New developments in LED illumination systems

15th December 2014

Posted By Paul Boughton

LED switching speeds under TTL trigger are in microseconds, making this solution an attractive option for end users
CoolLED wavelength grouping concept

Jim Beacher discusses the benefits of illumination systems that utilise the latest LED technology.

Recent developments in LED technology have resulted in illumination systems offering increased brightness across a very wide spectral band from the visible to the near-infrared (NIR).

A particular example is in microscope illumination and, in particular, for the fluorescence technique that is now widely used in life sciences and materials research. As well as requiring intensities at multiple defined wavelengths, these systems also require precise intensity control, with the ability to switch on and off quickly and repeatedly.

The goal of technology providers was to provide a high level of intensity from the mid-300nm region to the 700-800nm region. Work to address these joint needs of brightness and control has resulted in the development of a universal fluorescence illumination system from CoolLed, the pE-4000, which has 16 selectable LED wavelengths. This was considered adequate to satisfy almost all known applications and offer intensities in the NIR, where there is increased interest for in-vivo and optogenetic applications. Provision for subsequently incorporating lower wavelengths (such as 340nm, which requires the use of special optical materials that transmit at UV wavelengths) was taken into account.

To offer a system with extensive functionality such as stability, fast switching speeds and the ability to generate sinusoidal and square-wave functions, software communication via USB and TTL triggering via multiple BNC connections were incorporated. The ability to trigger in, trigger out and operate under analogue control was incorporated. Switching speeds under TTL are in microseconds (see Fig. 1.)

A challenge in developing a system with a wide spectrum of illumination is that losses incurred when combining LED light of different wavelengths can quickly compound and render the overall system performance unacceptable. To address this, the developers investigated how the microscopist would use the illumination system. It was recognised that illumination would be passed through optical filter sets installed within the microscope. These filters limit the wavelengths of light that will be transmitted.

As the filter sets are commercially available, it was possible to characterise their transmission wavelengths and group their performance into four bands of mutually exclusive wavelengths. Four motor-driven LED modules were then designed, each supporting the necessary wavelengths for each band.

As the optical path will require fewer combining optics, losses are dramatically reduced. The overall physical and environmental footprint of the system is reduced.

Achieving the intensities required for fluorescence microscopy requires the correct selection of the bare LED die to ensure that the wavelength peak is correctly positioned to match both the filter set and the absorption characteristic of the fluorophores. Active air-cooling technology ensures that the LED is held at a constant temperature. The LEDs can be over-driven for greater intensity without causing damage. Cooling increases useable lifetime and ensures that output remains stable.

LED semiconductor technology produces LEDs that have their peak intensities in either the blue or red regions of the spectrum.Phosphor technology has enhanced LED illumination systems by increasing intensity in the green region. A phosphor that is excited by a powerful LED peak can be selected, which emits at a wavelength where bare LEDs are weaker. 

The design of the pE-4000 has resulted in an intense, controllable and broad-spectrum illumination system that can be operated as a simple white light source or as an advanced research tool depending on the user’s requirements.

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Jim Beacher is business director at CoolLed in Andover, UK. 





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