The effect of varying the lighting model parameters
ψP and
π on the geometric correction function. (Left)
ΓM versus
ψT for different values of
ψP (with
ϕT =
ϕP = 0). The curve in blue corresponds to the case
ψP = 0 deg. The two dashed curves correspond to the cases
ψP = ±30 deg. Each curve represents the decrease in emitted luminance as a surface of constant albedo is rotated away from the punctate source. Changing the elevation
ϕT of the surface normal leads to a similar set of curves. These are plotted in Boyaci et al. (
2004). (Right)
ΓM versus
ψT for various values of the punctate-total ratio
π (with
ψP = 0 deg). The minimum of
ΓM is always at
ψT =
ψP, the direction to the light; changing
π affects only curvature. The blue, solid curve corresponds to
π = 0.67 (the value we use in
Experiment 1). The red, dashed curves correspond to values of
π = 0.87 (more curved) and
π = 0.2 (less curved). When
π = 0, the curve becomes a horizontal line (
ΓM ≡ 1), a readily interpretable result: if
π = 0 then the light is perfectly diffuse and changes in orientation do not affect the intensity of light absorbed by or emitted by the test surface.