Process for the colour measurement of printed samples including brighteners
View Patent ↗For the color measurement of samples printed on a substrate including a brightener, a raw spectral reflection factor of the sample is measured in a first measurement by illumination of the sample with light without UV portion. In a second measurement, a fluorescence spectrum of the sample is measured by illumination of the sample with only UV light. The measured fluorescence spectrum is recalculated as a corrected fluorescence spectrum by weighting with spectrally dependent correction factors and, finally, the measured raw spectral reflection factor and the corrected fluorescence spectrum are added to form a corrected spectral reflection factor from which the values characterizing the color of the sample are then calculated. The spectral correction factors are determined during the device manufacture for a certain set of light types and stored in the device.
1. Process for the colour measurement of samples printed on substrates with brighteners, whereby a spectral reflection factor of a sample is determined and made available for the calculation of values characterizing the colour, characterized in that the spectral reflection factor of the sample is determined by way of a double measurement, whereby in a first measurement a raw spectral reflection factor of the sample is measured by illumination of the sample with light without UV portion, a fluorescence spectrum of the sample is measured in a second measurement by illumination of the sample with only UV light, the measured fluorescence spectrum is recalculated as a corrected fluorescence spectrum by way of a correction model in which the fluorescence spectrum is spectrally weighted with a set of spectrally dependent correction factors, and whereby the measured raw spectral reflection factor and the corrected fluorescence spectrum are added to form a corrected spectral reflection factor which corrected spectral reflection factor is used for the calculation of the values characterizing the colour of the sample.
2. Process according to claim 1 , characterized in that individual sets of correction factors are used for the correction model for different light types and that respectively one corrected spectral correction factor is calculated for one or more select light types by using the individual set of correct parameters associated with the respective light type.
3. Process according to claim 1 , characterized in that the correction factors are determined from the corrected spectral reflection factor and a reference spectrum of a calibration sample as the result of an equalization calculation in which the deviations between the corrected spectral reflection factor and the reference spectrum are minimized.
4. Process according to claim 1 , characterized in that a continuous light source emitting also in the UV range and with exchangeable filters is used for the measurement of the raw spectral reflection factor and the fluorescence spectrum, whereby one of the filters blocks UV light and another filter blocks non-UV light.
5. Process according to claim 1 , characterized in that several selectively activatable light sources with spectrally different emission spectra are used for the measurement of the raw spectral reflection factor and the fluorescence spectrum, whereby at least one light source emits in the UV range.
6. Process according to claim 5 , characterized in that UV light emitting diodes and white light emitting diodes are used as light sources.
7. Process according to claim 5 , characterized in that the light source emitting in the UV range impinges the sample at an especially small angle of incidence deviating from the standard geometry.
8. Process according to claim 6 , characterized in that the light source emitting in the UV range impinges the sample at an especially smaller angle of incidence deviating from the standard geometry.
9. Process according to claim 1 , characterized in that the signal level of the UV illumination light is monitored by way of a UV reference measurement channel and taken into consideration in the measurement.
10. Process according to claim 1 , characterized in that the signal level of the UV illumination light is monitored by way of a fluorescence standard and taken into consideration in the measurement.
11. Process according to claim 1 , characterized in that one or more quality parameters are calculated for the colour reproduction of the sample from the corrected spectral reflection factor for one or more light types.
12. Process for the colour measurement of samples (M) printed on substrates with brighteners, whereby a spectral reflection factor of a sample (M) is determined and made available for the calculation of values characterizing the colour, characterized in that the spectral reflection factor ( 53 ) of the sample (M) is determined by way of a double measurement, whereby in a first measurement ( 45 ) a raw spectral reflection factor ( 48 ) of the sample (M) is measured by illumination of the sample (M) with light without UV portion, a fluorescence spectrum ( 43 ) of the sample (M) is measured in a second measurement ( 41 ) by illumination of the sample (M) with only UV light, the measured fluorescence spectrum ( 43 ) is recalculated as a corrected fluorescence spectrum by way of a correction model ( 51 ) in which the fluorescence spectrum ( 43 ) is spectrally weighted with a set of spectrally dependent correction factors ( 50 ), and whereby the measured raw spectral reflection factor ( 48 ) and the corrected fluorescence spectra are added to form a corrected spectral reflection factor ( 53 ), whereby the corrected spectral reflection factor ( 53 ) is used for the calculation of the values characterizing the colour of the sample (M), whereby individual sets of correction factors ( 50 ) for the correction model ( 51 ) are used for different light types ( 49 ) and that respectively one corrected spectral reflection factor ( 53 ) is calculated for one more selected light types using the individual sets of correction parameters ( 50 ) associated with the respective light type, and whereby the correction factors ( 50 ) are determined from the correct spectral reflection factor ( 53 ) and a reference spectrum ( 33 ) of a calibration sample as a result of an equalization calculation ( 34 ) wherein the deviations between the corrected spectral reflection factor ( 53 ) and the reference spectrum ( 33 ) are minimized.
13. Process according to claim 12 , characterized in that a continuous light source ( 1 ) with exchangeable filters ( 12 ) which also emits in the UV range is used for the measurement of the raw spectral reflection factor ( 48 ) and the fluorescence spectrum, whereby one of the filters ( 12 a ) blocks UV light and another filter ( 12 b ) blocks non-UV light.
14. Process according to claim 12 , characterized in that several selectively activatable light sources ( 1 , 5 ) with spectrally different emission spectra are used for the measurement of the raw respective reflection factor ( 48 ) and the fluorescence spectrum, whereby at least one light source ( 5 ) emits in the UV range.
15. Process according to claim 14 , characterized in that UV light emitting diodes and white light emitting diodes are used as light sources ( 1 , 5 ).
16. Process according to claim 14 , characterized in that the light source ( 5 ) emitting in the UV range impinges the sample (M) under an especially smaller angle of incidence deviating from the 45°/0° standard geometry.
17. Process according to claim 15 , characterized in that the light source ( 5 ) emitting in the UV range impinges the sample (M) under an especially smaller angle of incidence deviating from the 45°/0° standard geometry.
18. Process according to claim 12 , characterized in that the signal level of the UV illumination light is monitored by way of a UV reference channel ( 4 , 9 ) and taken into consideration for the measurement.
19. Process according to claim 12 , characterized in that the signal level of the UV illumination light is monitored by way of a fluorescence standard and taken into consideration for the measurement.
20. Process according to claim 12 , characterized in that one or more quality parameters for the colour reproduction of the sample (M) are calculated from the corrected spectral reflection factor ( 54 ) for one or more light types.
21. Process according to claim 12 , characterized in that the reflection of a white reference standard (W) is measured with a spectrophotometer of a receiver channel.
22. Process according to claim 21 , characterized in that the white reference standard (W) is a white ceramic tile or a white pill.