Replace this text with a brief descriptive title of the experiment

							               Chromatic discrimination with chroma-enhancing light sources

                        Wendy Davis, Cameron Miller, and Yoshi Ohno
                        National Institute of Standards and Technology

HYPOTHESIS
It is hypothesized that light sources that enhance object chroma (relative to object chroma under
a reference illuminant) enable improved chromatic discrimination in the color regions of the
chroma enhancement.
This notion of increased discriminability from expanded gamut is certainly not new (e.g,
Thornton (1972) and Fotios (1997)). Given that this was one of the reasons for the inclusion of
the Saturation Factor in the CQS, it is important that it be tested. Experiments are planned to
separately test discrimination of hue differences and chroma difference.

METHODS
Subjects: Though the observers for this experiment have not yet been selected, it is expected that
5-10 subjects will be tested, all with normal color vision. Variety will be sought with regards to
age, gender, and race.
Test samples: Several sets of reflective samples (1.75 cm x 1.75 cm; 2° visual angle) will be
tested in this experiment. The sets to test hue discrimination will be along the hue circle and of
constant chroma and lightness. The sets to test chroma discrimination will differ in chroma, but
be constant in hue and lightness.
Two sets of 15 samples will be used to test hue discrimination. They are nicknamed “orangish
samples” and “purplish samples” based on the direction that each set deviates from unique red.
Preliminary experimentation has been conducted to determine the optimal magnitude of color
differences between samples within each set. It was established that the desired degree of task
difficulty was achieved when the samples were ΔE*a,b ≈ 1 apart. Within each set, all samples
will have the same lightness (L* ≈ 40) and chroma; they will differ only in hue. The target
CIELAB coordinates for both sets of samples are plotted in the “target samples” worksheet in the
chrom_discrim_Experiment plans.xls spreadsheet.
Two sample sets will be used to test chroma discrimination. Each set will be consistent in
lightness and hue; individual samples within a set will differ only in chroma. Preliminary
experimentation will be necessary to determine the proper saturation difference between samples,
so only the lines that the sample sets will fall upon are shown in the plot in the “target samples”
worksheet. Each test set will be kept to ~20 or less samples, so it’s possible that each sample set
will be subdivided for data collection purposes.
The desired chromaticities will be first specified with software/printer ICC Profiles color
management system. The samples will be printed with a 9-ink thermal inkjet printer on satin-
matte photo paper. Spectroradiometric measurements will be taken and the samples will be
adjusted, as necessary, to achieve the desired CIELAB coordinates.
Test sources: Four light sources will be tested in this experiment. Within the defined work area,
the illuminance will be 300 lux, +/- 5%. All four sources will have a CCT of 4800K (with +/-
50K with the defined work area) and a uv < 0.002.
The spectral data and plots of SPDs for the four sources are shown in the “source spectra”
worksheet of the chrom_discrim_Experiment plans.xls spreadsheet. The sources nicknamed
“enhancing chroma,” “decreasing chroma,” and “super-enhancing” were created to mimic the
spectra of RGB LEDs, and were chosen for this experiment because, in simulations, they were
shown to enhance object chroma, decrease object chroma, and extremely enhance object chroma,
respectively. These simulation results are shown in the CIELAB plots (for the 15 CQS reflective
samples) in the “source simulations” worksheet.
 The enhancing chroma source has peak wavelengths at 464, 533, and 620 nm. The peak
wavelengths for the decreasing chroma source are at 464, 533, and 600 nm. The super-
enhancing source has peak wavelengths at 460, 530, and 627 nm. The “broadband” source is
fairly self-descriptive; it was created to be spectrally similar to the reference illuminant used in
the CRI and CQS calculations.
The CRI and CQS scores for all four sources are listed in the “source simulations” worksheet.
Experimental procedure: The experiment will be conducted in a booth that is approximately 102
cm (40”) wide, 65 cm (25.5”) deep, and 58.5 cm (23”) high. The horizontal surface of the booth
is painted N7 Munsell gray color. The walls of the booth are covered in black. A defined work
area in the center of horizontal surface is defined by a black perimeter and is approximately 50
cm (19.75”) wide by 35 cm (13.5”) deep, in which sufficient color uniformity is assured. Prior
to each experimental session, subjects will adapt to the broadband light source in the booth for at
least 10 minutes.
To test hue discrimination, observers will be asked to perform a hue sequencing task. They will
separately arrange the samples from the orangish and purplish sample sets. At the beginning of
each experimental trial, one sample will be set out for the subject as the “starter.” It is then the
subject’s task to select, amongst the remaining samples, the sample that is most similar in hue to
the starter. The selected sample is then placed next to the starter. The subject will then select,
from the remaining samples, the sample that is most similar to the one previously selected. This
process will be continued until all the samples are arranged in a sequence according to their hue.
Prior to data collection, subjects will be given hue sequencing instructions and a several practice
trials. Experimental trials will pseudo-randomly alternate between the four sources and the two
reflective sample sets (orangish and purplish). Each observer will complete at least five trials for
each sample set for each light source (more if time permits).
Observers will also be asked to perform a chroma sequencing task. They will separately arrange
the samples from sample set A and sample set B. At the beginning of each experimental trial,
one sample, either that with the highest or lowest chroma, will be set out for the subject as the
“starter.” It is the subject’s task to select, amongst the remaining samples, the sample that is
most similar in chromatic saturation to the starter. The selected sample is then placed next to the
starter. The subject will then select, from the remaining samples, the sample that is most similar
to the one previously selected. This process will be continued until all the samples are arranged
in a sequence according to their chroma. Prior to data collection, subjects will be given chroma
sequencing instructions and a several practice trials. Experiment trials will pseudo-randomly
alternate between the four sources and the two reflective sample sets (Set A and Set B). Each
observer will complete at least five trials for each sample set for each light source (more if time
permits).

RESULTS
A mean “error score” will be computed for each subject for each test source and for each
reflective sample set. The error score will take both magnitude and frequency of errors into
account by calculating the length of the line, in CIELAB, that is created by the sequence that the
subject arranges the samples in. The shortest possible line would be a 100% correct sequence.
Any errors will increase line length. The line length of a perfect sequence is subtracted from the
subject’s actual line length. The difference in length between the two lines is the error score.
The results of this experiment will help in understanding how chroma-enhancing light sources
effect chromatic discrimination. These results, in conjunction will other experiments, will
indicate the appropriateness of the Saturation Factor in the CQS.

TIMELINE
Now-December, 2007: create final sample set, spectroradiometric measurements, finishing
touches on light sources
January-June, 2008: Collect data
July-September, 2008: Analyze data
October 2008-February, 2009: Publish experiment

OTHER

References:

Thorton, W.A. (1972). Color-discrimination index. Journal of the Optical Society of America. 62, 191-
194.

Fotios, S.A. (1997). The perception of light sources of different colour properties. Doctoral dissertation,
University of Manchester Institute of Science and Technology.