Problem 5.11

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