Experiment
The reference instrument used in this work was a
spectroradiometer comprised of a scanning type, double-grating
monochromator in subtractive mode, that was equipped with
imaging optics (with a viewfinder) which provided an acceptance
angle of approximately 3°. Its three
slits were adjusted to provide a triangular bandshape with
5 nm half-width. The scanning interval was matched to the
bandwidth, which is an important requirement for accurate
colorimetry.4 The
relative spectral responsivity of the spectroradiometer
was calibrated immediately before and after the measurements
against two standard lamps traceable to the NIST spectral
irradiance scale. The spectroradiometer measurements on the standard lamps
reproduced to ± 0.0002
in x and y. The luminance of each sample color was also
measured with a reference luminance meter of known spectral responsivity.
The reference luminance values (Y) were obtained from the
luminance meter values, which were corrected for the spectral mismatch
of the instrument using the relative spectral power distribution of each
sample color obtained by the spectroradiometer. The Y values obtained
in this manner were more stable and reproducible than the Y values obtained
from the spectroradiometer itself which employed a photomultiplier.
The target instrument was a four-channel tristimulus colorimeter. It incorporated four colored glass filters that essentially matched its detectors’ relative spectral responsivities to the CIE color-matching functions. (Two of the filter-detector combinations were used to construct the  function.) The input optics included a holographic optical element that distributed the incident light to the four filtered detectors. The colorimeter head was equipped with a suction cup, which both held the instrument to the CRT screen and shielded ambient light. The linearity of the current-to-voltage converters in the colorimeter were verified to be within 0.5 % across their full operating range.
A broadcast-quality color CRT was used in combination with a video signal generator. The CRT was selected for spatial uniformity and stability. After changing to a new color, the monitor was allowed to stabilize for » 2 min. The colorimeter was located so that it measured the same position on the CRT as the spectroradiometer.
Eight colored glasses were selected that fell within the CRT’s color gamut. They were backlit by a 200 W, frosted quartz-halogen lamp operating at 2856 K approximately 50 cm away. A plastic diffuser (165 mm square and 5 mm thick) was placed in front of the glass being measured. (A ninth case consisted of the diffuser alone, without a colored glass.) A kinematic base was used to reproducibly position the tristimulus colorimeter head very close to the diffuser for the measurement of the various glasses. A significant amount of interreflection between the diffuser and the colorimeter head was noted. This caused the luminance of the diffuser, as measured by the colorimeter, to be 10.6% higher, on average, than the corresponding measurements made by the spectroradiometer. For the purpose of combining colorimetric measurements of the colored glasses (and diffuser) with those of the CRT, we have reduced the glass measurements by this factor to account for this effect.
To eliminate the effects of ambient light, all measurements were taken in a darkened environment. Measurements took place with each instrument sequentially. The spectroradiometer scan required »10 min; the tristimulus colorimeter gave results in < 1 min. The luminance for each color was recorded before and after the two measurements. When the two luminance values varied by more than 0.2 %, the measurements were repeated. |
