LED tube lights, designed as replacements for T8/T10/T12 fluorescent bulbs, represent a significant portion of the growing commercial LED lighting market. According to the United States Department of Energy (DOE), commercial forces are driving the development of LED technology, which is expected to represent 36 percent of luminaire sales for the general illumination market by 2020.

Considerations in Measuring LED Tube Lamps

Spectroradiometers like this one are frequently used as the measurement engine for LED testing.
The rise in development and production of LED tube lights has led to an increasing need for testing methods specifically tailored towards these lights. Typical LED tube lights are two feet to four feet in length, and require special instrument configurations to accommodate their large size.

LED manufacturers perform testing of their lights during the initial research and development phase and throughout the production process for quality assurance. Manufacturers are primarily concerned with measuring the brightness of the lamps, the color of the light produced, as well as their efficiency.

Brightness is calculated in terms of lumens, a derived unit of total flux that measures the total amount of visible light emitted by a source as perceived by the human eye. From this measurement, knowing information about the electrical input, the efficiency of the lamp – or luminous efficacy – can be derived in terms of lumens per watt.

Other important metrics of LED lamp measurements are the Correlated Color Temperature (CCT) and Color Rendering Index (CRI). Correlated color temperature is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source. The value is reported in degrees Kelvin. Warm white lamps actually have lower color temperatures than cool white lamps.

Open view of a 2-meter integrating sphere.
To give you an idea of the range of CCT’s, incandescent and warm white fluorescent lamps have a CCT around 3000 K, cool white and daylight compact fluorescent lamps have a CCT of about 5000K, and daylight is about 6500 K. Color rendering index measures the ability of a light source to accurately render all frequencies of its color spectrum when compared to a perfect reference light of similar type.

LED Measurement Systems

Light measurement system components for testing LED lamps consist of a measurement engine, an optical collection system, power supply and software. The overall quality of the system depends on the quality of the measurement engine, the type of optical system used, and the accuracy of the system calibration.

A variety of light measurement systems present alternate methods for performing LED testing of tube lamps. These methods include using integrating sphere systems, goniometer systems and optical integrating tubes for measurement. Each instrument has unique benefits and limitations. Frequently the decision on choosing an appropriate system will be determined by your testing application, required measurement properties and budget. System cost and footprint are significant considerations when building a test system, especially for LED tube lamps which are often four feet in length or longer.

Spectroradiometers

Integrating sphere vs. Integrating tube comparison.
Spectroradiometers are frequently used as the measurement engine for LED testing because they are the most accurate instrument for capturing spectral energy distribution of light sources. Spectroradiometers are used to determine radiometric, photometric and colorimetric quantities.

CIE Tristimulus values are calculated mathematically and used to compute CIE chromaticity coordinates. From these values the spectroradiometer can provide comprehensive color characterization that includes luminance, illuminance and spectral power. Accurate LED color measurement relies on spectroradiometers ability to deliver good spectral resolution, high dynamic range and excellent stray-light rejection.

Integrating Sphere Systems

Integrating spheres are the most common type of optical collection system used for LED testing. Integrating spheres excel at uniform collection of light, leading to applications such as measuring the total energy output and colorimetric properties of LED lamps and luminaires when paired with a spectroradiometer, power supply and light measurement software.

In the most general terms, an integrating sphere consists of a spherical shell, with a diffuse white, highly reflective, optical coating on its inside surface. Ports or apertures in the sphere wall admit or release light, which undergoes multiple diffuse reflections within the sphere cavity. Such reflection serves to spatially homogenize the light passing through the sphere, attenuating the optical signal significantly in the process. Typically, one or more baffles within the sphere cavity are positioned strategically to shield system components from undesired direct illumination, thereby enhancing sphere performance.

Integrating sphere systems provide an efficient way to measure total luminance flux and correlated color temperature (CCT) of LED tube lamps with relatively low uncertainty. Setup and measurement times are relatively quick, allowing for large throughput. Integrating spheres systems are also used to derive luminous efficacy and determine the efficiency of LED lamps.

Gamma Scientific goniometer.
However, there are limitations to using an integrating sphere for LED tube lamp testing. Since most replacement LED tube lamps are four feet in length they require a large integrating sphere that is at least two meters in diameter to accommodate the lamp for testing. A two meter integrating sphere requires a sufficiently large area for staging tests and is often expensive. Integrating spheres also cannot provide spatial light distribution measurements that are captured with a goniometric system.

Goniometer Systems

Goniometer systems are designed to analyze angle dependent spatial radiation properties of LED lamps, luminaires and modules. Since the spatial distribution of light is not homogeneous with LEDs, goniometers provide an effective method to capture full spatial distribution measurements. Goniometric systems typically provide the lowest uncertainty total flux measurements for LEDs because the system makes flux density measurements at many points in space.

For LED tube lamp testing a goniometric system is comprised of a spectroradiometer, goniometer, power supply and light measurement software. A goniometer system will provide measurements of spectral radiant intensity as a function of angle. From the measured angular distribution it can derive viewing angle, total included angle, partial flux and zonal flux.

Goniometric system software will provide control and data acquisition for all system components, synchronizing the spectroradiometer, goniometer and specified LED power supplies. From the spectral distribution measured at each angle, software automatically derives luminous intensity (cd), radiant intensity (W/sr), color values and full spectral characteristics. Spectral characteristics include: chromaticity, correlated color temperature (CCT), color rendering index (CRI), dominant wavelength and purity, peak wavelength, spectral bandwidth (FWHM), centroid wavelength and center wavelength.

Although goniometric systems are beneficial for LED testing because they provide full spatial distribution measurements, there are also drawbacks to using goniometers. Goniometer systems often take much longer to perform LED measurements because the system is recording dozens, if not hundreds of measurements to fully characterize the spatial distribution required to obtain accurate luminous flux measurements. Due to their complexity goniometric systems also require highly skilled operators, and like large integrating spheres, they can be costly.

Optical Integrating Tube Systems

Optical Integrating tube system with the lamp on.
An optical integrating tube is a new instrument specifically designed for testing LED replacement lamps. Like an integrating sphere or goniometer, the integrating tube couples with a spectroradiometer, power supply and light measurement software to form an LED test system. The system is referenced to a two meter integrating sphere and displays similar levels of precision.

Integrating tube systems are used to determine spectral and colorimetric measurements for LED tube lights up to four feet in length. The system measures total flux, chromaticity, correlated color temperature (CCT), color rendering index (CRI), peak wavelength and dominant wavelength.

The unique cylindrical design means that the entire length of the lamp is always the same distance from the integrating surface during testing, and therefore performs well at integrating the light emitted from the lamp.

The primary benefits to using an optical integrating tube are its size and cost relative to a two meter integrating sphere or goniometric test system. The integrating tube has a portable, table-top design (67.5" L × 13" H × 13" W) and costs significantly less than comparable systems for LED tube lamp testing, while still being referenced to two meter integrating sphere measurements.

However, like other systems for LED measurement, integrating tube systems also have constraints. While these instruments are specially designed for measuring tube lamps, they do not have the same flexibility that an integrating sphere or goniometric system possesses for testing different types of LED lamps and luminaires. But for LED tube lamps, it offers very repeatable, low-uncertainty measurements in a compact design.

LED tube lights, designed as replacements for T8/T10/T12 fluorescent bulbs, represent a significant portion of the commercial LED lighting market. Due to their longer life, increased efficiency and ecological benefits, the market for LED lamps, and demand for accurate test solutions is expected to continue growing. With a number of acceptable alternatives available in the market today for LED replacement tube testing, the decision on choosing an appropriate system will be determined by your testing application, required measurement properties and budget.

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Lighting Technology Magazine

This article first appeared in the July, 2013 issue of Lighting Technology Magazine.

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