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Monday, November 23, 2015

Difference Between Source & Illuminant

Source (or UV Level): A physical emitter of light.

Illuminant: Standardized, tabulated spectral distribution based on statistical

measurements or theoretical equations.

In any discussion of the measurement of the optical properties of paper, it is important to understand the difference between a light source (or UV Level) and an illuminant. The term light source refers to the actual light energy when striking the specimen during measurement or observation. For example, when paper is being viewed by a human being, the source of light striking the paper is generally either fluorescent light, incandescent light, daylight filtered through office windows or some combination of these.

On the other hand, the light striking the sample in a measuring instrument is generally provided by either a quartz tungsten halogen (QTH) lamp or xenon lamp, either of which may be filtered to alter their spectral energy distribution. These are physical sources of light and are therefore referred to as sources not illuminants. An illuminant is a light defined by a spectral power distribution, which may or may not be physically realizable. For example, illuminant D65 was arrived at by taking a large number of measurements of daylight throughout the world at various times of the year. The data was averaged to arrive at the D65 spectral distribution curve. There is no known lamp and filter combination that will produce an exact duplication of the D65 spectral curve, therefore, D65 is an illuminant (a table of spectral data describing the spectral distribution of light energy) but it is not a light source (a physical object that produces light). 

In most colorimeters, the source and the reflected light are filtered to Illuminant C. This is true of the Technidyne Brightimeter and Technibrite instruments where the light source is filtered quartz tungsten halogen (very similar in spectral distribution to Illuminant A), however, the standard observer to which all of the filter functions are adjusted is based on Illuminant C. Most spectrophotometers offer either source C or 65. This is done by using a xenon lamp which has lots of UV energy and then a cutoff filter is used during calibration to set the UV Level to C or D65, respectively.  The Technidyne Color Touch provides the user with a choice of three different sources, which differ by the amount of ultraviolet energy contained in each, D65 (high ultraviolet), C (moderate ultraviolet) and UV-EX (no ultraviolet). When this flexibility is available, and especially when dealing with the measurement of optically brightened papers, both the source (UV Level) and the illuminant must be individually specified.