Stimuli were rendered under four real-world light fields measured by Debevec (
1998). These are illustrated in
Figure 2, which also shows log luminance histograms for the image pixels corresponding to the sphere. We chose light fields measured both indoors and outdoors. The original light field measurements were reported in color, with separate values for nominal red, green, and blue (RGB) planes. To render the scenes spectrally within the RenderToolbox environment, the RGB light field images had to be converted to images that represented image intensity as a function of wavelength. This was done as follows. First, we converted the RGB values of the images at each pixel to
XYZ values on the assumption that the RGB values represented linearized RGB primary intensities with respect to the sRGB standard (International Electrotechnical Commission,
1999). A three-dimensional linear model for surface reflectance, computed in our laboratory from measurements of 462 Munsell papers (Newhall, Nickerson, & Judd,
1943; Nickerson,
1957; Nickerson & Wilson,
1950), was then used to convert the
XYZ values of each pixel to spectra. This was done by forming a 3 by 3 transformation matrix between
XYZ values and linear model weights, using standard linear model methods (e.g., Brainard,
1995). Spectra were produced by multiplying the basis functions by the obtained weights. The particular choice of linear model was not of deep theoretical significance and was motivated primarily by convenience; we currently know little about the spectral variation of real-world light fields. The rendering procedure resulted in an hyperspectral image with 31 planes, with each plane corresponding to one wavelength band. Sample wavelengths were between 400 and 700 nm at 10-nm steps. For the current experiment, only the 500-nm band resulting from this process was used. This single image plane was replicated three times to produce identical red, green, and blue image planes for display. Although the conversion to spectral light fields and subsequent hyperspectral rendering was not necessary for the current experiment, we implemented it in preparation for planned future experiments where the spectral properties of the stimuli will be manipulated.