Light sources, their standardization

The color of an object will also vary with the color of the light source. Other standard illuminants may radically change the color appearance of an object because the area under their curves usually varies considerably with wavelength (Fig. 11). 

The reaction now affords investigators another valuable tool because the absolute intensity of the chemiluminescence has been carefully measured. It can thus be used as a standard light source against which other chemiluminescent reactions can be measured without the need of detector calibrations and geometry corrections. This convenience is due largely to the fine work of Fontijn, Meyer, and Schiff,145,147 who used chemical actinometry to measure the emission from a discharge-flow system. In their first report on this reaction, they determined the value of k12 as 1.0 x 104 M x sec-1 for emission in the... 

Light sources can be described by their spectral power or energy distribution. This simply plots the amount of light (relative power) as a function of wavelength. Figures 2.1-2.5 are examples of standard light sources (Billmeyer and Saltzman, 1981). 

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. 

No better example of their confusion can be given than the statement, "The case for a single reference light source that could be used legally in the event of disputes was extensively debated but was extensively rejected, as there was concern that this reference would become the only light source used and accepted." If this were truly the case, there would be no need for any of the Pharmacopoeias and their reference standards, for the same recently given reason—which in reality is not the case. 

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. 

Wyszecki (13) proposed a "goodness-of-fit" criteria for the CIE in an effort to establish a better method for testing light sources with respect to their suitability to serve as a standard daylight source for colorimetry. While his method... 

During the first two decades of the 20th century, simple optical instruments were developed with which the analyst could visually compare the color intensity of a sample with a standard. These photometers with visual light source used colored glass filters to select a wavelength interval suitable for the substance to be analyzed, and their applicability was limited to colored compounds or colored derivatives. 

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