[11.5.20] In roadway lighting:

The luminance coefficient, q, multiplied by the cube of the cosine of the angle of incidence, (cos γ)3, of the light on the point (see Figure 11-1):

r = q(cos γ)³ ,
where:
q is the luminance coefficient in sr⁻¹
γ is the angle of incidence in degrees (the road surface is flat and not sloped up-down or right-left)

Unit: sr⁻¹

Note 1: The angle of observation α, affects the value of r. By convention, this angle is fixed at 1° for roadway lighting calculations.
Note 2: The reduced luminance coefficient is used for determining the luminance of a point of infinitesimal size in the roadway luminance formula.

Figure 11-1.

 

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