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Alissa Winkler, Charles Chubb, Charles E. Wright; Equiluminance Settings Interact Strongly With Spatial Frequency. Journal of Vision 2010;10(7):440. doi: https://doi.org/10.1167/10.7.440.
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© ARVO (1962-2015); The Authors (2016-present)
The minimum motion method is a standard tool used by psychophysicists obtaining perceptually equiluminant display settings for a light of hue A to another fixed light F. This method uses a 4 frame periodic stimulus, whose 1st and 3rd frames comprise counterphase, achromatic gratings and whose 2nd and 4th frames comprise counterphase square wave gratings alternating between lights A and F in quadrature with the square wave of frames 1 and 3. When the luminance of hue A is adjusted to make the motion of this stimulus ambiguous, the resulting light is taken as equiluminant to F. We document dramatic effects of the spatial frequency (SF) of the square wave used in the motion stimulus on the equiluminance settings obtained using this method. Some observers show the following pattern: when the square wave is low SF (3 cycles/deg), in order to be made equiluminant to a fixed gray, a saturated green needs to be made much lower in luminance than it does when the square wave is high SF (6 cycles/deg). For other observers, the reverse pattern holds: their equiluminant green settings are higher for the low than for the high SF square wave. Moreover, whichever pattern an observer shows in her equiluminant settings for green, she is likely to show the reverse pattern in her settings for red lights: i.e., if an observer produces higher equiluminance settings for green with the high than with the low SF square wave, then she tends to produce lower equiluminance settings for red with the high than with the low SF square wave. These findings underscore the importance of matching the SF of the minimum motion stimulus to the SF of context in which the equiluminant lights are to be used experimentally.
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