PDF Perceptions of White Light Sources of Different Color ...
Perceptions of White Light Sources of Different Color Temperatures
Light sources used for illumination have a "white" appearance with chromaticities close to the blackbody locus. Those with high CCT are informally called "cool white" and those with low CCT are called "warm white." surprisingly little research has been conducted, however, into the appearance of the light generated by "white" light sources used for illumination. To understand what constitutes perceptions of white lighting, the LRC conducted a
Chromaticities of the experimental SPDs (filled circles) for each CCT used. Also shown for reference are the ANSI tolerance quadrangles for solidstate light source and the center of each zone (open circles).
0.50 0.48 0.46 0.44 0.42 0.40
6500 K 0.38 0.36 0.34 0.32
5000 K
2700 K 3000 K
4100 K 3500 K
y y
series of experiments to measure perceptions of white
0.30
illumination from light sources of different CCTs.
0.28 0.28 0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 x
Experiment
Twenty subjects viewed a matte-white tabletop cube with an interior that was indirectly illuminated to 300 lx by combinations of eight independently controlled light sources. The matte-white hemisphere in the ceiling compartment mixed the light to create six groups of seven spectral power distributions (SPDs). All seven
SPDs within a group had the same CCT: 2700 K, 3000 K, 3500 K, 4100 K, 5000 K, and
0.50
0.48 2700 K
3000 K 0.46
4100 K
0.44
3500 K
0.42
5000 K
0.40 6500 K
0.38
0.36
0.34
0.32
0.30
2% 1% 0% -1% -2%
Linear fluorescent CFLs All families
0.28 0.28 0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 x
Iso-contours of 0% (i.e., white), ?1% and ?2% constant source tint for hue-magnitude judgments immediately upon seeing the illuminated box. Positive values are used to indicate green tint whereas negative values indicate purple tint. Similar results contours were found for judgments after 45 seconds and huechoice judgments.
Results
6500 K. Within Generally, perceptions of untinted white illumination
each CCT group, for sources with high CCTs (above 4000 K) are
one SPD had a associated with chromaticities that fall above the
chromaticity on blackbody locus. In contrast, perceptions of untinted
A subject views the experimental setup.
the blackbody white illumination for sources with low CCTs (below
locus; the other 4000 K) are associated with chromaticities that lie well
six SPDs included the range of practical light sources below the blackbody locus.
sold as white illumination.
The data show that every CCT (between 2700 K
For each condition (i.e., one SPD from the selected and 6500 K) has one or more chromaticities that are
CCT group), subjects were asked to immediately judge perceived as untinted white, as well as chromaticities
whether the light reflected off the interior walls was showing different degrees of purple or green tint. The
purple or green (hue choice) and what percentage of terms "warm" and "cool" also can be applied to every
that hue was present (hue magnitude). Then subjects CCT, and that these terms are not homologous with
made the same judgments after 45 seconds.
CCT designation. Further, change in CCT along the
Sponsor
blackbody is not a dimension of whiteness. Thus, this study can serve as a basis for future deliberations on
target chromaticities for untinted white illumination, as
well as for describing the source tints associated with
currently manufactured lamps.
? 2011 Rensselaer Polytechnic Institute. All rights reserved.
View LRC Project Sheets at lrc.rpi.edu/resources/newsroom/projectsheets.asp
Lighting Research Center, Rensselaer Polytechnic Institute ? 21 Union Street ? Troy, NY 12180 ? (518) 687-7100 ? lrc.rpi.edu
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