The color of an object can be described by brightness, saturation and hue, which are usually called Chroma. In order to describe various colors, it is necessary to test their luminance and Chroma coordinates.
Brightness is the brightness of an object, usually expressed in cd/m2 or nit. The chromaticity can be calibrated by two coordinates x and y of CIE1931-XYZ colorimetric system or by other colorimetric systems. Chromaticity coordinates can indicate the degree of shade (saturation) and type (hue) of a color.
Optical measuring instruments can generally be divided into spectrophotometer and colorimeter
according to their working principle. Light-type radiance meter generally adopts the method of spectrophotometric measurement. The spectrophotometric instrument is composed of sensor modules composed of sensor arrays to complete photoelectric conversion. Each unit of the sensor corresponds to a specific wavelength. The sensor measures the spectral power distribution or brightness characteristics of the luminous body at different wavelengths. Then the tristimulus value XYZ is calculated from the spectral data, which can measure the spectral energy distribution, radiance, brightness and Chroma of the light source.
The CIE standard spectral efficiency function and the CIE color matching function curve are stored in the instrument in the form of data. It can be used to process the spectral energy distribution data of the light source to be measured, so as to avoid the measurement error caused by the sensitivity matching of the sensor. Therefore, it is characterized by high accuracy, but slow measurement, high price and large volume. It can meet the needs of high precision data, and can calibrate the colorimeter according to the characteristics of the light source under test.
Colorimeter of chromatic type
Generally, the tristimulus measurement method is an instrument which uses three filters whose spectral sensitivity coincides with CIE tristimulus color matching function curve to measure the color of light source. This kind of instrument sensor is usually photodiode, which converts the incident light into electrical signal and directly generates the CIE standard XYZ tristimulus value. The accuracy of such sensors is limited, and the sensitivity curve of CEI has some errors. It is necessary to calibrate the Chroma meter with a standard light source similar to the spectral characteristics of the light source under test. In plane display correction, the spectrometer is usually used to calibrate the colorimeter according to the measured results of the light source under test.
The tristimulus measurement method has the advantages of fast speed, low price and poor accuracy. In order to reduce the measurement error, it is necessary to calibrate the data according to the spectral characteristics of the monitored display screen.