NOTE: Originally published in January of 2011, this article is still relevant, but a few things have changed. It was updated on April 6, 2016.
The recent explosion in LED based lighting products (hereafter referred to as SSL) has resulted in confusion amongst those not familiar with the photometric testing procedures necessary for such products. However, before discussing absolute photometric principles and why they are used, it is useful to describe the photometric testing process that most lighting professionals are familiar with when using lighting design software. This is a photometric file produced by the relative photometric testing process.
In relative photometric testing, the lamp (HID, fluorescent, incandescent) is tested separately prior to testing when installed in the luminaire as a “system” to determine its total luminous flux (lumen output). There are strict guidelines for lamp testing that are not critical to this discussion, although it is worth mentioning that photometric testing facilities typically maintain a stock of commercial lamps that are properly seasoned (per IES LM-54-1999 standard) so that their light output is reasonably constant.
After lamp testing, the lamp manufacturers published lumen output for the reference lamp can be divided by the measured output to determine a “Lamp Factor”. This factor is then applied to all subsequently measured luminous intensity values (candela) to normalize the output of the luminaire (lamp + fixture system) to published lamp manufacturer data. The result, when output to an electronic file, is what we commonly see in an IES standard (LM-63) photometric report. The lamp manufacturers published lumen value is the basis for the luminous intensity data (candela) contained in the report. This lumen value can be altered in application software such as AGi32 to scale the output of the luminaire for different published lamp lumen ratings.
Relative luminaire testing procedures for HID, fluorescent and incandescent luminaires are described in detail in IES documents such as LM-46-04: IESNA Approved Method for Photometric Testing of Indoor Luminaires Using High Intensity Discharge or Incandescent Filament Lamps.
Why SSL products are different
The field photometric performance of an SSL source is dependent on its integration with the fixture housing for thermal, optic and driver reasons. Of primary concern is heat generated by the SSL module when installed in the fixture housing, as it may substantially degrade photometric performance. For many SSL products, the light source and the housing are an integrated unit and cannot be separated. Additionally, output of SSL sources are typically less consistent than conventional sources. This may necessitate the need for more frequent product sampling for verification of product performance.
The need for Absolute Photometry
Unlike relative photometric testing, the absolute photometric testing process does not test the source independently from the system (source + housing). There is no “Lamp Factor” to be applied to normalize the output to the source to manufacturers published lumen output. The SSL source is completely dependent on its function within the system to be accurately measured.
The absolute testing process measures the luminous intensity data for the system and reports this data as an absolute, unconditional measurement. The test data cannot be accurately scaled for different SSL source performance. The indication that the photometric testing process has been performed using the absolute method is the presence of the negative one (-1) in the lumen output space of the electronic photometric data file.
IES document LM-79-08: Electrical and Photometric Measurements of Solid State Lighting Products, directs that all SSL product photometric testing be done using the Absolute Photometry method.
How software responds
While we cannot speak for all software, Lighting Analysts application software programs AGi32 and ElumTools (Revit Add-in) will load absolute photometric files and utilize them for calculation of Illuminance or Luminance on a point-by-point basis in exactly the same manner as a relative photometric file. The difference is that the output of the SSL luminaire tested by the absolute method is fixed. It cannot be altered to resemble photometric performance of any SSL source other than the LED module that was originally tested.
Ramifications of using absolute testing with some common lighting calculations:
- Luminaire Efficiency – Since there is no total luminous flux rating of the source external to the system, the calculation of efficiency is typically expressed as N.A.
- Rated lumens – There is no lumen rating, it is shown as -1 per IES LM-63-00.
- LER – is based on “luminaire lumens” and not “lamp lumens”.
- Coefficients of Utilization (CU) – are based on “luminaire lumens” and not “lamp lumens”.
- LCS/BUG rating – are based on “luminaire lumens” and not “lamp lumens”.
Can the test report lumens be changed?
Due to confusion by software users upon discovering the negative one (-1) value in the software lumen cell, some manufacturers have elected to assign a lumen value to the photometric test report after the fact. By using the total zonal lumen output of the luminaire (luminaire lumens) as a lumen value, some software issues common in older program versions can be avoided, eliminating user questions. Unfortunately, any IES LM-63 report for an SSL product with a non-negative lumen value is INVALID and does not follow industry standards established by LM-79-08. End users encountering such data should contact the manufacturer and request a copy of the original test report as produced by the photometric laboratory. The negative one in the lumen cell should send a signal to all users of “Hands Off”.