The characteristics of the LEDs on the wall ("SKY-LEDs") in the sound-localization hoop setup at the department of Biophysics, Donders Institute for Brain, Cognition and Behaviour.
The sky-LEDs are organized in a radial, exponential fashion, with an LED on the cross-section of 12 spokes (30 deg separation) and 7 concentric rings. The rings have a radius \( r \) of 14, 25.5, 40, 58.2, 81.5, 112, and 149.5 cm. In the old setup the SKY was 160 cm away from the center of a subject's head, so the radial distance \(R = \arctan (r)/160 \) was 5, 9, 14, 20, 27, 35 and 43 deg.
The luminance of the sky-LEDs was measured on 19-Feb-2013 with a Minolta LS-100 lux-meter, while sitting in the chair, and focus adjusted. Measurements were taken at a distance of 106 cm with the following settings of the lux-meter: non color-corrected, preset, continuous, absolute, slow response. Luminance values when the intensity was set at 1 (complete darkness) and 255 (room lights on lowest possible intensity) were determined, as shown in figure 2A and B, respectively. The LEDs did not fill the entire measurement area, so these values are not absolute, and as the measurements were taken at a fixed position (the subjects head), this changed the area measured as a function of distance. This leads to a decrease in observed luminance as the radius increases. In figure 1C and D, normalized luminance values are plotted as circle size for every ring and spoke (normalization is achieved by dividing with the mean luminance per ring).
Note that the center LED can be red or green (measured in green condition), while the other LEDs are always green (\( \lambda = 585 nm\)).
After repositioning certain LEDs (so that they point towards the center of the set-up), luminance was measured again ( 106 cm to central fixation LED, up to 183 cm to far left LED, Fig. 3). This time the lux meter was positioned on a standard, which allowed for slightly more stable recordings, but allows only for measurements of the lower hemisphere. The LED on ring 1, spoke 5 was twice as bright than the other LEDs (this is actually the ring 1, spoke 3 outlier in the previous setup).
The luminance of the recorded signal depends linearly on the intensity as given in the exp-file (between 0 and 255). This is shown in figure 4 for the central fixation LED.