O P T I C A L M E T R O L O GY
Characterization of LEDs
Improving quality through LED characterization PHOTOMETRY AND COLORIMETRY IN LED TECHNOLOGY Cabin lighting in airplanes, status displays in smartphones or state-of-the-art LC TV backlighting would not be possible today without the use of LEDs. Fine nuances in color coordinates or color temperature make a product appear to be of lower quality – there is no tricking the human eye here. HELGE BRÜGGEMANN
he combination of long service life, extreme mechanical resilience and the expectation of high light yield are unique to LEDs. In addition to the 3 mm LEDs that are produced by the billion and the extremely bright 400 mA half-space light sources, organic LEDs are also rising in popularity. Due to the eye’s extreme color perception, there is good reason why the requirements on color similarity for bundled LEDs are very high. For high intensity lighting or for the illumination of large
T
CONTAC T Laser 2000 GmbH 82234 Wessling, Munich, Germany Tel. +49 (0)8153 405-0 Fax +49 (0)8153 405-33 www.laser2000.de
28
Laser+Photonics
spaces, only LEDs with similar emission and color characteristics can be combined effectively. This not only applies to color LEDs that generally exhibit a temperature dependent center wavelength, but also to white LEDs, where an irregular phosphorous coating can result in yellow or blue tints (Figure 1).
Quality assurance with photometry and colorimetry Photometry puts the electromagnetic radiation measured by a detector in relation to the visual impression of the human eye. It measures the eye's light perception based on brightness sensitivity curves V(l) that describe the average physiologic sensitivity to brightness of the human eye depending on wavelength l. The detectors are calibrated in lumen, lux, candela (cd), cd/m2. Colorimetry (color measurement) uses standardized mathematical formulas to
1 In this CIE color diagram, ›c1b‹ and ›c2‹ represent the closely defined color space for white LEDs
represent the visual result of a color inspection or color comparison numerically. Three standardized evaluation
4 | 2009
Characterization of LEDs
O P T I C A L M E T R O L O GY
INFO: color rendering index, CRI
Tristimulus curves
The CRI is a value that can be calculated and refererences 14 test colors. In the case of an LED, the CRI displays the ability of that LED to represent a certain color. For example, CRI-14 (blue) for a red LED is close to zero. As some white LEDs do not have a continuous spectrum, inappropriate LEDlighting can lead to considerable color contamination. http://en.wikipedia.org/wiki/color_rendering_index
The radiant flux (measured in watt) emitted by an LED or LED array in general is termed the ›total flux‹. If a luminaire actually emits light into 4π space, the ideal measuring assembly is for an LED situated at the center of an integrating sphere on a mounting assembly intended for this purpose. The complete system, consisting of the sphere, its integrated sample mount and (for example) a fiber-coupled spectroradiometer, is calibrated to make measuring of the LED features possible (Figure 3). Typical parameter values measured include radiant flux, luminous flux, dominant wavelength, center wavelength, chromaticity coordinate, CRI et cetera. LIV curves can be recorded and efficiency can be measured easily if power supply
units can be controlled by the software. In addition to cw measurements, pulsed measurements can additionally be carried out. This capability is important for the mass production of LEDs, primarily due to the demand for high speed electronic, mechanical and optical testing on the production line. SLEDs (very bright or super-luminescent LEDs) are also measured in an integrating sphere, but with the LED installed at the rim of the sphere. Due to the requirement for LED cooling, light is, in this case, emitted into a half-space of 2π (and termed ›forward flux‹). The supply lines for LED power and cooling do not have to be introduced into the sphere, and the LED can be replaced more quickly when it is installed from the outside. Both 4π and 2π measurements can be conducted with integrating spheres of various diameters (for example, 15 and 30 cm). The system is complemented by matching LED adapters with or without TEC stabilization and calibration lamps of V appropriate power.
3 Components for luminous flux measurement
4 ›CIE 127‹ approved LED measurement tools
Wavelength (nm) 2 CIE tristimulus curves
functions (tristimulus filters, Figure 2) for red, green and blue are used to turn the three color signals received into a color impression that can be displayed in the standard color space as a vector with the two components x and y. These measurement systems are specifically intended for determining the color behavior of screens and other selfluminous sources.
Measuring light quality and quantity Basically, there are the following three types of measurements for LEDs: ■ How much light does the LED emit in total? ■ How much light does the LED emit in a particular direction? ■ How much light does the LED emit as a function of angle? To answer these questions, one can make use of the special measuring devices as detailed below. Primarily it is important that the captured light is processed with spectroradiometers. That is, while photometric devices can prodide data characterized by units of lumen, lux, cd, or cd/m2, and do so with a relatively simple and inexpensive system set-up, spectroradiometric systems provide both photometric and colorimetric values, including chromaticity, correlated color temperature (CCT), color rendering index (CRI) and dominant wavelength, as well as radiometric values (W, W/sr, W/m2sr) and spectral details such as peak wavelength, bandwidth et cetera.
4 | 2009
How much light does an LED generally emit?
Laser+Photonics
29
O P T I C A L M E T R O L O GY
Characterization of LEDs
INFO: integrating spheres Integrating spheres are an established, versatile and efficient medium for measuring light. Some applications would be impossible without them. A basic requirement for their function is the use of materials for the sphere’s inner walls with reflection characteristics that are as diffuse as possible and a reflection coefficient that is as high as possible. Integrating spheres are mostly used for spatial homogenization of radiation sources, either for characterization purposes or alternatively in order to provide a uniform source. The total radiation of a light source is introduced into the sphere and mixed there by multiple reflection from the sphere's inner walls. This radiation can then be measured or utilized at one or several outlets. It is not important if the light is emitted directly, as from a laser or light bulb, or indirectly, as for transmission or reflection from a sample.
LED emission in a particular direction
LED emission as a function of angle
The ›CIE 127‹ [1] standard must be adhered to during characterization of LEDs when the strength of the beam is the relevant feature to be determined. Most LEDs use the photometric unit candela (cd) for specification on their datasheets. W/sr and cd are commonly measured with so-called ›intensity heads‹ (Figure 4). The CIE 127 specification distinguishes between two measurement setups: for a constant measuring area of 1 cm2, the distance to the LED tip is either 100 or 316 mm. This leads to the solid angles of 0.01 sr (›Condition B‹) and 0.001 sr (›Condition A‹), respectively. Special measuring heads comply with this requirement in terms of distance and active area, while minimizing the stray light that could negatively affect the results. A diffuser is used to homogenize the collected light and couple it to the sensor. This device is also compatible with either a spectroradiometer or a singlechannel photoelectric sensor.
LED goniometers are used to record angle dependent intensity distribution. Gamma Scientific offers the ›940-LED‹, a two-axis LED goniometer (Figure 5). The 940-LED pivots the LED automatically to permit a quick characterization. At maximum resolution, the lowest step width is 0.1°, while the software can easily be used to create custom measuring intervals and measuring macros.
Processing of collected light with spectroradiometers Laser 2000 offers spectroradiometers for LED characterization suited to the tasks described above. The most obvious performance feature of a spectroradiometer is spectral resolution, which should be ideally better than 5 nm. The importance of other parameters, such as long-term stability, suppression of stray light and temperature stability, are typically only recognized after commissioning. This is
where simple and compact spectrometers differ from high end devices. Gamma Scientific offers its ›RadOMA‹ line for four different wavelength ranges (Figure 6). Each of these devices has a variable slit for five different resolutions. RadOMA spectroradiometers also offer state-ofthe-art stability, signal-to-noise and stray-light performance.
Spectral analysis software ›Light Touch‹ is an interactive Windows 2000/XP spectral analysis software program (Figure 7). The program controls any of Gamma Scientific's spectroradiometer systems, and can also be used as a stand-alone data analysis package. Photometric values are given for all common units (Imperial, CGS and MKS). Colorimetric quantities such as tristimulus values, chromaticity coordinates, correlated color temperature, and color rendering index are also automatically calculated and shown in a measurement workspace. Complex data manipulation functions, file editing and related subroutines, as well as blackbody curve generations are included in the software package. Import and export capability to common spreadsheet formats allows individual editing and preparation of results for reports.
Summary: improving quality via LED characterization In the automotive field, LEDs are already on their way to success. They are now also expanding into other areas that used to be the exclusive domain of traditional lamps. Because of this, there is a need for complete metering
5 LED-Goniometer with spectroradiometer (left) and an example for the angle dependent intensity distribution of an LED (right)
30
Laser+Photonics
4 | 2009
Characterization of LEDs
6 High end spectralteleradiometer for sophisticated measurements
7 Windows compatible control and data acquisition software
systems that are able to quickly and reliably determine the photometric values of LEDs, including luminous flux and luminous intensity. Other important parameters include chromaticity coordinate, dominant wavelength, and color temperature. Determining these with an accuracy of 1 percent poses a challenge that can only be met by state-ofthe-art, high quality measuring devices. Laser 2000 offers complete high performance solutions for LED measurement that will prove their worth after only a short time in use. LITERATURE 1 The International Committee on Illumination (French: Commission Internationale de l'Éclairage; CIE) is an independent non-profit organization targeted at international cooperation and information exchange in all areas of lighting, imaging and coloring research. www.cie.co.at.
AUTHOR Dr. HELGE BRÜGGEMANN is Sales Engineer at Laser 2000. He is responsible for the specialized areas of optical measurement, beam analysis, light measurement, spectroscopy and IR technology.
■ www.laser-photonics.eu You can find this article online by entering the document number eLP110017
4 | 2009
Laser+Photonics