Introduction

Artificial lighting would not be required if our buildings were not occupied or visited by human beings. The purpose of lighting installations is to allow people to perform physical or visual tasks, and the quantity and the quality of the lighting system is very important. It should be noted that lighting is an extremely topical subject and ‘Correct’ lighting requirements are hard to define due to the extremely high psychological effect that lighting has on human beings. One of the basic terms used in lighting by designers, engineers, contractors and sales people is the “Lumens” of the light fitting.

Lumen

Lumen is the measurement of light produced by a light source or lamp. The number of lumens quoted is the amount of light that is given off in all directions from the lamp. When we compare two lamps we compare how many lumens they produce. E.g. a 100 watt incandescent lamp produces 1200 lumens. Similarly, a 20 watt compact fluorescent lamp also produces 1200 lumens.

Typically, the lumen output of a lamp is set by the manufacturer and tested via the agreed international testing method. Once the lamp lumen output has been determined, then the lamp is installed in a luminaire (light fitting) and then it is tested again to see how efficient the light fitting is. The test of the luminaire determines how many of the lamp lumens are used outside the fitting. The ratio of the original lamp lumens compared to the lamp lumens used outside the luminaire is known as the Light Output Ratio (LOR).

In a number of luminaires such as highbay luminaires and floodlights, only around 70% of the original lamp lumens are used outside the luminaire. In the new age of LED luminaires, the process to determine the lumen output of a single chip and that of a luminaire are different. The lumen output of a chip in open air and when it is first turned on is known as the ‘flash test lumen output’. Unfortunately some of the lighting industry use this figure as the basis of their luminaire total lumen output. They simply multiply the number of chips installed by the flash test lumen output and quote this as the lumen output of the luminaire. This figure is usually well above the true lumen output of the luminaire.

Testing Procedures

The correct testing procedure (which all LEDified LED luminaires adhere to), is to test the lumen output of the LED’s in the luminaire after it has been constructed and it has been running in the laboratory for a minimum of 30 minutes at the required ambient temperature (usually 25 degrees Celcius). Then only the lumens that are measured outside the luminaire are recorded. This is known as testing in absolute photometry. The current internationally recognized testing procedures are:

LM-79 – Electrical & Photometric Measurements of Solid State Lighting Products

It is the Illuminating Engineering Society of North America (IESNA) approved method for the Electrical and Photometric Measurements of Solid-State Lighting. It measures an LED luminaire or integral lamp as a whole system according to a standard process using specified equipment.

Who can perform testing?
LM-79-08 testing should be performed by by NATA (or international equivalent) laboratories. All LEDified product testing are performed according to LM-79-08 in a NATA recognised laboratory (or international equivalent) and reports are available upon request.

What does it provide?
The testing report issued according to a standard format will provide:

– Total Luminous Flux
– Luminous Intensity Distribution
– Electrical Power Characteristics
– Luminous Efficacy (calculated)
– Color Characteristics (CRI, CCT)

LM-80 – Measuring Lumen Maintenance of LED Light Sources

It is the Illuminating Engineering Society of North America (IESNA) approved standard for measuring lumen maintenance of LED light sources. LM-80-08 apply to the LED package, array, or module alone, not a complete system, it is testing a component level.  The standard does not provide guidance for extrapolation of testing results.

Who can perform testing?
LM-80-08 testing should be performed by NATA (or international equivalent) laboratories. All LEDified products use LEDs with available LM-80-08 test report from recognised laboratories.

What does it provide?
The testing report issued according to a standard format will provide luminous flux for a given current over a 6,000 hours period with interval measurements. Luminous flux will be measure for 3 different LED case temperatures: 55ºC, 85ºC and a third temperature to be selected by manufacturer. Besides, the lumen maintenance, the chromaticity shifts over the measured period.

TM-21 – Projecting Long Term Lumen Maintenance of LED Light Sources

It is the Illuminating Engineering Society of North America (IESNA) approved method for taking LM-80 data and making useful LED lifetime projections. The standards apply to lifetime projection of LED package, array or module alone. The results can then be used to interpolate the lifetime of an LED source within a system (luminaire or integrated lamp) using the in-situ LED source case temperature.

Who can perform testing?
There is not such a thing as testing, it is a mathematical method based on LM-80-08 collected data.
Among other thing, TM-21-11 will consider:
– If total LM-80 data period is between 6,000 and 10,000 hrs, we consider the last 5,000 hours
– If total data period is above 10,000 hours, we use the last half of collected data
– In situ case temperature interpolation using Arrhenius equation between LM-80 temperature
– Projections are limited to 6 times the available LM-80 data period so projected and reported lifetime may or not be the same

What does it provide?
The method will provide a projected lifetime for the LED source or system.
Life notation results will then use the following standardized nomenclature: Lp (Yk)
– P: Lumen maintenance percentage. For LED luminaire we consider L70 to be the standard. After 30% lumen depreciation, we consider the system is not performing its duty anymore and should be replaced
– Y: Length of LM-80 data period in thousands of hours Example: L70 (6k) = 36,000 hours

The lumen (light) output of LED’s also decreases as the LED’s age. The economical lamp life of an LED lamp is rated at a lumen depreciation of 30% (known as the L70 value).

So when dealing with the lumen output stated by the myriad of LED suppliers in the market, you should ask for confirmation that the absolute lumen output of the luminaire has been stated.