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Luminous Flux Calculator

Work out luminous flux instantly with clear inputs, formula shown and shareable results.

About Luminous Flux Calculator

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.

Formulas

Luminous flux

flux = I x omega

Cone solid angle

omega = 2 pi (1 - cos(half angle))

Worked example

With the default values below, Luminous Flux Calculator returns Luminous Flux In The Beam of 568.38. Change any input above and every figure here is recalculated in your browser.

Inputs used

Luminous intensityminimum 0 cd
1,500 cd
Beam angleaccepts 0–360 deg
40 deg

Results

Luminous Flux In The Beam
568.38 lm
Beam Solid Angle
0.3789 sr
Flux If It Radiated In All Directions
18,849.6 lm
Efficacy If The Lamp Drew 10 W
56.838 lm/W

Step by step

  1. 1Values used
  2. 2Luminous flux
  3. 3Cone solid angle
  4. 4Luminous Flux In The Beam= 568.38 lm
  5. 5Beam Solid Angle= 0.3789 sr
  6. 6Flux If It Radiated In All Directions= 18,849.6 lm
  7. 7Efficacy If The Lamp Drew 10 W= 56.838 lm/W

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does the same intensity give different flux?

Because flux counts total light emitted while intensity counts light per direction. A narrow beam is intense but carries little total flux.

What efficacy should a modern lamp reach?

About 80-120 lm/W for LEDs, 15 lm/W for incandescent and 60-100 lm/W for fluorescent.

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