Candela (cd) to lumens (lm) 計算機 and how to 計算.
Enter the luminous flux in lumens, apex angle in degrees and press the 計算 button
to get the luminous intensity in candela:
For uniform, isotropic light source, the luminous flux Φv in lumens (lm) is equal to the luminous intensity Iv in candela (cd),
times the solid angle Ω in steradians (sr):
Φv(lm) = Iv(cd) × Ω(sr)
The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the cone apex angle θ in degrees (°):
Ω(sr) = 2π(1 - cos(θ/2))
The luminous flux Φv in lumens (lm) is equal to the luminous intensity Iv in candela (cd),
times 2 times pi times 1 minus cosine of half the apex angle θ in degrees (°):
Φv(lm) = Iv(cd) × ( 2π(1 - cos(θ/2)) )
So
lumens = candela × ( 2π(1 - cos(degrees/2)) )
Or
lm = cd × ( 2π(1 - cos(°/2)) )
