2.8 × 10^-5 Joules
Considering the base area of the 'space' to be 1m²
Therefore; it's 'height' h = 6.0 m
Time taken (t) for all the light in this space to travel down 6.0 m ..
t = h/c
= 6.0 m / 3.0^8m/s
t = 2 × 10^-8 s
Energy reaching this 1 m² base = 1.40 × 10^3 J/s .. (1400W)
Net energy (E) travelling through this space = 1400J/s x 2 × 10^-8 s
E = 2.8 × 10^-5 J