Characterizing LED Parameters using Spectroscopy
Ocean Optics spectrometers and accessories can be configured to measure absolute or relative irradiance of LEDs and other radiant sources, with a variety of optical fixtures, calibrated sources and other tools for convenient measurements.
In this application note, we’ll describe how a Flame spectrometer, radiometrically calibrated light source and fiber optic integrating sphere can be used to measure absolute spectral intensity values of LEDs.
When used in a teaching lab, the procedures described here can help students to understand principles of absolute and relative irradiance, peak metrics and power and energy output. For researchers and other users, similar procedures can be applied to measure LED characteristics in testing and sorting facilities.
LEDs are often defined by their CWL (center wavelength) and FWHM (full width at half maximum) characteristics, as well as their power (obtained by integrating the intensity under the spectral curve in irradiance mode).
CWL and FWHM measurements can be made in either relative irradiance or absolute irradiance mode, but power measurements can only be made in absolute irradiance mode. Several wizard functions in OceanView software are available to guide users through relative and absolute irradiance, peak metrics and more.
Not all LEDs have a single peak. White light LEDs have two peaks – one from a blue LED, which excites a phosphor coating inside the LED package; and a less intense, broader peak at a longer wavelength from the phosphor emission. White light LEDs are more often characterized using correlated color temperature (CCT), which fits the LED’s spectral output to a blackbody curve (essentially comparing it to the sun or a tungsten halogen bulb) and reports the corresponding color temperature.
For this experiment, we will focus on determining CWL, FWHM and Power for various LEDs.
Ocean Optics modular spectrometers and accessories make it possible to configure dozens of different setups for relative and absolute irradiance measurements. For a teaching lab setting, the compact and robust Flame-Vis-NIR spectrometer (350-1000 nm) is an excellent option, with the FOIS-1 integrating sphere and 400 µm optical fibers to collect and transmit signal. (The STS-VIS spectrometer is another good option for radiometric measurements.)
Our system also comprises a power supply/controller (LED-PS) to operate the LED and adjust its drive current. Power and other LED characteristics are determined by comparing the LED to a radiometrically calibrated VIS-NIR source designed for use with an integrating sphere (HL-3-INT-CAL). Learn more about radiometrically calibrated sources.
For sample LEDs, we used bare bulbs of 9.52 mm diameter or smaller, with 2.77 mm lead spacing. This size is compatible with the LED-PS power supply/controller.
LED Measurement System
Flame-S-Vis-NIR (350-1000 nm)
|Radiometrically calibrated light source:||HL-3-INT-CAL tungsten halogen source, calibrated for use with FOIS-1 integrating sphere|
|Optical fibers/probe:||QP400-2-VIS-NIR 400 µm patch cord|
|Sampling optics:||FOIS-1 integrating sphere that collects light from the LEDs|
|LED power supply/controller:||LED-PS powers the LED, displays the LED drive current, and holds the LED in place|
|Software:||OceanView spectroscopy software|
|Also notable:||An alternative to the integrating sphere for signal collection is a cosine corrector, which collects light from 180° FOV; be aware that choosing a different sampling optic requires a different calibration routine|
This experiment takes advantage of OceanView software irradiance wizards to streamline your measurements. Users will be able to locate LED peaks, identify CWL and determine power output.
More advanced tasks can be added to the experiment, including adjusting the current drive on the LED-PS and observing its effect on LED peak shape and center wavelength; and using the OceanView Schematic View to display integrated power and other photometry values in separate scalar views.
Application Notes and Technical Resources
- Emissive Color Measurement: Measuring Chromaticity and Color Temperature of White LEDs
- LED Color and Photometry Measurements
- LED Color Measurement with Compact Spectral Sensor
- Measurement Techniques: Irradiance
- Miniature Spectrometers for Narrowband Laser Characterization
- Miniature Spectrometers Address Challenges of LED Research and Production
- Turning Up the Heat on Solid State Lighting