Luminous Flux
About Luminous Flux Conversion
Luminous flux measures the total amount of visible light emitted by a source in all directions—essentially the "total light output" of a lamp or LED. It's what you see on light bulb packaging to compare brightness. Unlike intensity (candela) which measures light in one direction, or illuminance (lux) which measures light received on a surface, luminous flux captures the complete light production regardless of where it goes. This makes lumens the fairest way to compare light sources: a 1000-lumen LED and 1000-lumen incandescent produce the same total visible light, though their efficiency (lumens per watt) differs dramatically.
The SI unit is the lumen (lm). Lumens replaced watts as the standard brightness measure when energy-efficient lighting made watts misleading—a 10W LED can match a 60W incandescent's 800 lumens. The lumen is defined in terms of candela: 1 lm = 1 cd·sr (candela-steradian). Luminous flux is weighted by human eye sensitivity, peaking at 555 nm (green-yellow)—this is why lumens measure perceived brightness, not raw optical power.
Our converter handles luminous flux units and helps understand the relationships between different photometric quantities.
Luminous Flux Conversions and Relationships
| From | To | Multiply By |
|---|---|---|
| lumen (lm) | cd·sr | 1 (equivalent) |
| kilolumen (klm) | lm | 1,000 |
| lm | klm | 0.001 |
| megalumen (Mlm) | lm | 10⁶ |
| lm | millilumen (mlm) | 1,000 |
| cd (isotropic source) | lm | 4π ≈ 12.57 |
| lm | cd (isotropic) | 1/(4π) ≈ 0.0796 |
| lm | watt (at 555nm) | 1/683 ≈ 0.00146 |
Key relationships: 1 lumen = 1 candela·steradian. For a point source emitting uniformly in all directions: total lumens = 4π × candela ≈ 12.57 × candela. At 555nm (peak eye sensitivity): 1 watt = 683 lumens exactly.
Luminous Flux Unit Reference
Lumen (lm) – The SI unit of luminous flux, representing total perceived visible light power emitted by a source. By definition, 1 lm = 1/683 watt at 555nm (peak human eye sensitivity). This definition ties lumens to the photopic luminous efficacy function V(λ). Typical light source outputs: candle ~13 lm, 40W-equivalent LED ~450 lm, 100W-equivalent LED ~1600 lm, car headlight ~1000-3000 lm per bulb, stadium floodlight ~100,000 lm.
Candela steradian (cd·sr) – Mathematically equivalent to the lumen by definition. This notation explicitly shows the relationship between luminous intensity (candela) and the solid angle (steradian) it illuminates. A 1-candela source emits 1 lumen into each steradian of solid angle; since a complete sphere is 4π steradians, an isotropic 1-cd source emits 4π ≈ 12.57 lumens total.
Kilolumen (klm) – 1,000 lumens. Convenient for high-output sources: professional projectors (3-10 klm), stage lighting (10-50 klm), stadium lights (50-200 klm). Increasingly used as LED technology enables higher outputs from compact fixtures.
Megalumen (Mlm) – 1,000,000 lumens. Used for large-scale lighting installations: sports arena total lighting (50-200 Mlm), major concert stages (10-100 Mlm). Rarely applied to single sources.
Luminous efficacy (lm/W) – While not a flux unit, this ratio is essential for comparing light sources: incandescent 10-17 lm/W, halogen 15-25 lm/W, CFL 50-70 lm/W, LED 80-200 lm/W, theoretical maximum 683 lm/W (at 555nm only). Higher efficacy means more light per watt of electricity.