EBV LIGHTLAB I Special Focus
market leader in the field of LED metrology and works closely with various international commissions. The devices meet the ILC (International Lighting Commission) requirements for precise measurements of the luminous flux, which is measured in lumens. An excerpt from the list of Instrument Systems’ reference customers reads like a “who’s who” of the industry, containing not only leading LED manufacturers but also users that include: Apple, Avago, Everlight, Osram OS, Vishay and Zumtobel.
Measuring technology A core element of all the measuring technology in the EBV LightLab is the CAS 140CT spectroradiometer from Instrument Systems, which is designed to perform industry reference measurements under continuous 24-hour operation. This high-precision device measures the spectral power distribution of a 1,100 nm light source (infrared) within milliseconds through the visible light spectrum into the UV range at 380 nm – with a data point interval of 0.8 nm over a very high intensity measuring range of nine decades, which corresponds to a dynamic range of 1:1,000,000,000. All of the measured values are outputted via the USB bus to a PC/laptop/notebook, where the SpecWinPRO software then handles further processing of the data in Windows (XP and higher). Not only does this software deliver the measurement data in the form of individual items of data in a table, it also displays the spectral distribution and position in the colour diagram in graphic form. In this way, the software also calculates the colour temperature and colour rendering index (CRI) on the basis of the measured values. Here, the device measures ‘only’ the radiometric output; to determine the photometric output, the software evaluates the radiometric spectral measurement data on the basis of the sensitivity curve of the human eye and outputs the luminous flux in lumens.
Using a plug-in, the SpecWinPRO software controls a Keithley
source meter. This means that the entire measurement process can be controlled via software – from the electrical activation of the LED to the details of the data output and data processing. The light to be measured reaches the analysis device via CIE- compliant optical probes and a fibre-optic cable.
In practice
The following example, which describes the development of a light with a total light flux of 500 lm, demonstrates how a high- precision spectroradiometer in a 4π sphere with a diameter of 1,000 mm can help engineers in their work. Theoretically, five single 100-lumen LEDs are sufficient for this design, but the use of diffusers and lenses leads to a certain amount of loss. In the 1000-mm integrating sphere in the EBV LightLab, manufacturers can measure finished systems and identify whether five LEDs are sufficient for ensuring compliance with the specifications or whether a sixth one is needed. Alternatively, the light can be measured with a modified diffuser-lens system to see whether the specifications can potentially be met with just five LEDs. Differential measurements with the aid of auxiliary lights in the 4π sphere enable the SpecWin PRO software to partial out any measured value distortions caused by unwanted absorption by the light housing. In practice, however, this absorption is completely irrelevant because it only affects the semi-spherical space that does not receive any light anyway – in other words, the area behind the lamp.
Colour spectrum and reproduction Light and lamp designers are very interested in the spectral distribution of light. In this context, investigations focus not only on the location of intensity peaks and spectral distribution, but also on the colour reproduction index and colour temperature.
For example, the CRI value describes how an object appears when irradiated with a certain light. A good example of this is the bright street lamps which emit a yellowish light from their sodium vapour lamps. Sodium vapour lamps are very efficient and deliver a high luminous flux (measured in lumen) for the amount of electric power they use, which means that they offer a high luminous efficiency (lm/W). However, the yellowish light emitted by sodium vapour lamps is relatively mono-spectral. Although under this light you can clearly see whether or not there are any people, animals or vehicles on the road, it is not possible to distinguish green lines from red ones on a map under this type of street lighting. Similarly, a number of different light-emitting diodes are limited by certain
compromises in terms of colour. However, it is now possible to compensate quite effectively for these limitations, and with suitably clever designs it is currently quite possible to generate light with an excellent colour reproduction index well above 90. However, a 4π integrating sphere and a spectroradiometer are required in order to measure this type of light source. With the aid of a goniophotometer of type LEDGON 100 from Instrument Systems, EBV Elektronik is able to determine the angle-dependent directional characteristics of individual LEDs and smaller LED modules – and this with an angular resolution of 0.1° throughout the entire half-space in the beam direction of the LED. In combination with the spectroradiometer CAS 140CT already mentioned above, this will also enable EBV Elektronik to measure various spectral parameters such as colour co-ordinates or colour
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temperature as a function of angle. Since a number of different parameters of white LEDs – such as the colour temperature – vary significantly with the beam angle, these measurements play a very important role. While geometry-related measurement errors can occur in measurements performed in an integrating sphere, the goniophotometer – which is equipped with servo motors and controlled fully automatically by the SpecWinPRO software – is suited for precisely determining the radiant power and luminous flux and, in turn, the all-important luminous efficiency quotient, which is specified in lm/W.
The SpecWinPRO software is again used for the goniophotometer for
processing and presenting the measurements, which are then available in the IES and EULUMDAT export formats for further processing. These data formats represent the characteristic beam data for an LED, and they can then be used as the starting data point for the simulation of a complete lamp.
Source meter The 2440 source meter from Keithley is essentially a high-precision, programmable constant current and constant voltage source for actuating LEDs and LED modules under test conditions in the EBV LightLab. The current and voltage can be precisely set via the serial interface. To minimise the load on the light sources during the measurements, the
source meter can be software-controlled so that the LEDs are switched on only during the actual measurement but remain switched off during gaps between measurements. The 2440 source meter is also an ultra-high-precision volt and ampere meter that transfers the current, voltage and power data to the measurement software via an interface.
The range of measuring equipment in the EBV LightLab also includes an
infrared camera. This camera allows ‘hot spots’ on circuit boards to be identified in good time so that thermal management (i.e. the proper dissipation of thermal losses) can be optimised on the basis of this data. In addition to these measuring instruments, EBV’s new LightLab also
features two lighting measurement devices, the Mavo-Spot 2 light density measuring device and the Mavolux 5032B luxmeter. By focusing on objects such as roads, tunnels, sports grounds or airfields from a distance, the Mavo- Spot 2 ‘light density camera’ enables corresponding light density measurements (unit: cd/m2) to be performed. It can also be used for determining the light density of displays, monitors or projection screens. The angle of detection of the Mavo-Spot 2 is just 1°, which allows for a highly precise selection of the measurement points or pixels on which the device focuses. The measurement object can be located anywhere from 34 cm to an infinite distance away from the device. The Mavolux light meter allows users to calculate the illumination levels (measured in lux) at the workplace and on other surfaces. The EBV LightLab also features a range of universal multimeters that
measure not only the actual effective values of voltages and currents, but also the effective power values. These devices also enable the measurement of resistances, frequencies and capacitances. Also featured by the EBV LightLab is equipment for measuring and monitoring temperature. Various pieces of equipment ranging from small tools through miniature drills to special stations for the temperature-controlled soldering of LEDs onto MCPCBs ensure that users themselves can carry out small modifications or repairs directly in the EBV LightLab.
As you can see, EBV takes care of the little things as well. EBV Elektronik |
www.ebv.com
Components in Electronics October 2012 7
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