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AS73211

Application Notes

Datasheet

• PUBLIC

DS000556

• v3-01 • 2018-Feb-07

│ 62

13.5

Sensor Calibration

Since the main variables of color change upon the arrangement of the observer, the object and light -

it is essential to optimize and calibrate color measurement tasks to the specific application, especially

for absolute color measurements according to the CIE 1931/DIN 5033.The calibration has three

functions. It converts the measured values from light-to-digital converter into the color space

XYZ/xyY/Luv/Lab or others. Secondly, it compensates production-related tolerances of the individual

sensors. Thirdly, the accuracy of the XYZ sensor is extremely sensitive to the opto-mechanical design

and variations of the system in, which it resides. These influences need to be corrected as other (e.g.

external) effects like temperatures or others can influence other components in the sensor system and

therefore the overall sensor response. Depending on the application and system accuracy, a sensor

calibration will be possible by an individual system or by an in-series calibration. In the process of

calibration there are conditions, which are required to receive reliable results in the CIE 1931 color

space. Using a standardized illumination source such as A, F2 and D65 as reference, the angle of

incidence as well as the arrangement of sensor and illumination are important input variables of the

calibration and determine the quality of the XYZ transformation. ams Sensors Germany offers special

white papers and application notes to find an optimized application-specific solution (time, costs and

quality) for calibration. Please ask our sales team.

For calibration the (color) target, measured by a spectrometer (n * XYZ as absolute color values and

reference) and the color sensor (n * ADC measured) must be known. By a simple coefficient matrix

method the relationship between the measured sensors values and absolute color coordinates in

CIE 1931 color space can be made: T (1) is the matrix of the reference measurement (XYZ values of

spectrometer), S (EQ1) is the sensor signal matrix (ADC values of Sensor) and K is the transformation

matrix (EQ2). After the transposition of S, a transformation matrix K (linear regression) is calculated

(EQ2). The result (EQ3) is the correction matrix K, which is used to transform measured sensor values

(ADC result of a color target) into the color space XYZ based in CIE 1931 (EQ4).

Information

Matrixes are not set up as a square matrix and depend on the number of targets n.

Equation 7:















...

;















adcZ

adcY

adcX

adcZ

adcY

adcX

adcZ

adcY

adcX

...

Equation 8:



 

 1





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