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How LM-79 testing can help evaluate LED luminaire quality & performance
Feb 23, 2017

How LM-79 testing can help evaluate LED luminaire quality & performance

LM 79 is the Illuminating Engineers Society North America (IESNA) approved testing method to generate electrical and photometric measurements of solid state lighting (LED) products of 400w led flood light.

What is the LM-79 test methodology?

LM 79 is an approved method that describes the procedure to be followed and precautions to be observed in performing reproducible measurements of total Luminous flux, electrical power, luminous intensity distribution, chromaticity of Solid-State lighting products for illumination purposes under standard conditions.

How is LM-79 different from other test methods used for conventional lighting products?

Due to the unique thermal and electrical characteristics of LEDs, standard test methods using relative photometry, cannot measure the lumen output of LED light sources. The LM-79 test remedies this problem by using absolute photometry. (see below for clarification)

What is relative photometry?

Relative photometry is used for testing standard filament or fluorescent lamps. Lamps and ballasts are tested separately from luminaires. The final output is the luminaire efficiency, calculated by dividing the total lumen output of the luminaire by the product of the rated output of the lamps and the ballast factor.

Relative photometry reduces the volume of luminaire tests, as the specifier can mimic lamp data from existing tests.

What is absolute photometry?

Absolute photometry is used to directly measure the total lumen output of complete lighting systems including lamp, ballast, driver, thermal management and all other fixture components. The efficiency of an integrated LED luminaire cannot be calculated in a standard manner. Instead luminous efficacy is calculated by dividing the total light output produced by total input electrical power.

What are the LM-79 test outcomes ?

IESNA prescribes the following uniform parameters to present the data obtained during LM-79 testing.

Photometric Properties

Electrical Properties

Luminous flux (lumens)

Input RMS AC voltage

Luminous efficacy (lumens/W)

Input RMS AC current

Luminous intensity distribution (candelas)

Input AC power

Chromaticity coordinates

Input voltage frequency

Correlated colour temperature (CCT)

Power factor

Colour rendering index (CRI)

Similar for DC powered SSL

What are the limitations of LM-79?

As we all know about the strength of LM 79. Everything has its pros and cons. Let us have a look on the drawbacks of LM 79.

LM 79 depicts the parameters of the individual products. So the obtained Parameters cannot be used to access alike products.

Other limitation is the sample size. It does not state any sample size. So, the manufacturer provides only one sample instead of supplying more. This thus increases the vulnerability of lesser quality products.

This can be used only to determine complete luminary instead of LED used ( as based on absolute photometry) , which can produce different results due to quality of accessories used like glass, thermal effect, lenses etc.

LM-79  Scope

Applies to LED-based products incorporating control electronics and heat sinks: – Products requiring only line voltage or DC power supply – Includes complete LED luminaires and – Integrated LED products (LED chips with heat sinks) .

Does not cover – LED products requiring external operating circuits or heat sinks (bare LED chips, pkgs, and modules) – Fixtures designed for LED products but sold without a light source.

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