how do i work out the illuminance of a wall light that is 5 metres up on a wall?

Question

the wall light is 1500 candela so the illuminance directly below the light would be 1500/5 squared = 60
what would be the illuminance 3 metres on the ground away from the wall

Answer 1

You see there is a right triangle with legs 5 m and 3m. The light beam from the wall light to the spot forms the hypotenuse, whose length is sqrt(5^2 + 3^2) (meter). Hence the luminance 3 metres on the ground away from the wall is:
1500/(5^2 + 3^2) = 1500/34 (candela/m^2)