Application Setup: LED Analysis

The USB4000 spectrometer is optimised for LED measurements with Grating #2 (350-1000nm wavelength range), a 25µm entrance slit and an L4 detector collection lens to increase light efficiency and reduce stray light. With this configuration, optical resolution is ~1.33nm (FWHM). An OFLV-350-1000 order-sorting filter eliminates second- and third-order effects. This optical bench configuration maximises system sensitivity, mitigating the light loss inherent with the use of an integrating sphere - the sampling optic of choice for most LED applications. The P400-2-VIS-NIR optical fiber collects the light from the LED and funnels it to the spectrometer. The fibre is equipped with SMA connectors which simply screw into the spectrometer at one end and into any sampling optics you order at the other. As standard, a 2m fibre is supplied, but other lengths are also available.

This is the least expensive approach. You can measure the relative spectral power distribution using the USB4000 by simply pointing the optical fibre at the LED under test. Ideally, however, you would fit the CC-3 cosine diffuser to the end of the fibre to minimise alignment sensitivity. This is based upon an opal glass diffuser with a spectral range of 350-1000nm. Although the radiometric response of the system is relative, you will still obtain accurate spectral and colorimetric data from your spectrometer by referencing the spectrometer against a blackbody source (for example, a tungsten bulb). The colorimetric parameters reported include the chromaticity, colour temperature, dominant wavelength and colour rendering.

You can apply an absolute spectral irradiance calibration to the spectrometer system equipped with the CC-3 diffuser using the LS-1-CAL reference source. This is a light source that has a known spectral irradiance in the range from 300-1050nm. Please note that if you calibrate the cosine diffuser, your measurements will be of spectral irradiance (W/m².nm) in the direction of view. You will not be able to measure the total flux from the LED - for that you will need an integrating sphere (see below). Also note that some white LEDs show a strong variation of colour with direction of view - if you use an integrating sphere to collect the light from the LED in all directions, you will measure the average colour from the sample.

Most people choose an integrating sphere when measuring LEDs. An integrating sphere will provide an alignment-insensitive measurement of the forward or total flux from the sample.

With a sphere as your sampling optic, and provided you have calibrated the system with a reference source, you will be able to measure the absolute spectral radiant flux (W/nm) as well as the total luminous flux (lumens) from your LED. We offer a range of integrating spheres to suit different sample powers and sizes.

The Ocean Optics FOIS-1 integrating sphere measures the forward (or partial) flux from a single LED. The sphere has a diameter of 38mm and an entrance port size of 9.5mm. You calibrate the radiometric axis of the spectrometer with this integrating sphere using the LS-1-CAL-INT, which is a NIST-traceable light source calibrated from 300-1050nm.

For measuring the total flux from LEDs, LEDs packaged with lenses or in reflectors, as well as from LED clusters and luminaires, we offer a range of larger integrating spheres from Labsphere with diameters from 150mm up to 2m.

The 150mm Labsphere integrating sphere was purpose-designed for measuring forward flux from LEDs and features a 38mm input port, making it suitable for use with individual LEDs (including those fitted with lenses or in a reflector) as well as small clusters. The sphere is fitted with a 5W calibration lamp (plus separate, stabilised power supply) which means that you can compensate for the self-absorption or reflection errors arising from the sample placed at the sphere input.

To measure the total flux from an LED source, you normally place the sample in the middle of the integrating sphere. To that end, Labsphere's 0.25, 0.5, 1.0 and 2.0m spheres feature interior access. A sphere is normally chosen such that the sphere diameter is roughly twice the length of the linear luminaire to be placed within it, or roughly ten times the maximum dimension of the sample.

Typically, the 25cm sphere is good for up to 100W and the 50cm sphere for up to 400W. The 0.25 and 0.5m spheres are calibrated with a 35W standard of spectral radiant flux placed centrally within the sphere, driven by a separate, stabilised power supply. An auxiliary lamp can be fitted to these spheres which allows you to correct for the absorption and reflection errors that arise when samples are placed both within the sphere or at the 50mm external input port.

The USB4000 with CC-3 cosine collection optics can be set up to perform measurements of luminous intensity in candelas per square meter (cd/m²) from an LED cluster or signal. For advice on how to perform intensity measurements with your spectrometer, please contact Pro-Lite.

LED Power Supply: Secures, Powers & Drives the LED

Software

SpectraSuite spectroscopy operating software provides for measurements of both absolute and relative spectral flux of LEDs, and calculates absolute irradiance, chromaticity (L*a*b*, XYZ, xy u'v') as well as colour temperature, dominant wavelength, colour rendering and more. For those who wish to write their own software, for example to perform automated production line testing, we offer OmniDriver software. This allows users to control their spectrometer from within their own process-control software written within most common popular languages (e.g. C, C++, Pascal, Delphi, LabView, Visual Basic, etc.). OmniDriver can be used within Windows, Mackintosh and Linux operating systems.