Light intensity

Intensity corresponds to the amount of light emitted by a light source and can also represent the amount of light received by a plant.

This is an important factor, because plants' growth may suffer if they do not receive sufficient light. They may :

• lose their characteristic shape
• grow thin, leggy stems

Further :

• New leaves may also be smaller and turn yellow.
• Some plants may lose their colour (crotons and variegated plants) or turn dark green.

Plants exposed to excessive light, on the other hand, may not flower properly or may turn pale green.

Different species of plants require different amounts of light. For instance, orchids, bromeliads and succulents need brighter light than do African violets, Begonia rex and foliage plants.

Light intensity decreases dramatically with distance from the light source. For example, plants located 15 cm from two 40-watt fluorescent lamps installed in a fixture fitted with a reflector receive about 9250 lux, but only 5380 lux if they are 30 cm from the lamps.

The human eye is a very poor "instrument" for measuring light intensity, because the pupil adjusts constantly in response to the amount of light it receives. To accurately measure the light intensity in a given spot, it is best to use a light meter. This will make it easier to tailor the lighting to the needs of the plants you are trying to grow.

Light intensity may be measured in lux (metric system) or foot-candles (Imperial system). Note that 1 foot-candle = 10.76 lux and 1 lux = 0.0929 foot-candles.

The following chart shows the indoor light intensity for different exposures :