LED Opal Diffuser Light Source
Research into LED and light properties determined that an opal glass diffuser would achieve a lambertian distribution as close as possible to ideal. The natural optical properties of the opal glass cause any incident light from a point source to disperse in a lambertian distribution. We use small surface mount LEDs to provide a source (at a given wavelength) located about 10mm behind the diffuser. The LEDs are mounted on an aluminum heat sink which keep them from over heating and also provides intensity stability. The combination of LEDs and opal glass guarantee that the light source distribution remains stable, and that repeatability is consistently achieved.
LED light sources are found to be superior to the flash lamp light source due to:
- LED's emit light at a specific wavelength, eg 525nm, 450nm, 635nm thus no bandwidth filters are required
- A single light source can be constructed of multiple LED's and controlled to sequentially emit light of different wavelengths
- Only one PMT required as light source is sequential not simultaneous activated.
- Sequencing of light source maximises the light's intensity as it eliminates the need to split light into three seperate paths
- The use of a single PMT eliminates the need to tune three seperate PMT's
- Life of LED's far outlast flash lamps, which are generally replaced yearly
- Heat generated by the LED light source is a fraction of a flash lamp, minimising changes in sample RH
The multi-wavelength light source is pulsed on and off at the same intensity during its measurement process, which is controlled by the instrument’s microprocessor. The LED intensity is optimized for the response of the photomultiplier tube at each wavelength. This is adjusted initially during manufacture, however it can easily be altered in the field.
With the introduction of the opal glass, the aperture of the light source window has been dramatically reduced to only 10mm x 13mm. This reduction in aperture has significantly improved the uncertainty due to angular truncation. The diagram below shows a cross section of the Aurora 1000 & 3000 cell without backscatter. The truncation angle is now from 10° to 171°, providing a full 161° of illumination.
Evolution of the Ecotech light source
The light source used in Ecotech's nephelometers have undergone significant evolution over the past ten years since the M9003 Integrating Nephelometer was introduced in 2003.
This paper led directly to the evolution of the latest light solurce used in our Aurora Nephelometers which is described above.
Results produced by this new light source are detailed in a 2010 paper