For purposes of uniform illumination, a circular aperture
could be selected in the bottom test pentagon. Fig. 5(b) demonstrates
nonuniformity varying with the circular aperture radius.
The maximum nonuniformity reaches about 3.1% when the
aperture is the inscribed circle of the pentagon. By decreasing
the radius, the uniformity can be improved. The nonuniformity
can be reduced to below 2% when the radius of the aperture is
less than 3.4 cm in our prototype. The radius length is 85% of
the 4 cm inradius of the inscribed circle, which agrees with the
result given by the Monte Carlo ray tracing method described in
the last section. In this case, the aperture area occupies 72% of
the inscribed circle of the bottom test plane. Across the aperture,
a mean flux of 2100 W/m2 is reached.