Light Information - Short Version
Visible light can be easily measured with a light meter (G. E. use to make a popular, inexpensive one that still can be found at used camera shops). Note that a light meter measures light at one time, that is instantaneously. It's not over a period of time. Footcandle is an instantaneous measurement of light. If you start to think about how bright certain levels of footcandles are, then you will start to understand the environment and light levels that indoor palms require. In the evening, inside your family room, with a TV on and a table lamp you might have 25 footcandles. The light in an office where it is to easy to read a book is usually about 125 footcandles. Sun light entering a south window often has levels of 500 - 800 footcandles. A typical shadehouse where indoor palms are grown has about 2000 footcandles. Outside, on a bright sunny summer day when the sun is directly overhead is around 10,000 footcandles. As you can see, you will almost never have as much light inside your house as in the nursery where the palms were grown, even if they came from a shadehouse with 80% shade.
Light Information - Long (professorial) Version
Luminous intensity (or candlepower) is the light density within a very small solid angle, in a specified direction. In other words, this is the total number of lumens from a surface emitted in a given direction. The unit of measure is candela. In modern standards, the candela is the basic of all measurements of light and all other units are derived from it.
Imagine a candle flame centered within a sphere that is one foot in radius. That point source, of uniform intensity equal to one candela, produces an amount of light flowing through one square foot of the sphere's area that is defined as one lumen. The amount of light falling on that one square foot of the sphere is equal to one footcandle. Since the area of the sphere is equal to 4p, then the amount of light flowing through the sphere is 4p lumens, or about 12.57 lumens. In other words, a one candela source produces 12.57 lumens and an illuminance of one footcandle at a distance of one foot.
Luminous flux is the time rate of flow of light. The unit of measure is the Lumen. "In Flux" means "everchanging" just like the light in the morning, at noon, and evening. If you graphed the light levels through the time of a day it would be a bell shaped curve. One lumen may be defined as the light flux emitted in one unit solid angle by a one-candela uniform-point source. The lumen differs from the candela in that it is a measure of light flux irrespective of direction. The lumen is used to express a quantity of light flux: total output of a source, output within a specific angular zone, amount of absorbed light, etc.
Illumination is the density of luminous flux on a surface This parameter shows how "bright" the surface point appears to the human eye. The appropriate units of measure are Footcandle and Lux. One footcandle is the illumination produced by one lumen uniformly distributed over one square foot of a surface, or conversely this is the illumination at the point of a surface which is one foot from, and perpendicular to, a uniform point source of one candela. So, footcandles incident on a surface=Lumens/Area(sq.feet). Lux is used in the International System. Both have a similar objective, but meters are used for Lux and feet are used for Candelas. Therefore, one lux=0.0929 footcandles. Or, approximately, 1 Fc=10 Lux. (Note: Footcandle & Lux are instantaneous readings, not readings taken over a period of time)
Luminance or Brightness is a luminous intensity of a surface in a given direction per unit of projected area of the surface. Actually, our eye sees brightness, not illumination. Every visible object has brightness. Usually, brightness is proportional to the object's illumination, so a well illuminated object seems brighter.
Footcandles to Lux conversion
One footcandle is the illumination produced by one lumen uniformly distributed over one square foot of surface, and lux is the illumination over one square meter of surface. Therefore, one lux=0.0929 footcandles. Or, approximately, 1 Fc=10 Lux.
Http://www.apogee-inst.com/ Apogee Instruments is a private company that sells low cost meters that measure light for plant growth. This web site has useful technical information on the difference between Photosynthetic Radiation and visible light seen by the human eye..
