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Setting up an LM-79 test lab: preliminary testing done in-house can get products to market quicker.

The LM-79-08 (Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products) testing protocol established by the IES is widely used to certify LED-based luminaires, and is required for compliance with the Design Lights Consortium, LED Lighting Facts and Energy Star programs in the U.S. Although lighting manufacturers must utilize independent laboratories for LM-79 certification (because the IES does not allow self-auditing), it is still sometimes advantageous for manufacturers to perform their own preliminary LM-79 testing in-house.

This in-house testing can help manufacturers produce more optimized designs and reduce their time-to-market. With rapid in-house testing having essentially no incremental cost, lighting developers can retest a design as frequently as they wish. The effect of even the smallest design change can be precisely quantified, leading to superior results.

LM-79 provides guidance for low uncertainty measurement of luminous flux, electrical power, luminous intensity distribution and chromaticity. The most common photometric measurement systems used to perform measurements in accordance with LM-79 utilize an integrating sphere, a goniometer or both. Since more than one testing approach is allowed by LM-79, there are trade-offs to be considered in selecting the proper equipment when planning for a new photometric lab. Smaller companies may not have the budget for a full-blown LM-79 photometric lab, so they might build out their lab in stages, as budgets become available for expansion.


For companies on a limited budget, the greatest value is achieved by having in-house measurement capabilities for luminous flux and chromaticity. These are measured most conveniently using an integrating sphere and spectroradiometer. While LM-79 does allow use of a photometer, which is a less expensive alternative than a spectroradiometer, this configuration provides no chromaticity data.


The integrating sphere itself is a hollow sphere with a white, highly diffuse, reflecting coating on its inner surface. Light emitted from a source placed inside the sphere undergoes numerous reflections, such that it becomes completely spatially homogenized (Figure 1). A small port in the sphere surface allows this averaged light to be sampled. A properly calibrated integrating sphere and spectroradiometer system can be very fast and highly accurate, for lamps and luminaires up to a certain size limit.

It is important that the sphere diameter is sufficiently large for the size of the lamps and luminaires that will be tested. If the device under test is too large relative to the sphere, the sphere may not be able to completely homogenize the light, compromising measurement accuracy. LM-79 provides sphere sizing guidelines. Typically, for 4n measurements, a sphere of 1 meter diameter or larger is used for compact lamps (size of typical incandescent and compact fluorescent lamps), and 1.5 meter or larger for larger lamps (e.g., size of 4-ft linear fluorescent lamps and HID lamps). LM-79 also includes guidance on maximum port dimensions for 2[pi] measurements.

The integrating sphere output is fed into a spectroradiometer, which performs the actual quantitative measurement of light output and spectral content. A range of commercial spectroradiometers are available which vary widely in terms of cost and capabilities. For the buyer, the two most important specifications are usually speed and accuracy. There's always a trade-off between performance and cost, but trying to use a low-accuracy spectroradiometer can result in unanticipated costs. Accuracy is critical because data produced by the lab must ultimately correlate with data from independent test laboratories, and system errors in terms of spectral purity and wavelength (color) or amplitude (flux) can be costly if third-party testing has to be repeated due to unexpected failures. Use of a spectroradiometer that allows field-calibration from accurate flux standards is critical to long-term accuracy. Measurement speed is important as well, as this will directly affect the throughput of the lab and the time frame in which the lab will generate a return on investment.


A goniometer pairs a light detector with a motorized system for rotating a source around its center point in two axes, allowing light output to be measured as a function of angle (the spatial power distribution). The proper goniometer type must be utilized, to support the required coordinate system used in industry-standard data file formats. LM-79 calls for the use of Type C moving mirror (or moving detector) goniometers. However, these goniometers are expensive and require a large lab space for accurate results. New goniometer types, such as the so-called "Type D" goniometer (Figure 2), move the lamp under test relative to a fixed detector to capture the relative luminous intensity values at different angles.


These goniometers are lower cost, require less space and can produce the same level of accuracy as a true Type C goniometer, especially when an auxiliary photometer is used to correct for variations in output at different physical orientations of the lamp. This type of goniometer is allowed under the newly released CIE S 025 standard for SSL testing, which is intended to be released as a harmonized, global SSL test standard in the future. Type D moving source goniometers work well with LED sources, but cannot be used for discharge lighting sources that are orientation-sensitive, such as HID and fluorescent.

The goniometer can be paired with either a photometer (goniophotometer), for angular luminous intensity measurements only, or with a spectroradiometer (goniospectroradiometer) if angular color information is required. Photometers have a faster measurement speed, because a spectroradiometer may require a significant integration time in order to deliver adequate signal-to-noise color data, particularly when measuring lower light levels (which becomes a problem at the edges of the luminaire radiation field). For this reason, companies will often choose to use a spectroradiometer with an integrating sphere, but will use a photometer with their goniometer to obtain the spatial power distribution.


Whether considering an integrating sphere system or goniometer, the importance of the system software should not be overlooked. The software should seamlessly integrate all of the system components, including ancillary components such as precision power supplies, power analyzer, thermo-electric cooling systems and temperature monitor, and the software should produce the required output file formats and reports. The software should also facilitate use of light standards' calibration files, support transfers of calibrations and streamline procedures for self-absorption correction. The software will often be the primary contributor to the system's ease of use and overall utility.


Instruments and entire testing setups are readily available for any luminaire manufacturer that wants to conduct their own LM-79 testing. Companies with budget constraints can take a phased approach to the transition to in-house testing by evaluating their specific needs and determining where to make initial investments. By working with an equipment supplier that offers the right level of performance through modular system solutions that can be upgraded over time, companies can better manage their capital investments and deliver the best return on their investment.

Jonathan Lipscomb is an electro-optical engineer at Gamma Scientific, where he is responsible for in-house, third-party testing of customer supplied systems for IES accreditation, as well as system characterization testing to FDA and CalTrans specifications.
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Title Annotation:PRODUCT TESTING
Author:Lipscomb, Jonathan
Publication:LD+A Magazine
Date:Aug 1, 2016
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