Luminous Flux Converter

Luminous flux measures the total amount of visible light emitted by a source.

Key units:

• Lumen (lm): total light output from a source
• Candela (cd): light intensity in a specific direction
• Lux (lx): lumens per square meter (illuminance)
• Foot-candle (fc): lumens per square foot (used in the US)

Relationships:

• 1 lux = 1 lumen per m²
• 1 foot-candle = 1 lumen per ft² = 10.764 lux
• 1 candela (uniform sphere) = 4π lumens ≈ 12.566 lumens

Lux to Foot-candle conversion:

• lux = foot-candles × 10.764
• foot-candles = lux × 0.0929

Typical illuminance levels:

The three units here describe genuinely different things, and a spotlight makes it click. Lumens measure the total light a source pumps out in every direction. Candela measure how concentrated that light is in one direction. Take a bare bulb and a spotlight of the same lumen output: the spotlight has a far higher candela rating because it focuses those lumens into a narrow beam, while the bare bulb spreads them everywhere and reads low in candela despite emitting just as much total light. Lux is the third piece, what actually lands on a surface once distance and spread are accounted for.

This is why bulbs are now sold by lumens rather than watts. Watts only ever measured power drawn, and it tracked brightness only because old incandescent bulbs were all similarly inefficient. LEDs broke that link: a 9-watt LED and a 60-watt incandescent both put out about 800 lumens, so lumens are the honest brightness number. The figure worth comparing on the box is lumens per watt, the efficacy, where a good LED reaches 80 to 100-plus lumens per watt against an incandescent’s dismal 15 or so.