Power is not output
Watts
A watt only tells you how much electricity the lamp draws
Same watts, different light
A 20 W fluorescent can match the visible output of a 75 W incandescent because the technologies convert power differently
Compare fixtures
Judge lamps by usable output and placement, not by the number printed on the bulb alone
Garden math
Many growers still size rooms by watts per square foot, then check the canopy with a meter
Human-eye units
Candela
The base unit of emitted visible light, historically tied to one candle
Lumens
Visible flux in a beam. One candela radiating equally in all directions is about 12.6 lumens (4π)
Example CFL
A 23 W compact fluorescent emits about 1,600 lumens
Foot-candle
Light at one foot from a one-candela source
Lux
One lumen per square meter. 100 lumens on 1 m² is 100 lux; the same 100 lumens on 10 m² is 10 lux
Photon units plants care about
Moles
Count photons. One mole is 6.02 × 10²³ photons
Why moles
Candelas, lumens, lux, and foot-candles ignore UV and far-red and overweight yellow-green
Quantum light
PAR expressed in moles (often µmol/m²/s) is the check that plants are getting usable photons
Instant versus day
Lumens and lux are snapshots. Outdoor and greenhouse light need a daily total because intensity changes all day
Meter placement
Read at canopy height, where the tops should receive the strongest light
When a meter helps
Optional tool
A strong garden can be planned from watt input alone, but a meter removes guesswork
Evenness
Walk the canopy to find dim corners and overlapping hot spots
PAR over lux
Foot-candle and lux meters miss plant-useful bands; a quantum meter is the better plant tool
PAR limits
Even PAR skips far-red and UV and does not weight how efficiently each wavelength is used
Still worth it
PAR remains more meaningful for growers than lumen-based ratings