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How
is Light Measured?
The "color" of
light sources comes from a complicated relationship
derived from a number of different measurements,
including correlated color temperature (CCT) or Kelvin
temperature (K), color rendering
index (CRI), and spectral distribution.
However, color is most accurately
described by a combination of CCT
and CRI.
The first factor in choosing a lamp color
is the correlated color temperature.
CCT is defined as the absolute temperature
(expressed in degrees Kelvin) of a theoretical
black body whose chromaticity most nearly resembles
that of the light source. The CCT rating is
an indication of how "warm" or "cool" the light
source appears. The higher the number, the
cooler the lamp color will appear. The lower
the number, the warmer the lamp color will
appear.
The K rating in a lamp model
is a generalized form of addressing the color
output of a lamp or CCT. The hotter the kelvin
temperature gets, the bluer.
10k lamps seem to be a nice crisp white, while higher
kelvin can go from a blue/white to very blue and
lower kelvin seem more like that of sunlight
(6500k). Metal Halide bulbs go up to 20,000K
(commonly used in aquariums) providing the
bluest light.
Kelvin
Rating Chart
| Kelvin
Rating |
Description |
Uses |
Bulb Types |
2000-
3000K |
Light looks red/orange
spectrum and is not pleasant to the eye |
Grow Lights (budding & flowering
Parking lots, street lights, garages etc. |
HPS,
T5HO Fluorescent
T8, T12 |
| 4000K |
Neutral
white light |
General lighting,
factories, parking lots, warehouses |
Metal Halide
T5 Fluorescent, T8, T12 |
| 5000K |
White
- daylight |
General
and retail lighting applications |
Metal Halide
T5 Fluorescent, T8, T12 |
| 6500K |
A high CCT white
daylight source that looks the most like
natural sunlight |
Aquariums, greenhouses,
general office lighting
Used to simulate
average outdoor light conditions |
Metal Halide
T5 Fluorescent, T8, T12 |
| 10,000K |
Crisp white light |
Aquariums & greenhouses |
Metal Halide
T5HO |
| 20,000K |
Blue
light |
Aquariums |
Metal Halide
T5HO |
Spectral
Energy Distribution & PAR Watts
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total visible spectrum is perceived by us humans
as white light, but
the "white light" is actually separated
into a spectrum of colors from violet
to blue, to green, yellow, orange and
red made up of different wavelengths.
Plants use the blue to red part of the
spectrum as their energy source for photosynthesis.
The different combinations and the
relative intensity of various wavelengths
of light determines the
CRI of a light source. |
Spectral
Distribution for Sunmaster Blue CDX
Bulb
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The output of a 400 watt incandescent
bulb is about 25 watts of light, a 400
watt metal halide bulb emits about 140
watts of light. If PAR is considered
to correspond more or less to the visible
region, then a 400 watt metal halide
lamp provides about 140 watts of PAR. A
400 watt HPS lamps has less PAR, typically
120 to 128 watts, but because the light
is yellow it is rated at higher lumens
(for the human eye).
Color Rendering Index
In general, CRI is a numeric indication of a lamp's
ability to render individual colors accurately. The
CRI value comes from a comparison of the lamp's
spectral distribution to the standard (e.g. a black
body or the daytime sky) at the same color temperature.
The higher the CRI the more natural and vibrant the
colors will look. A bulb with a CRI of 85 or higher
is excellent being that the sun has a CRI of 100. Eye
Hortilux makes 90 -92 CRI bulbs that are used in
aquarium, horticulture and other applications.
Color Shift and Variation
Different colors are produced
in metal halide lamps by using various
arc tube shapes and metal halide salts.
In new lamps these halides need to "burn-in" for
approximately 100 hours before they reach
their optimum color. This is why new
lamps can sometimes be unstable or vary
in color.
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