Test probe P100-M3
Photocoupler

1. Instrument Calibration Principle of Photoelectric Integral Colorimeter

As mentioned earlier, due to the CIE spectral tristimulus value function x(λ) having two peak wavelengths, x(λ) can be divided into long-wave regions x₁(λ) and short-wave regions xb(λ). The xb(λ) curve is similar in shape to z(A), and there is a proportional relationship between them: xb/Z = K. In this case, the spectral matching of the photoelectric integral detector can be done in two different ways: one is self-matching, where the spectral response of one detector is matched to x(λ), and the other is to match the spectral responses of three detectors to the true x(λ), y(λ), z(λ) curves, using a scale factor K to obtain the total stimulus value X = X₁ + KbZ = X₁ + KZ.

Different photodetector spectral matching methods lead to different calibration approaches for the corresponding colorimeters. The following two distinct chroma correction processes are discussed below:

(1) The spectral response of the three photodetectors of the colorimeter are respectively matched to x(λ), y(λ), z(λ) curves.

As shown in Figure 6-30, the detector of the color measurement instrument to be calibrated is aligned on the photometric guide. The X, Y, Z tristimulus values of the standard color source (standard swatch) are used to adjust the three current-voltage converters acting as photoelectric signal processing circuits (operational amplifiers). The size of the feedback resistor or the gain of the amplifier is adjusted, such as R₁, Rv, R₁₂ in Figure 6-31, so that the displayed reading matches the tristimulus values of the standard color source, X, Y, Z. At this point, the instrument reaches its calibration state.


Some photoelectric integral colorimeter probes are designed with a flat receiving surface. In such cases, the Y channel can be calibrated by illumination. As shown in Figure 6-32, the Y channel acts as an illuminometer detector, and the instrument is calibrated and adjusted on the photometric guide using a luminous intensity standard lamp. The Y channel correction potentiometer adjusts the amplifier’s gain.



2. Application Examples

The calibration process of a photoelectric integral color measuring instrument is illustrated through the example of a color luminance meter.

(1) Calibration of the Color Luminance Meter

The color luminance meter uses brightness as the calibration unit. Figure 6-33 shows the schematic diagram of the calibration device using the CIE Standard Light Source A (with luminous intensity I). The distance between the instrument and the magnesium oxide (barium sulfate) whiteboard is r, and the illuminance of the surface of the magnesium oxide whiteboard is E = I / r². Additionally, magnesium oxide whiteboards are known as standard light sources. When illuminated by the CIE Standard Light Source A, the reflectance is p, which allows us to calculate the tristimulus values of the magnesium oxide whiteboard under illumination: X₁, Y, Z.


When the illuminance of the magnesium oxide whiteboard surface is E, the brightness of the surface is L = PE / π. During calibration, the first step is to adjust the Y channel of the color luminance meter so that the displayed reading equals L₀. Usually, the distance between the standard lamp and the standard whiteboard is adjusted to ensure that L = 100 cd/m², making the calibration easier. Then, adjust the corresponding X₁ and Z channels to make the tristimulus values X₁, Y, Z match the calculated tristimulus values of the magnesium oxide whiteboard under the CIE Standard Light Source A.

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July 28, 2025