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Common Lighting terms, measurement techniques and general information.
Figure 1. Candela
Figure 2. Footcandle
Figure 3. End Footcandle
Figure 4. Lumens
Figure 5. Lumens
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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LUMINOUS INTENSITY AND FLUX:
The unit of luminous intensity I
is the candela (cd) also known as the international candle. The
intensity of a light source is commonly referred to as its candlepower.
The unit of luminous flux F is the lumen (lm). One lumen is equal to
the luminous flux which falls on each square meter (m2)
of a sphere one meter (1m) in radius when a 1-candela isotropic light
source (one that radiates equally in all directions) is at the center of
the sphere. Since the area of a sphere of radius r is 4 pr2,
a sphere whose radius is 1m has 4pm2
of area, and the total luminous flux emitted by a 1-cd source is
therefore 4p1m.
Thus the luminous flux emitted by an isotropic light source of
intensity I
is given by:
F = 4 pI
where
Luminous flux (lm) = 4 p × luminous
intensity (cd)
ILLUMINATION:
The illumination (or illuminance) E of a surface is the
luminous flux per unit area that reaches the surface:
E = F/A Illumination
= luminous flux /
area
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Luminous Intensity & Luminous Flux: |
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lumens = 4π × cd cd = lumens
4π
lumens = 4π × (Mean
Spherical Candlepower)
fc = cd cd
= fc × d ²
d ²
lumens = fc 4π × d
² fc = lumens
4π × d²
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Illumination: |
Inverse Square Law:
E = I
d² |
Cosine Law:
E = I
cos θ
where
θ is angle of incidence d² d² |
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Related Formulas |
10.764 × fc = lux |
1 lux = 0.0929 fc |
10.764 ×
lumens / sq. ft = lux |
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lumens / m²
= lux |
1 cd / sq.
ft. = π × fL (foot-Lambert) |
3.426 × fL =
nits = cd / m² |
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1 fL = 1
lumen / sq. ft. |
1 fL = 3.426
cd / m² |
1 cd = 1
lumen per steradian (unit solid angle), where “steradian”
unit solid angle is a cone. Unit solid angle is
photometric brightness, where the spherical surface is:
S = 4π²
If r = 1, then there are 4π lumens in the sphere, (12.566
lumens) |
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Lighting in the work place
Light Level or Illuminance, is the amount of light measured in a plane.
The work plane is where the most important tasks in the room or space
are performed. Measuring Units Light Level - Illuminance
Illumenance is measured in foot candles (ftcd, fc) (or lux in the
metric SI system). A foot candle is actually one lumen of light density
per square foot, one lux is one lumen per square meter.
- lux = fc 10.752
- fc = lux / 10.752
Common Light Levels Outdoor
Common light levels outdoor at day and night can be found in the
table below:
| Condition |
Illumination |
| (ftcd) |
(lux) |
| Sunlight |
10,000 |
107,527 |
| Full Daylight |
1,000 |
10,752.7 |
| Overcast Day |
100 |
1,075.3 |
| Very Dark Day |
10 |
107.53 |
| Twilight |
1 |
10.75 |
| Deep Twilight |
.1 |
1.08 |
| Full Moon |
.01 |
.108 |
| Quarter Moon |
.001 |
.0108 |
| Starlight |
.0001 |
.0011 |
| Overcast Night |
.00001 |
.0001 |
Common and Recommended Light Levels Indoor
The outdoor light level is approximately 10,000 lux on a clear day.
In the building, in the area closest to windows, the light level may be
reduced to approximately 1,000 lux. In the middle area its may be as low
as 25 - 50 lux. Additional lighting equipment is often necessary to
compensate the low levels.
Earlier it was common with light levels in the range 100 - 300 lux
for normal activities. Today the light level is more common in the range
500 - 1000 lux - depending on activity. For precision and detailed
works, the light level may even approach 1500 - 2000 lux.
The table below is a guidance for recommended light level in
different work spaces:
| Activity |
Illumination
(lux, lumen/m2) |
| Warehouses, Homes, Theaters, Archives |
150 |
| Easy Office Work, Classes |
250 |
| Normal Office Work, PC Work, Study Library, Groceries,
Show Rooms, Laboratories |
500 |
| Supermarkets, Mechanical Workshops, Office Landscapes |
750 |
| Normal Drawing Work, Detailed Mechanical Workshops,
Operation Theatres |
1,000 |
| Detailed Drawing Work, Very Detailed Mechanical Works |
1,500 - 2,000 |
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