Work out luminous flux instantly with clear inputs, formula shown and shareable results.
Luminous flux is intensity multiplied by the solid angle it fills. A 1500 cd source confined to a 40 degree cone subtends 0.379 sr and therefore emits about 569 lumens. The same intensity radiating in every direction would need 18,850 lumens, which is why reflectors are so effective at boosting apparent brightness.
Luminous flux
flux = I x omega
Cone solid angle
omega = 2 pi (1 - cos(half angle))
Because flux counts total light emitted while intensity counts light per direction. A narrow beam is intense but carries little total flux.
About 80-120 lm/W for LEDs, 15 lm/W for incandescent and 60-100 lm/W for fluorescent.