The color of the color printed image is formed by overlapping the yellow, magenta, and blue primary inks in different proportions. Instead of measuring the color on the screen when measuring the color of a printed image, the quality control strip printed at the same time as the printed image is measured. The control strip is generally placed at the tip of the printed sheet. With the measuring instrument, the corresponding color patches of the control strips can be used to obtain the printing quality information, such as the solid density, overprinting rate, dot gain, dot density, neutral ash reduction, contrast and other parameters of each primary color ink to judge the image tone and tone reproduction.

There are three measurement methods for measuring the color of printed images, namely densitometer measurements, colorimeter measurements, and spectrophotometer measurements.

1. Densitometer measurement

Densitometer is the main instrument in color separation, plate making and printing. This measurement method has always been the most commonly used form of objective evaluation of quality in the printing industry. Densitometers are inexpensive and widely used. When the densitometer measures the color surface, only the relative amount of a certain color ink in the printing can be obtained. It cannot indicate the hue of the measured color. Densitometer measurements are not associated with a variety of color systems and therefore cannot be described in color language. The density meter has certain limitations in color measurement and evaluation. It is not a standard color measurement instrument.

2. Colorimeter Color Measurement

The colorimeter is to obtain the visual response proportional to the color tristimulus values ​​X, Y and Z by directly measuring the surface of the measured color. After conversion, the X, Y and Z values ​​of the measured color can be obtained, and these values ​​can also be obtained. Convert color parameters into other color spaces. The colorimeter is a special density meter with three broadband filters. Due to the fact that the instrument itself and the principle are thick in certain errors, the absolute accuracy of the color measurement value is not good. However, due to its low price, it is still a widely used color measuring instrument.

3. Spectrophotometer (Spectrophotometer) Color Measurement

Spectrophotometers measure the reflectance of light at each wavelength of the visible spectrum of the color surface. The light of the visible spectrum is irradiated with the color surface at a certain step distance (5 nm, 10 nm, 20 nm), and the reflectance is measured point by point. The spectrophotometric curve of the surface of the measured color can be obtained by plotting the reflectance value of each wavelength of light with each wavelength. Each spectrophotometric curve uniquely expresses a color. The measured value can also be converted to other color system values. Spectrophotometer is a flexible, ideal color measurement instrument. At present, color measuring instruments for color quality inspection of some printing machines equipped abroad are using spectrophotometers.