How-to Understand LED Light Photometry Data

Here at Environmental Lights, we provide Integrating
Sphere Test Reports for all of our LED Strip light products. These reports include photometry
data generated from scanning the product in our Labsphere illumia plus Light Measurement
System. This system includes a one meter sphere with integrated high end spectrometers, auxiliary
lamps and other sensors for the correction of light absorption to ensure accurate test
results. The test reports include a Chromaticity Chart, photometric measurements, spectral
power distribution and spectral response characteristics. One of the most important measurements for
white LEDs is the Correlated Color Temperature or CCT. The color temperature indicates if
the light appears warm like a candle or cool like bright daylight. CCT is expressed in
degrees Kelvin and means that a black body heated to that temperature would emit that
specific color of light. As a black body gets hotter, the light it emits changes from red
to orange to yellow to white to blue. This black body curve can be seen on the CIE 1931
Chromaticity Chart. (Show the CCT Temp below and black body curve on 1931 chart.) Our white
LED products range from 2400K as very warm white to over 6500K, which is considered to
be daylight white. If the measured LED product is colored, the
peak wavelength is the most common parameter used to indicate the color. UV LEDs are around
400 nanometers and below. Blue, green and red LEDs are around 463, 520, and 630 nanometers
respectively. Luminous flux is a measure of luminosity,
which is the perceived brightness of light. Luminosity reflects the differences in sensitivity
of the human eye to various wavelengths of light along the electromagnetic spectrum.
The test reports show lumens per test sample length, which is listed at the end of the
report. For example, most of our LED strip lights are measured in the Labsphere with
0.15 meters exposed inside the sphere. In order to calculate the estimated luminous
flux per meter, simply take the luminous flux value you see on the test report and divide
by 0.15. Color Rendering Index, or CRI, is an industry
standard method of measuring the quality of a light based on the ability of the light
being tested to recreate eight colors. The CRI value is calculated by averaging the chromaticity
of eight wavelengths and the results of testing each color is listed as R1-R8 on the test
report. Ratings generally range from a perfect 100 to zero, but can be negative for colored
LEDs. Colored LEDs are not expected to have high CRI because rendering colors accurately
is not their purpose. They typically focus on rendering only one color well.
Color Quality Scale, or CQS, is a scale to measure light quality that has been developed
by the National Institute of Standards and Technology. Although not yet an industry standard,
it is intended to be an improvement on the current CRI measurement. It averages the chromaticity
of fifteen wavelengths (rather than the eight of CRI). 100 is the maximum, and negative
ratings are possible although not common for white lighting. Each R1-R14 refers to a specific
color and the ability of the light being tested to recreate them. R1-R8 are used in generating
the overall CRI, but R9-R14 are also useful. We have provided both CRI and CQS values in
our test reports for your reference. For any additional questions on how to interpret
Integrating Sphere Test Reports, please feel free to give us a call. We would be happy
to help.

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