Philips Hadco-Information Brief Absolute and Relative Photometry

7/28/2019 Philips Hadco-Information Brief Absolute and Relative Photometry

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Information Brief

Information BriefRelative and Absolute Photometry (May 2011, Rev. 3) Page 1 of 3

Understanding Relative and AbsolutePhotometry

Photometry is the measurement of radiation in terms of human visual response1; in other words, it is themeasurement of light. As technology has evolved over the years so have photometric measurement methods.LEDs or Light Emitting Diodes are at the forefront of the digital lighting revolution; therefore, a greaterunderstanding of photometry and the appropriate measurement method for this growing technology is required.To ensure accurate and precise photometric measurements of our entire range of productsparticularly LEDluminaireswe use a custom designed photometric laboratory that complies with national standards.

Test methods and equipmentThere are different test methods and equipment that can be used to perform photometry. One device is agoniophotometerthat measures the light output of sources (such as bare lamps) and luminaires.2 Agoniophotometer uses 3-D spherical photometric coordinates to define a web of photometric data surroundingthe luminaire being tested.3 Think of goniophotometric measurements in terms of a globe. See Figure 1. Theluminaire rotates about a vertical polar axis that is similar to our Earths north-south polar axis. Photometric datais collected at angles in horizontal (H) and vertical (V) planes to identify location on the spherical web similar tohow we use lines of latitude and longitude, respectively, on a globe.

Figure 1. Spherical globe and photometric coordinate systems.

Illustration on the right courtesy of the IESNA Lighting Handbook, 9 th Edition.

Polar Axis

North Pole

South Pole


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Goniophotometers and coordinate types are normally divided into three categoriesTypes A, B, and C. Type Aphotometry is typically used for automotive lighting; therefore, it will not be discussed. Type B photometry istypically used for floodlight, bullet and ingrade / inground luminaires. Type C photometry is typically used forroadway, post top and bollard luminaires. The differences between Types B and C include how the luminaire ismounted to the goniophotometer, test parameters, and the resulting photometric data. See Figure 2.

Figure 2. Type B (left) versus Type C (right) photometry.Illustrations courtesy of the IESNA Lighting Handbook, 9th Edition.

Custom designed photometric lab

Our entire photometric lab is custom designed from the room itself to our custom built moving mirrorgoniophotometer. See Figure 3. Our custom system enables precise testing of a wide range of luminaires withnumerous sizes and shapes. While the luminaire spins 360 about the fixed polar axis, the mirror rotates 180around the luminaire, reflecting the luminaires light output to a stationary photodetector positioned at the back ofthe lab a fixed distance away from the luminaire s lamp center. Stray light inside the photometric lab is cancelled

so the photodetector only records the light output coming directly from the luminaire via the mirror.

The end result: A 3-D globe of light measurements saved as an .ies photometric file.

Figure 3. Our moving mirror goniophotometer (left) and photodetector (right).


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Our environmentally controlled custom lab is compliant to ANSI (American Nationals Standards Institute) andNIST (National Institute of Standards and Technology) standards and it undergoes annual review. Our labpersonnel are trained to perform photometric measurements per ANSI / NIST standards and IESNA procedures,and they also undergo annual review. The lab is recognized to participate in ETLs Satellite Program. We submitkey luminaires to certified independent photometric labs as a validation of our lab, procedures and personnel.

With this oversight and recognition, we ensure repeatability, accuracy, and precision of our .ies photometric files.

Proper photometric testing of LED luminairesWe test luminaires using one of two methodsrelative or absolute photometry. In relative photometry therelative total lumen output of the bare test lamp is measured at the time of test. Then the lumen output of theluminaire with the test lamp installed is measured and the luminaires relative efficiency is calculated with respectto the test lamp4. This is the total efficiency (net) reported in the .ies photometric file. In absolute photometrythe total absolute lumen output (gross) of the luminaire with lamp installed is measured. 5 Thus, lamp lumens andefficiency are not applicable and cannot be calculated or reported.

We test LED luminaires using absolute photometry. Like all solid state electronic devices, LED performance isimpacted by heat. Too much heat affects light output and life, so we design heat sinks to effectively control heat.

For more on this topic see the Information Brief LED Thermal Management. Often the luminaire itself is animportant part of the total heat management solution. If you remove the LED light engine from the luminaire, youtake away part of the heat sink. As a result, the LEDs will overheat and not perform as desired. Therefore, wetest LED light engines in-situ, meaning while they are installed in the luminaire using absolute photometry. This isalso compliant with IESNA LM-79-08 Approved Method: Electrical and Photometric Measurements of Solid-State LightingProductsfor more on this topic see the Information BriefLM-79 and LM-80.

SummaryLED lighting provides dramatic impact and energy savings. To optimize performance and put the light you needwhere you want it, make sure to use photometric files derived from the proper testing method with the rightequipment. Ensure that LED luminaires are tested using absolute photometry per LM-79 to maintain the integrityof the thermal system and thereby provide optimum performance.

References

1. Rea, M.S. (editor), IESNA Lighting Handbook,9th Edition, New York: Illuminating Engineering Society of NorthAmerica, 2000.

2. Ibid.

3. IESNA Testing Procedures Committee, Goniophotometer Types and Photometric Coordinates, LM-75-01, NewYork: Illuminating Engineering Society of North America, 2001.

4. IESNA Testing Procedures Committee, IESNA Approved Method for Photometric Testing of Floodlights Using HighIntensity Discharge or Incandescent Filament Lamps, LM-35-02, New York: Illuminating Engineering Society of NorthAmerica, 2002.

5. Ibid.

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Philips Hadco-Information Brief Absolute and Relative Photometry