Study Of Spectrophotometer For On-line Color Measurement

Spectrophotometer is the most basic instrument in color measurement industry. Because of its high accuracy, it has become the most widely used instrument. On-line color measurement means that, the spectrophotometer installed on the production line can timely get the color information of products and continually monitor the quality of products on the production line. It is helpful to reduce the production waste and improve production efficiency. During the measurement, it also can greatly save time and labor cost without leaving production out of the production line or stopping production. According to the requirement of color measurement of paper on the production line, it is important to do researches on the key technology in on-line spectrophotometer and develop a prototype.According to the application requirements, a set of instrument specification is proposed. Based on the colorimetry theory and error theory, color measurement error and instrumental precision and accuracy are analyzed in-depth. The influences of spectral reflectance factor photometric error, wavelength error and photometric stochastic error on color measurement are researched. The relationship between color measurement precision and accuracy and instrument design precision and accuracy is concluded. After comparison and selection, the instrument design scheme is established——forty-five degree annular / normal geometry(45°a:0°), combination of pulsed xenon lamp and dual-beam spectrometers, complementation of internal calibration and external calibration, switching different spectral illumination for measurement of fluorescent specimens, and utilization a distance measuring sensor to solve the problem of object flutter.After selecting appropriate detector, the spectrometer optical system of Czerny-Turner configuration is designed,whose spectral range is 340~740 nm, spectral resolution is 2 nm. A filter is used to eliminate the second order diffraction spectrum, and a glare stop is used to solve the contradiction of unmatched numerical apertures between fiber optics and the spectrometer. The collection optical system of the sample detection branch is designed, which adopts a design method that the end face of fiber is placed on the image focal plane of the optical system. It is helpful to uniformly collect spectral information of the sample measured area, in case of uneven color of local area. The light energy collected varies insensitively with the object flutter. After selecting light source, 45 degree annular illuminating system is designed. By comparison of ellipsoid, toroidal and cylinder, cylinder is chose as the annular mirror of illuminating system. Further analysis shows that, at present the way only depending on the optical system to achieve that the light energy collected varies insensitively with the object flutter is not feasible.What’s more, the data processing algorithm of the dual-beam spectrometers is studied, which effectively corrects the color measurement error led by light source output energy fluctuation and ensures the stability of the measurement data of the instrument itself. Also, the data processing algorithm of distance compensation with the distance measuring sensor is studied, which effectively compensates the color measurement error led by the object flutter and ensures the stability of the measurement data of the instrument in on-line measurement. According to the design scheme, mechanical structure and electronic circuit is design, and software is programmed. In the course of development of the prototype, firstly, each module is assembled and tested; then, combination of modules is tested; finally, each module is installed and assembled in the instrument box. The actual performance of the two spectrometers of the prototype is tested. The spectral resolution is 2 nm, wavelength calibration precision is less than 0.2 nm, and signal to noise ratio is greater than 300(the relative error is less than 0.333%) when signal digital number is more than 10000. The actual performance of the prototype is tested. The repeatability color difference ?E*ab is 0.0644, inter instrument agreement color difference ?E*ab is less than 0.3, distance variation error color difference ?E*ab is less than 0.15. The performance evaluation results satisfy requirements.The successful development of prototype of spectrophotometer for on-line color measurement fills the domestic blank, and lays a technology foundation for development of the next engineering prototype and commercialization in the future.