As in Rucker et al., lighting conditions were produced with LEDs that consist of independently controlled red, green, and blue components (Lamina Ceramics, Westhampton, NJ, USA: Atlas Light Engine; peak wavelengths: 619 ± 20, 515 ± 35, and 460 ± 35 nm). The lights were placed above the center of the cage and had a beam spread of 36°. Lamina Titans RGB LEDs driven by an eight-channel, 12-bit Access I/O, USB-DA12-8A digital to analog converter with waveform generator functionality connected to BuckPucks (LuxDrive: 3021 D-E-500; LED Dynamics, Randolf, VT, USA) were used to create the stimuli.
Light output was calibrated, and a sinusoidal output was produced digitally using lookup tables and confirmed by recording illuminance output (Newport Model 818-SL serial number: 6915; Newport Corp., Irvine, CA, USA). Because we were interested in the role of blue light, which does not contribute significantly to luminance measures, the irradiance of the light source was used to equate the lighting components. The red, green, and blue components of each light source had a mean irradiance of 50 μW/cm
2. Small adjustments were made to the mean illuminance for both conditions using neutral density filters so that they were equivalent to 680 human lux. In a previous study,
9,11 a unit of Illuminance “chick lux” (E
c) was calculated as the irradiance of the light source multiplied by the chick photopic spectral sensitivity function (V
c).
40 The irradiance value above is equivalent to 214 chick lux for red, 191 chick lux for green, and 64 chick lux for blue.
The modulated lights produced 80% contrast in the red, green, and blue components of the illuminants. Contrast was calculated as Michelson contrast: Contrast % = (L
max − L
min)/(L
max + L
min) × 100. For the experiments reported in this paper, the relative contrast of the RGB components and mean illumination levels were kept constant in all conditions to test the hypothesis that a color signal was temporal frequency dependent. Temporal contrast is unaffected by defocus, and a temporal stimulus does not affect the modulation transfer function of the eye with regard to spatial frequency.
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