Work out luminous intensity instantly with clear inputs, formula shown and shareable results.
Luminous intensity is flux per unit solid angle, measured in candela. A 60 degree beam subtends 0.842 steradians, so 1200 lumens concentrated into it gives about 1425 cd. Narrowing the beam raises intensity without changing the lamp's output, which is why spotlights read so high in candela.
Luminous intensity
I = flux / solid angle
Cone solid angle
omega = 2 pi (1 - cos(half angle))
Because the same flux is squeezed into a smaller solid angle. A torch rated at thousands of candela may emit only a few hundred lumens.
4 pi, about 12.57 steradians. An omnidirectional lamp divides its lumens by that figure.