Footcandle Equations
 Unit of illumination equal to one lumen uniformly
incident upon an area of one square foot.
Also, equal to the illuminance at a point one foot
distant from a one candela source.

Lumen
The international unit of luminous flux or quantity of
light. If a uniform point source of one
candela is at the center of a sphere of one foot radius,
having an opening in the surface of one
square foot area, the quantity of light that passes
through is called a lumen. The sphere has
a total surface area of 12.57 square feet. Since, by
definition, a lumen flows to each square
foot of surface area, a uniform point of one candela
produced 12.57 lumens.

Lumens per Watt
A ratio  expressing the luminous efficacy ofa light source;
light output divided by powerwatt.
Examples: Edison's first lamp 1.4 L/W
Fluorescent lamps 35-100 L/W
Infrared lamps 6-9 L/W
Incandescent lamps 10-40 L/W
Mercury lamps 50-60 L/W
Metal Halide lamps 80-125 L/W
High Pressure Sodium lamps 100-140 L/W
Theoretical max for white light 225t L/W

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