lx 与cd 的计算方法
Candela to lux calculation with distance in meters
The illuminance Ev in lux (lx) is equal to the luminous intensity Iv in candela (cd),
divided by the square distance from the light source d2 in square meters (m2):
Ev(lx) = Iv(cd) / (d(m))2
So
lux = candela / square meters
Or
lx = cd / m2