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Photometry and Geometrical Optics

Problem 5.11

An illuminant shaped as a plane horizontal disc of radius R=25 cmR=25 \mathrm{~cm} is suspended over a table at a height h=75 cm.h=75 \mathrm{~cm} . The illuminance of the table below the centre of the illuminant is equal to E0=70E_{0}=70 lx. Assuming the source to obey Lambert's law, find its luminosity.

Reveal Answer
M=E0(1+h2/R2)=7⋅102lm⁡/m2M=E_{0}\left(1+h^{2} / R^{2}\right)=7 \cdot 10^{2} \operatorname{lm} / \mathrm{m}^{2}
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