|
How is lighting
classified?
Traditional lighting is frequently
discussed in terms of Lumens, CCT, CRI and Watts.
Here we will have a brief look at these concepts and
discuss how to compare.
Lumens
Technically
“the
SI unit of
luminous flux is the
lumen (lm).
One lumen is defined as the luminous flux of light
produced by a light source that emits one
candela of
luminous intensity
over a solid angle of one
steradian.
In other systems of units, luminous flux may have
units of
power.”
In practical
terms this is a measurable way of comparing how well
a device illuminates an object. Most light devices
will have a lumen rating published on its
packaging. The table below lists several items and
their approximate lumen rating and energy
efficiency.
|
Category |
Type |
Efficiency |
lm/W |
|
|
candle |
0.1% |
0.3
8 |
|
Incandescent |
40-60 W tungsten
filament |
5% |
12.6-14.5
6 |
|
|
100 W tungsten
filament |
7% |
17.5
6 |
|
|
glass halogen |
6.5% |
16 |
|
|
quartz halogen |
10% |
24 |
|
|
tungsten-halogen |
7%-10% |
18-25
5 |
|
|
high-temperature
incandescent |
14% |
35 |
|
Fluorescent |
13 W twin-tube
fluorescent |
23% |
56.3
1 |
|
|
compact fluorescent |
18-24% |
45-60
3 |
|
Light-emitting diode |
white LED (low power) |
6%-18% |
15-42
5 |
|
|
white LED (high
power) |
11%-24% |
26-60
5 |
|
|
white LED
(prototypes) |
24%-41% |
60-100
5 |
|
Arc lamp |
xenon arc lamp |
12%-60% |
30-150
4 |
|
|
mercury-xenon
arc lamp |
20%-22% |
50-55
4 |
|
Ideal radiators |
ideal
black-body
radiator at 4000 K |
20% |
47.5
7 |
|
|
ideal black-body
radiator at 7000 K |
38% |
95
7 |
|
|
ideal white light
source |
100% |
242.5 |
|
|
monochromatic 555.6
nm source |
100% |
683
6 |
CCT the
Correlated Color Temperature
Imagine a piece
iron being super heated until it glows. As the
temperature of the bar changes colour of the light
emitted will also change.
The CCT rating
refers to the color a pure black substance would
glow if heated to a specific temperature in Kelvin.
CCT is measured in Kelvin, the scientific unit of
temperature measurement.
The graph below
gives us an indication of this scale and shows that
5000 K is close to day light. Interior warm white
for general lighting is 3200K, cool white for
display or industry is 5500K.
Some common
examples:
-
1200 K: a candle
-
2800 K: tungsten lamp (ordinary household bulb),
sunrise and sunset
-
3000 K: studio lamps, photofloods,
-
5000 K: electronic
flash, average daylight. A designation of D50
stands for "Daylight 5000K" and is the most
common standard for professional light booths
for
photography,
graphic arts,
and other purposes.
-
6000 K: bright midday sun
-
7000 K: lightly overcast sky
-
8000 K: hazy sky
-
10,000 K: heavily overcast sky
CRI Color
Rendering Index
Why do some
colours appear different under different lights? The
answer is that different types of light illuminate
differently.
The color
rendering index (CRI), is a measure of the ability
of a light source to reproduce the colors of various
objects being lit by the source.
The best
possible rendition of colors is specified by a CRI
of one hundred, while the very poorest rendition is
specified by a CRI of zero. In general, CRI values
higher than 80 are considered good for indoor
lighting. |
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