CCT and CRI for common white light sources are given in Table 1 for comparison purpose (Misra et al 2006). 

You might question why we need a light source to measure an emitter. The light source acts as a standard for comparison, using the reflectcmce from a standardized reflectance material so as to be able to use the STANDARD OBSERVER response we have already developed for color matching. 

Formulating to minimize shrinkage differences has its drawbacks as well. The first is metamerism. Metamerism occurs when two color samples (a standard and batch) match under one light source but no longer match when the light source is changed. This is important in automotive interiors, for example, where the OEM wants all of the materials in the vehicle to... 

The conditions necessary for the use of color chips are relevant to all visual color-matching and assessment procedures. Such tasks must be done by individuals with normal color vision, using standard viewing and lighting conditions. The illumination should be white and diffuse, and there should be no interference from extraneous light sources, or reflections from the ceiling or other objects in the room. 

Conditional match n. A pair of colors, which appear to match only under limited conditions, such as under a particular light source and a particular observer a meta-meric match. 

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