Following the process described in
Calibration process, we initially characterized the display and the software.
Figure 7 shows the relationship between
RGB reflectances and luminance output for the three channels separately (red, blue, and green). In addition, the sum of the
RGB luminance (i.e., gray) is also shown in the rightmost panel; the dashed line represents the luminance additivity (sum of luminance independently measured for red, green, and blue channels), and the bold line shows the measured luminance when all
RGB channels were presented simultaneously. The red, green, and blue panels also show the results of fitted lines (linear regression), the coefficient of determination (
R2), and the root mean square error (RMSE). Please note that the fitted line is used solely to illustrate the luminance linearity in the presented figures and was not used to construct the LUT tables described in
Calibration process. As previously mentioned, a one-dimensional piecewise linear interpolation was used to build the LUT tables. The three primary channels are not increasing linearly, which would have been the expected behavior, as explained in
Calibration scene and measurements. The fitted lines for the red, green, and blue channels show poor linearity with relatively high RMSE. Additionally, with excellent additivity behavior, the dashed line would ideally perfectly align with the solid line in the rightmost panel. However, the luminance additivity is found to be very poor in this setup.
Figure 8 shows the spectra of the three channels individually for varying
RGB reflectances. The continuous lines represent the measured spectra for the different
RGB reflectances, while the dashed lines represent the spectra predictions obtained by multiplying the lowest measured spectrum by a linear scaling factor. The linear scaling factor was calculated by dividing the radiance of the test reflectances (0.5, 0.75, 1.0) by the radiance of the reference reflectance (0.25) at specific wavelengths (450 nm for blue channel, 550 nm for green channel, and 630 nm for red channel). Linear scalings of the lowest spectrum result in spectra that perfectly fit the spectra measured for higher
RGB reflectances for the red and blue channels. This behavior suggests very good channel constancy for those channels. However, the green channel seems much less constant. Two extra peaks of radiance that are not linearly increasing can be observed. In other words, the red and blue channels seem to interfere with the green channel. This suggests a lack of independent scaling for the green channel, which may interfere with calibration. Finally,
Figure 9 shows the chromaticity coordinates (
x,
y) of the three channels with normalized
RGB reflectances higher than 0.20 on the CIE 1931 chromaticity diagram. In the case of good chromaticity constancy, the dots of the same color should overlap. The chromaticity constancy is satisfactory for the red and blue channels but poor for the green channel, as expected from the spectrum in
Figure 8.