While In Tandem Designs was working on a fluorescence measurement system, 2 fundamental design questions arose with regard to using an LED Source :-
It’s well known that the output power of an LED reduces as the temperature increases. It is also common for the peak wavelength to increase with temperature. To use and LED without controlling its temperature adds uncertainty and potentially drift to the measurement being made. But how can we make temperature controlled LED light source without adding significant cost ? In Tandem Designs hit upon the idea of simply operating the LED a little above ambient and using a µController to control a heater and fan – simple, inexpensive and reliable – and stable.
LEDs are not perfect light sources: they frequently produce light at wavelengths significantly different to their peak wavelength. This is particularly noticeable with UV LEDs which often generate light well into the visible range partly due to fluorescence of the LED’s substrate. Of course there are many ways to add filtering to a light source, but adding a focusing assembly between the LED and the outgoing fibre is simple and efficient mechanism that maximises the power delivered by the fibre. In Tandem Designs solution was an X-Y-Z mini optical bench within the Phos-4® to allow the LED output to be focused through a filter into the fibre. There is another subtle advantage to this : substrate fluorescence can be minimised by focusing the light from the centre of the LED die on to the fibre which has the smallest content of substrate fluorescence.
The development of multi-die LEDs opened up the opportunity to combine LED wavelengths into one fibre. These devices provide options such as comparative fluorescence using two excitation wavelengths or broadband emission from 400nm to 700nm for transmission spectroscopy measurements. So why not make this device capable of driving 4 LED dies completely independently ? That’s what we did.
It need only one more thing – a stable, flexible interface to drive the LED allowing multiple modes of control :- A USB interface for MatLab® and LabView® users; Logic controls to allow inward or outward synchronisation; A UART connection for µcontrollers; And an analogue voltage control option to control intensity.
Phos-4® is a registered trademark of In Tandem Designs Pty Ltd
MatLab® is a registered trademark of The MathWorks, Inc
LabView® is a registered trademark of National Instruments Inc