Download Teufel colors

Teufel colors
We have developed 16 colors that are isoluminant, equally
detectable and perceptually equidistant. They cover the hues
available on a standard color monitor and provide a convenient
representation for illustrating a difference between receptoral and
postreceptoral mechanisms.
The position of these colors in the space spanned by the
excitations of S-, M- and L-cones (cone space) has been
determined using psychophysical methods such as flicker fusion
photometry and detection experiments. In conventional cone space
(MacLeod & Boynton 1979) these colors form an ellipse situated
on a plane of equal luminance. Such ellipses were described first
by MacAdam (1942) plotting his results in CIE space. Luminance
is often defined as the sum of L- and M-cone signals.
Using flicker fusion photometry we first determined the plane of
equal brightness for 5 color normal observers. Due to a small
contribution of S-cones to the perceived brightness the plane of
equal brightness is tilted with respect to the sum of L- and M-cone
signals (see Figure a). Generally, scaling of the three base vectors
of cone space (the cone fundamentals) is set arbitrarily to 1 (Kaiser
& Boynton 1996). Rescaling cone fundamentals with
s = .06
m = .37
l = .63
yields the simplest representation of colors using the polar
coordinates r (radius) and  (azimuth angle) in cone space or cone
difference space. Equally detectable colors lie on a circle with
radius r being equal to the detection threshold in a plane of equal
luminance in cone difference space (see figure a). Increase of
azimuth angle  corresponds to going counterclockwise around the
circle. The 16 colors shown at the end of this page are generated by
advancing in steps of equal increments of  around a circle with a
radius of five times the detection threshold (as measured in five
observers).
Following the thin arrows in the figure - going from a to b illustrates how the cone excitations of two of the colors situated at
azimuth angles 180° and 270° are transformed into S-cone
contrasts (cS), M-cone contrasts (cM) and L-cone contrasts (cL) at
corresponding azimuth angles . To test the validity of this
representation the results of many detection experiments of five
subjects are plotted (see figure at the right hand side above).
Observers had to indicate the occurrence of small rectangular
chromatic and isoluminant stimuli to the left or right of the fixation
point. Thresholds were computed at 75% of psychometric
functions. Each point (cS, cM, cL) is the result of 300 stimulus
presentations. These data are well approximated by the cone
contrast functions shown in b. The best-fitting sinusoidal functions
in cone contrast space that approximate the detection circle are
cS = 0.1006 sin ( - 289.1)
cM = 0.0098 sin ()
cL = - 0.0040 sin ( - 48.8).
These functions are shown in the figure (panels a – c) below. They
describe the contributions of the three receptoral mechanisms to
the detection of the respective colors indicated by stars. Using cS,
cM and cL the contributions to the detection of the 16 colors can be
expressed in terms of the postreceptoral chromatic mechanisms
blue- yellow { cS- (cM+ cL)} and red-green {cM - cL}. These are are
shown in panels d and e in the figure below. Since the colors are
isoluminant, the receptoral mechanisms S-cone contrast plotted in
a. and the post receptoral mechanism blue-yellow plotted in d. are
indentical for the 16 colors considered here.
We use these colors to study percepts in human observers, for
example the shift in color of a gray test field surrounded by either
of the 16 colors displayed on a color monitor. The three monitor
guns (R,G,B) are calibrated with respect to their spectral
radiances. Psychophysical thresholds can then be transformed into
values of c S, c Mand c Land plotted as a function of the azimuthal
angle Φ. From these the contributions of the postreceptoral
mechanism red-green {c M-c L} can be computed. We can predict
the outcome of experiments for a range of hypotheses related to
the chromatic mechanisms.
As mentioned above use of these colors in psychophysical
experiments requires a calibrated monitor and an otherwise well
defined setting. Monitors used for browsing the internet generally
do not satisfy these conditions. The picture below qualitatively
mirrors their detectability and hue range.
The CIE coordinates of these colors are published in Teufel &
Wehrhahn (2000). In this paper also their derivation from flicker
fusion photometry and detection experiments as well as
geometrical considerations of a color space similar to that of the
MacLeod & Boynton (1979) cone space can be found.
Literature:
Kaiser PK Boynton RM (1996) Human Color Vision. Optical
Society of America, Washington D.C.
MacAdam DL (1942) Visual sensitivities to color differences in
daylight. J Opt Soc America 32: 247-274
MacLeod DIA Boynton RM (1979) Chromaticity diagram showing
cone excitation by stimuli of equal luminance. J Opt Soc America
69: 1183-1186
Teufel H Wehrhahn C (2000) Evidence for the contribution of Scones to the detection of flicker brightness and red-green. J Opt
Soc America A 17: 994-1006