Key Criteria to Evaluate LEDs for your Next Lighting Design

LED lamps, manufactured by many companies, are now widely available for use. The engineers who implement them need to know about and compare specifications to make sure they meet the performance parameters required for their next design. The successful implementation of LED technology requires an intimate understanding of these performance metrics (as well as of the many industry standards for solid-state lighting; see also the TechZone article “The Latest Testing Requirements for Solid-State Lighting”).

This article will review LED and luminaire specifications that are likely to appear on a datasheet and explain the criteria you need to consider when evaluating different manufacturers’ claims.

For LED modules, the International Electrotechnical Commission/Publicly Available Specifications (IEC/PAS) recommends that designers look at the:

1. Rated input power
2. Rated luminous flux
3. LED luminaire efficacy
4. Luminous intensity distribution
5. Photometric code
6. Rated color rendering index (CRI)
7. Correlated color temperature (CCT)
8. Rated chromaticity co-ordinate values
9. Lumen maintenance code
10. Rated life in hours
11. Failure fraction corresponding to the rated life of the LED module
12. Ambient temperature for a luminaire

Here are some short definitions to help you understand the criteria when you are reviewing datasheets for LEDs in your next design:

a) Rated input power
The rated input power shows the amount of energy consumed by a luminaire, including its power supply. It is expressed in watts.

b) Rated luminous flux
It corresponds to the light emitted by the luminaire which is expressed in lumen (units of light output).

• For traditional (non LED) luminaires it is not very common to measure and publish the rated luminous flux. This is normally calculated as the lamp flux multiplied by the light output ratio (LOR) of the luminaire.
• To make a technical comparison between ‘traditional’ and ‘LED’ luminaires, it is recommended to take the actual application into account and compare both lighting designs.

c) LED luminaire efficacy
The measured initial luminous flux divided by the measured initial input power of the same individual LED luminaire. It is expressed in lumens per watt.

d) Luminous intensity distribution
The spatial distribution of the luminous flux graphically depicted in a luminous intensity distribution curve, which is usually expressed in a polar coordinate diagram representing the light intensity as a function of angle about a light source. It is expressed in cd = lm × sr-1.

e) Photometric code
A six digit photometric code that displays the important ‘quality of light’ parameters: CRI, CCT, chromaticity co-ordinates, and luminous flux.

f) Rated Color Rendering Index (CRI)
The color rendering of an LED module giving white light is the effect on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.

g) Correlated Color Temperature (CCT)
The color temperature of an LED module giving white light is determined by comparing the light emitted by the LED module with light of an ideal black-body radiator at the given temperature. It is expressed in Kelvin.

h) Rated chromaticity co-ordinate values (both initial and maintained).
The behavior of the chromaticity co-ordinates of an LED module expressed in two measurement results of both initial and maintained chromaticity co-ordinates.

i) Lumen maintenance code
The measured initial luminous flux (initial value) is normalized to 100% and used as the first data point for determining the LED module life. The maintained luminous flux (maintained value) is measured at 25% of rated life-time up to a maximum of 6.000 hours, and expressed as percentage of the initial value. The maintained value determines the lumen maintenance code.

j) Rated life of the LED module and the associated rated lumen maintenance (Lx).The length of time during which a population of LED modules provides more than the claimed percentage (x) of the initial luminous flux, always published in combination with the failure fraction. It is expressed in hours.

k) Failure fraction (Fy), corresponding to the rated life of the LED module in the luminaire The percentage (y) of a number of LED modules of the same type at their rated life designates the percentage (fraction) of failures. This failure fraction expresses the combined effect of all components of a module including mechanical, as far as the light output is concerned. The effect of the LED could either be less light than claimed or no light at all.

l) Ambient temperature (tq) for a luminaire.
The ambient temperature around the luminaire related to the specified performance. For a given performance claim the ambient temperature (tq) is a fixed value. It is possible to specify performance claims at different ambient temperatures. It is expressed in degrees Celsius.

Real world examples

Let us now look at a datasheet to see what information it provides. The Cree XLamp ML family of LEDs offers information on Luminous flux range (10.7 to 13.9 lumens in “F” group code) and extend to “N3” group code with luminous flux from 56.8 to 62 lumens. CCT range for cool white extends from 5,000 to 8,300 K. The typical CRI for cool white is 75. You can also get information on relative flux versus junction temperature (Fig. 1). Other important pieces of information include how to get the longest life from the LED in your designs. There is also a link to an application note to find out how thermal design, ambient temperature, and drive current affect the LED junction temperature and lifetime.¹

Cree provides an excellent description of how to select LEDs from various bins and kits. “All XLamp LEDs are tested and sorted by color and brightness into a unique bin. Each bin contains LEDs from only one color and brightness group and is uniquely identified by a bin code. White XLamp LEDs are sorted by chromaticity (color) and luminous flux (brightness). Color XLamp LEDs are sorted by dominant wavelength (color) and luminous flux (brightness). LEDs are shipped on reels containing LEDs from one bin and are always labeled with the appropriate bin code.”

In summary, lighting engineers need to take the time to research details in the datasheet to find the right LED for their project. It is also a good idea to look for application notes and other reading material to discover the nuances of the LEDs you are considering. To further aid you in this effort, the IEC has published IEC/PAS 62717 performance requirement document for LED modules,² as well as the IEC/PAS 62612 for LED lamps self-ballasted for >50 V.³

For more information on the products mentioned here, use the links provided to access product pages on the DigiKey website.

References:

1. Cree Applications Note Xlamp long-term lumen Maintenance
2. Federation of National Manufacturers Association
3. IEC 62610