Light Measurement
The whole discipline of optical measurement techniques can be roughly subdivided into photometry and radiometry.
Whereas photometry focuses on determining optical quantities that are closely related to the sensitivity of the human eye, radiometry deals with the measurement of energy per time (= power, given in watts) emitted by light sources or impinging on a particular surface.
Thus, the units of all radiometric quantities are based on watts (W). According to CIE regulations, symbols for radiometric quantities are denoted with the subscript “e” for “energy”. Similarly, radiometric quantities given as a function of wavelength are labelled with the prefix “spectral” and the subscript “λ” (for example spectral radiant power Φλ).
To choose the proper light measurement instrument user should understand the fundamental differences between photometers and spectral light meters, exploring their unique functionalities, applications, and the contexts in which each is most effectively used.
Colorimetry
Colorimetry aims to quantify the physiological perception of color caused by a spectral color stimulus. This stimulus can be the spectral radiant power of a light source or the spectral irradiance on an object multiplied by its reflectance or transmittance. The CIE (Commission Internationale de l’Éclairage) has defined standard illuminants for consistent color characterization.
For more detailed description of analysis method, refer to our blog post
Spectral Resolution Measurement
Spectral resolution measurement devices like spectroradiometers are the optimal light measurement technique to perform color evaluations and analyses.
Key Concepts and Metrics
MacAdam Ellipses and Binning:
MacAdam ellipses illustrate that equal distances in the xy color space do not represent equally perceived color differences. Binning, often using ANSI bins in the u’v’ diagram or custom manufacturer bins, is used to sort LEDs based on color properties. Recent CIE recommendations suggest using n-step circles in u’v’ color space for better interpolation and representation of LED diversity. Precise spectrometers are required for these measurements.
RGB and XYZ Color Matching Functions:
Based on the tristimulus theory, these functions quantify the stimulation of red, green, and blue cones in the human eye. The CIE 1931 XYZ color matching functions are preferred for their positive values and the fact that y(λ) matches the CIE spectral luminous efficiency function V(λ).
Tristimulus Values Calculation:
The XYZ tristimulus values are calculated using integrals involving the spectral color stimulus function and the color matching functions.
(x, y) and (u’, v’) Chromaticity Diagrams:
These diagrams provide two-dimensional representations of color. The (u’, v’) diagram offers more uniform color spacing compared to the (x, y) diagram.
Correlated Color Temperature (CCT):
Characterizes the spectral distribution of a light source, indicating whether it’s „warm” (reddish, low CCT) or „cool” (bluish, high CCT).
Color Rendering Index (CRI):
A numerical measure of a light source’s ability to render colors accurately compared to a reference source.
Color Quality Scale (CQS):
An alternative to CRI that considers color preference in addition to rendition, potentially valuing increased saturation.
Planckian Locus:
Represents the emission of a black body radiator relative to color temperature.
Dominant Wavelength:
Determined by the intersection of a line from the white point through the color coordinates of the radiator with the spectral locus on the CIE 1931 color space diagram.
Purity:
Defines how close a radiator’s color coordinate is to the spectral locus.
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