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Calculate VL (visibility level)
5/26/94
Taken from Bailey, Clear, Berman paper, JIES Summer 1993
}
{ User must set:
L Adaptation luminance in cd/m^2
dL Difference in luminance between background and foreground
S Target size in minutes of arc
Function computes:
dLt Visible luminance threshold
Sr Size below which Ricco's law applies
VL Visibility level
}
sq(x) : x*x;
fit(a1,a2,n) = a1 * (L^(1/n) + a2)^(n/2);
A = fit(.40743,1.6074,2.8723);
B = fit(.0840155,.43369,1.7188);
Sr = (4 + 2*L)/(1 + L);
dLt = if(Sr-S, sq(A/S), sq((B*(S-Sr)+A)/S));
VL = dL / dLt;
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