Light is measured in a standard unit called a foot-candle (fc). A foot-candle
is the equivalent amount of light that is produced by an international
candle at the distance of one foot.
A lumen is the term used to measure the intensity of the light. It is
the total amount of light energy given off by a light source. (As you
move your plant away from the light source, the lumens striking the leaf
surface will correspondingly decrease with more distance.) Foot-candles
(fc) ÷ 10.76 = Lumens (lux).
The Light Meter is commonly used to measure how much light a particular
plant is currently receiving. It measures in foot-candles, and it usually
comes with a light chart specifying the light requirements of a wide variety
of common plants.
INDOOR LIGHT DURATION AND INTENSITY
Duration: A general rule of thumb is that 12-16 hours of natural or artificial
light, or a combination of the two, are needed to promote active growth.
Not enough hours of light will slow or stop plant growth.
Intensity: Intensity varies enormously from plant to plant.