Although most of the stimuli were presented using a cathode ray tube (CRT), our initial observations were obtained using a standard Maxwellian-view optical system. Given the importance of establishing that the color phenomena we explored were not dependent on the method of stimulus generation, we briefly describe the Maxwellian-view system and its use. Full details of the system can be found elsewhere (Stockman, Sharpe et al.,
2007; Stockman, Smithson et al.,
2007; Stockman et al.,
2008).
The Maxwellian-view system had five channels all illuminated by a 900-W Xe arc lamp. The radiance in each channel was controlled by the insertion of fixed neutral density filters (Ealing, Holliston, MA; Oriel, Stratford, CT, or Melles Griot, Irvine, CA) or by the rotation, under computer control, of circular, variable, neutral-density filters (Rolyn Optics, Covina, CA). Wavelengths were selected by interference filters with full-width-at-half-maximum bandwidths of between 7 and 11 nm (Ealing or Oriel). Sawtooth and other flickering waveforms were produced by pulse-width modulation of fast, ferro-electric liquid crystal light shutters (Displaytech, Longmont, CO) at a carrier frequency of 400 Hz. Frequencies near the 400-Hz rectangular-pulse frequency were much too high to be resolved, so that observers saw only the temporally changing stimuli produced by the variation of the pulse width. The desired sawtooth and triangular waveforms were calculated in real time using counters and loops. The maximum rise-time for the sawtooth flicker was 2.5 ms (i.e., the period of the 400-Hz carrier) and the maximum usable contrast for a given channel was 92%. Head positions were maintained by a dental bite bar and viewing was monocular.
We refer to the magnitude of each stimulus as the
depth of modulation or the
modulation for short. By that, we mean the ratio of the maximum deviation of the waveform to its mean, usually expressed as a percentage. For a given cone type, the cone modulation is thus the ratio of the maximum cone excitation produced by the waveform to the mean cone excitation produced by the waveform, again usually expressed as a percentage. This can be calculated simply from the stimulus calibrations using the CIE cone fundamentals of Stockman and Sharpe (
2000).
In the Maxwellian-view system all five channels were optically superimposed and used to produce circular targets, the diameter of which subtended 4° of visual angle. The specific wavelengths in each channel were chosen to isolate either L- or M-cones responses, and to have little effect on S-cones. To produce L-cone-isolating waveforms, a 650-nm target of 9.49 log
10 quanta s
−1 deg
−2 and a 521-nm target of 7.68 log
10 quanta s
−1 deg
−2 were modulated in opposite phase (the two lights were M-cone equated, so produced little or no M-cone flicker). A 520-nm target of 8.52 log
10 quanta s
−1 deg
−2 and a 658-nm target of 9.40 log
10 quanta s
−1 deg
−2 were flickered in opposite phase to produce M-cone–isolating flicker (these two lights were L-cone equated, so produced little or no L-cone flicker). The relative radiances to equate the targets for either L- or M-cone isolation were based on the CIE cone fundamentals of Stockman and Sharpe (
2000). The calculated maximum L-cone and M-cone modulation was about 15% in both cases. Because the combined targets appeared orange when unmodulated, a fifth, steady 529-nm target of 8.90 log
10 quanta s
−1 deg
−2 was added to make the mean hue in the absence of flicker more yellow, although while some observers saw the unmodulated target as yellow, others saw it as slightly orange. The overall retinal illuminance was approximately 3.06 log
10 photopic trolands, thus precluding rod involvement (Sharpe, Fach, Nordby, & Stockman,
1989).
Observers preadapted to the unmodulated stimuli in Maxwellian-view or on the CRT for 2 min prior to making observations.