Colour measurement method and colour measurement device
A colour measurement device includes a measurement array (MA) which includes: a plurality of illumination arrays ( 20, 30, 40 ) for exposing a measurement spot (MS) on a measurement object (MO) to illumination light in an actual illumination direction ( 2, 3, 4 ) in each case, and a pick-up array ( 50 ) for detecting the measurement light reflected by the measurement spot (MS) in an actual observation direction ( 5 ) and for converting it into preferably spectral reflection factors; and a controller for the illumination arrays and the pick-up array and for processing the electrical signals produced by the pick-up array. The controller is embodied to process the measured reflection factors on the basis of a correction model, such that distortions in the measurement values as compared to nominal illumination and/or observation directions, caused by angular errors in the illumination arrays and/or the pick-up array, are corrected.
1. A method of correcting reflectance measurements of a color measurement device having a measurement array defining a plurality of measurement channels i, each measurement channel i comprising a paired combination of illumination and observation directions and each illumination and observation direction having a nominal direction based on a measurement geometry of the color measurement device, each illumination and observation direction further having an actual direction as measured from the device as manufactured, comprising:
calculating nominal flake normal angles φ ni from the nominal illumination and observation directions of the plurality of measurement channels i;
determining actual illumination and observation directions of a plurality of measurement channels i of the color measurement device;
calculating actual flake normal angles φ ai from the actual illumination and observation directions of the measurement channels i;
measuring actual reflectance spectra R i (λ) for the plurality of measurement channels i at a plurality of wavelengths λ;
calculating average actual reflection factors R(φ ai ) for the plurality of measurement channels i;
determining a correction function C(φ) on the basis of the actual flake normal angles φ ai and the actual reflection factors R(φ ai );
calculating the nominal reflection factors R(φ ni ) on the basis of the correction function C(φ) and the nominal flake normal angles φ ni ;
correcting the reflectance spectra R i (λ) on the basis of the actual reflection factors R(φ ai ) and the nominal reflection factors R(φ ni ).
2. The color measurement method according to claim 1 , wherein the method further comprises storing the nominal flake normal angles φ ni and actual flake normal angle φ ai on the color measurement device.
3. The color measurement method according to claim 1 , wherein the correction model C(φ) and the nominal brightness reflection factors R(φ ni ) are calculated in the space of the flake normal angles.
4. The color measurement method according to claim 3 , wherein the nominal brightness reflection factors R(φ ni ) are calculated from the respective residual between the actual brightness reflection factors R(φ ai ) and the correction model C(φ).
5. The color measurement method according to claim 1 , wherein the correction model C(φ) is uniform and is used for all the measurement geometries.
6. A color measurement device with flake normal dependent correction factors, the color measurement device comprising:
a plurality of illumination devices;
at least one pick-up device; the plurality of illumination devices and the at least one pick-up device defining a plurality of measurement channels i, each measurement channel comprising a paired combination of illumination and observation directions and each illumination and observation direction having a nominal direction based on a measurement geometry of the color measurement device, each illumination and observation direction further having an actual direction as measured from the device as manufactured; and
a controller comprising a microcontroller and a non-volatile memory; and
the non-volatile memory including:
stored nominal flake normal angles φ ni based on nominal illumination and observation directions of each of the measurement channels i;
stored actual flake normal angles φ ai based on the actual illumination and observation directions of the measurement channels i; and
stored computer executable instructions which, when executed by the microcontroller, cause the color measurement device to:
measure actual reflectance spectra R i (λ) for the measurement channels i at a plurality of wavelengths λ;
calculate average actual reflection factors R(φ ai ) for the plurality of measurement channels i;
determine a correction function C(φ) on the basis of the actual flake normal angles φ ai and the actual reflection factors R(φ ai );
calculate the nominal reflection factors R(φ ni ) on the basis of the correction function C(φ) and the nominal flake normal angles φ ni ;
correct the reflectance spectra R i (λ) on the basis of the actual reflection factors R(φ ai ) and the nominal reflection factors R(φ ni ).
7. The color measurement device according to claim 6 , wherein the instructions further cause the color measurement device to calculate the correction model C(φ) and the nominal brightness reflection factors R((φ ni ) in the space of the flake normal angles.
8. The color measurement device according to claim 7 , wherein the instructions further cause the color measurement device to calculate the nominal brightness reflection factors R(φ ni ) from the respective residual between the actual brightness reflection factors R(φ ai ) and the correction model C(φ).
9. The color measurement device according to claim 6 , wherein the instructions further cause the color measurement device to calculate the measured reflection factors R i (λ) on the basis of the difference between the actual brightness reflection factors R(φ ai ) and the nominal brightness reflection factors R(λ ni ).
10. The color measurement device according to claim 6 , wherein the correction model C(φ) is uniform and is used for all the measurement geometries.
11. The color measurement device according to claim 6 , wherein the pick-up device is a spectrometer.
12. A color measurement device with flake normal dependent correction factors, the color measurement device comprising:
a plurality of illumination devices;
at least one pick-up device; the plurality of illumination devices and the at least one pick-up device defining a plurality of measurement channels i, each measurement channel comprising a paired combination of illumination and observation directions and each illumination and observation direction having a nominal direction based on a measurement geometry of the color measurement device, each illumination and observation direction further having an actual direction measured from the device as manufactured; and
a controller comprising a microcontroller and a non-volatile memory; and
the non-volatile memory including:
stored nominal illumination and observation directions of each of the measurement channels i;
stored actual illumination and observation directions of the measurement channels i; and
stored computer executable instructions which, when executed by the microcontroller, cause the color measurement device to:
calculate nominal flake normal angles φ ni from the nominal illumination and observation directions of the plurality of measurement channels i;
calculate actual flake normal angles φ ai from the actual illumination and observation directions of the measurement channels i;
measure actual reflectance spectra R i (λ) for the measurement channels i at a plurality of wavelengths λ;
calculate average actual reflection factors R(φ ai ) for the plurality of measurement channels i;
determine a correction function C(φ) on the basis of the actual flake normal angles φ ai and the actual reflection factors R(φ ai );
calculate the nominal reflection factors R(φ ni ) on the basis of the correction function C(φ) and the nominal flake normal angles φ ni ;
correct the reflectance spectra R i (λ) on the basis of the actual reflection factors R(φ ai ) and the nominal reflection factors R(φ ni ).
13. The color measurement device according to claim 12 , wherein the actual illumination and observation angles are measured by a manufacturer of the color measurement device and the corresponding data are stored in the color measurement device by the manufacturer.
14. The color measurement device according to claim 12 , wherein the actual illumination and observation angles are measured by a user of the color measurement device and the corresponding data are stored in the color measurement device by the user.