![]() The XYZ->CIELAB conversion (taken from CIE 15.2 1986, “Colorimetry”) bounds L* to the range of 0-100 as follows: That is, at no point can the reflection spectrum exceed the intensity (which shows up as the L* value in CIELAB) of the reference illuminant. Thus, the resulting X, Y, and Z values (which are the starting point for other color spaces such as CIELAB) are all relative to this normalized reference illuminant. The reflection or transmission spectrum (0-100% at each wavelength) is multiplied by this illuminant before the individual color values (X, Y, Z) are computed. When doing a relative color measurement, the result is usually referenced to these illuminants: These illuminants are normalized to a value of 100 at 560 nm. I P = ( S P - D P) * C P / ( T * A * dL P)Ĭ p = Calibration file, in μJ/count (specific to the sampling optic)Ī = Collection area, in cm 2 (A=1 for an integrating sphere)ĭL P = Wavelength spread (how many nanometers each pixel represents)Ī: Typically, CIELAB is computed with some reference illuminant - e.g., A, D65, or D75. The calibration process generates a file with energy response data for each pixel in the CCD, given in μJ/count. Factoring in the surface area of the sampling optic and the integration time allows irradiance measurements in μW/cm 2 to be reported (power=energy/time). Calibration is only possible if the absolute power output of the calibration light source is known, so if the light source calibration data is not given in the units μW/cm 2/nm, it may not be a light source capable of calibrating for absolute irradiance measurements.Ĭalculating absolute irradiance takes into account the collection area of the sampling optic, and is corrected using the calibration data for each pixel, C P. ![]() ![]() Measurements in absolute irradiance mode require a calibration using a source with known power output. A spectrum is measured with the sampling optic (fiber tip, CC-3 cosine corrector, etc.) connected to the calibration light source, and is then compared to the known output power of the calibration light source. Remember to always use the light source calibration file specific to the sampling optic being used, and calibrate just prior to measurement if possible. Irradiance is the measurement of radiant flux per unit area hitting or passing through a surface. An absolute irradiance measurement results in a spectrum that is accurate in both shape and magnitude. The y-axis scale becomes scaled in power or flux units like μW/nm or μW/cm 2/nm, making it easy to calculate other power or energy values.
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