Can we modify IES files for LED products?

March 8, 2011

Let’s start by specifying that in photometry, the way to test the LED fixtures differs from the HID lamps. While HID lamps are tested in a relative way, the LED lamps are tested in an absolute way. IES files absolutely prevent us from changing the lumens as we do so with the HID lights.

For cost reasons, manufacturers cannot test all the combinations of product-power-optical system. But there has been a big debate within the industry for several months now as to whether we can modify the IES files of LED lamps in order to obtain different powers (other than the ones tested).
 

For decades now, in order to get IES files, we test a power in the lights so that we can then use the same file for different powers. For example, we test a light of 100W and we use it afterwards for 70 W and 150W by changing the lumens in the lighting software.
 

Since the advent of LED lighting and absolute testing, it is impossible to directly modify the lumens. To do so, we need to change the candela multiplier inside the file itself. Some people think that we should never use a modified file because LEDs are very sensitive to heat (something that varies depending on powers) and that the current used will affect the performance of the luminaire.
 

It is also important to mention that the lack of standard from IESNA gives way to a major debate on the appropriateness of using IES modified files for LED products. Some unscrupulous manufacturers do not hesitate to increase the performance of their luminaires by changing the files. As a result, many clients have lost confidence in these files and ask for the results of photometric tests done in independent laboratories.
 

With all that in mind, the question we need to ask ourselves should not be “Can we edit IES files of LED products?” but rather “How should we modify IES files?”
 

It is no secret that no manufacturer has the financial means to test the photometry of all these light fixtures with all the powers and optical systems available on the market. Such as no manufacturer has been able to afford to do so for HID lamps for that matter.
 

In that perspective, a very simple rule should be applied to modify IES LED files: always start from a higher power to generate an IES file of a lower wattage. By doing so, you won’t be faced with a thermal problem: a lower power of light will always generate less heat than a higher power. This way, the lumens of the modified file will not be higher than it is supposed to in reality. Furthermore, as the lights of lower power are more efficient in lumens per watt, when using them in such a way, the reality will be slightly higher than the modified file. And in the end, the client, the end user, will not be fooled or even cheated.
 

By changing the candela multiplier, we just do what the lighting software does when we change the lumens of a HID file, which is apply a factor at all candelas of the IES file.
 

A final detail which is also crucial to know: a modified file should always be clearly identified as such.