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Candela:
The unit of luminous intensity. One candela is defined as the
luminous intensity of 1/600,000 square meter of projected area
of a blackbody radiator operating at the temperature of solidification
of platinum under pressure of 101,325 Newtons per square meter.
Footcandle:
A footcandle is a measure of light intensity. A footcandle is
defined as the amount of light received by 1 square foot of
a surface that is 1 foot from a point source of light equivalent
to one candle of a certain type.
End Footcandle:
End Footcandle measurements are based on the focused light beam
only. The spherical energy or surrounding light output is not
captured by or reflected back to the surface of the footcandle
light meter. End footcandle is the focal light beam measurement
from point A to point B at one-foot distance.
Lumen:
A unit of light flow or luminous flux. The lumen rating of a
lamp is a measure of the total light output of the lamp. The
most common measurement of light output (or luminous flux) is
the lumen. Light sources are labeled with an output rating in
lumens. For example, a R30 65-Watt indoor flood lamp may have
a rating of 750 lumens. Similarly, a light fixture's output
can be expressed in lumens.
As lamps and fixtures age and become dirty, their lumen output
decreases (i.e., lumen depreciation occurs). Most lamp ratings
are based on initial lumens (i.e., when lamp is new).
End Lumens:
End Lumens measurements are based on a spot of light only. The
spherical energy or surrounding light output is not captured
by or reflected back to the surface of the lumen light meter.
End lumens is the light measurement from point A to point B
at one-foot distance.
Luminance:
Luminous Flux (light output). This is the quantity of light
that leaves the lamp, measured in lumens (lm). Lamps are rated
in both initial and mean lumens.
Initial lumens indicate how much light is produced once the
lamp has stabilized; for fluorescent and high-intensity discharge
(HID) lamps, this is typically 100 hours.
Mean lumens indicate the average light output
over the lamp's rated life, which reflects the gradual deterioration
of performance due to the rigors of continued operation; for
fluorescent lamps, this is usually determined at 40% of rated
life.
Luminous (Light Level):
This is the amount of light measured on the work plane in the
lighted space. The work plane is an imaginary horizontal, tilted
or vertical line where the most important tasks in the space
are performed. Measured in footcandles (fc or lux in metric),
light levels are either calculated, or in existing spaces, measured
with a light meter. A footcandle is actually one lumen of light
density per square foot; one lux is one lumen per square meter.
Like lumens, footcandles can be produced as either initial or
maintained quantities.
Work Plane:
The level at which work is done where illuminance is specified
and measured. For office applications, this is typically a horizontal
plane 30 inches above the floor (e.g., desk height).
Beam Lumens:
The total flux in that region of space where the intensity exceeds
50 percent of the maximum intensity.
Field Lumens:
The total flux in that region of space where the intensity exceeds
ten percent of the maximum intensity.
Lux:
The metric unit of measure for illuminance of a surface. One
lux is equal to one lumen per square meter. One lux equals 0.0929
footcandles.
Light Level:
Light intensity measured on a plane at a specific location is
called illuminance. Illuminance is measured in footcandles,
which are workplane lumens per square foot. You can measure
illuminance using a light meter located on the work surface
where tasks are performed. Using simple arithmetic and manufacturers'
photometric data, you can predict illuminance for a defined
space. (Lux is the metric unit for illuminance, measured in
lumens per square meter. To convert footcandles to lux, multiply
footcandles by 10.76).
Efficacy:
A measure of the luminous efficiency of a radiant flux, expressed
in lumens per watt as the quotient of the total luminous flux
by the total flux. For daylighting, this is the quotient of
visible flux incident on a surface to radiant flux on that surface.
For electric sources, this is the quotient of the total luminous
flux emitted by the total lamp power input.
Efficacy of a Light Source:
The total light output of a light source divided by the total
power input. Efficacy is expressed in lumens per Watt.
Watt:
The unit of measuring electrical power. Watts does not relate
to the light output level. It defines the rate of energy consumption
by an electrical device when it is in operation. The energy
cost of operating an electrical device is calculated as its
wattage time in hours of use. In single-phase circuits, it is
related to volts and amps by the formula: Volts x Amps x Power
Factor (PF) = Watts. (Note: For AC circuits, PF must be included).
Kilowatt Hour (kWh) Formula:
The measure of electrical energy from which electricity billing
is determined. For example, a 100-Watt bulb operated for 1000
hours would consume 100 kilowatt hours (100 Watts x 1000 hours
= 100 kWh). At a billing rate of £0.10/kWh, this bulb
would cost £10.00 (100 kWh x £0.10/kWh) to operate
over 1000 hours.
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