Light is commonly measured in foot-candles (ft-c) or lux. One lux is equal to one lumen per square meter and one foot-candle is equal to one lumen per square foot. To convert measurements use the following formula:

foot-candle = lux x 0.0929

lux = foot-candle x 10.764

Measuring light levels are important for greenhouses, hydroponics, and aquaponics. Plants differ greatly in their light intensity requirements. Indoor plants are often classified by the amount of light necessary for growth:

  • Low (minimum 1.1 Klx, 0.8 to 2.1 preferred for good growth)
  • Medium (minimum 1.1 to 1.6 Klx, 2.1 to 5.4 preferred)
  • High (minimum 1.6 to 10.8 Klx, 5.4 to 10.8 preferred)
  • Very high (minimum 10.8 Klx, 10.8+ preferred)

About 1.1 Klx for 12 hours per day are necessary simply to maintain plant quality for one year and at least 2.1 Klx for 12 hours per day are necessary for foliage plants to manifest any benefit from fertilization. While lack of sufficient light results in poor plant growth, too much light can also be harmful. Shade plants cannot tolerate excessively high light levels. When a plant receives too much direct light, the leaves bleach or scald, sometimes dying. This often happens after moving a plant outdoors in direct light. Any changes in light intensity should be gradual.

Hanna Instruments offers a portable meter for measuring Lux.