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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Spectrally Programmable Light Engine Technology

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FEATURE ARTICLE

April 2004

Spectrally Programmable Light Engine Technology:

The Coming Revolution in Digital Illumination

By Ulrich Stange

Figure 1: Spectral shaping capabilities of the SPLE: The blue curve shows the spectrum of the Cermax lamp powering the SPLE as modified by the optical transfer function of the diffraction grating used in this system. The green curve indicates the target spectrum that the SPLE tries to approximate. The red curve shows the spectrum of the actual SPLE output. The insets show the DLP patterns that produce the corresponding output. Spectral flattening: The SPLE produces constant intensity over the entire selected spectral range.

Imaging requires illumination. Advanced illumination enables advanced imaging. Since digital electronic imaging began in the late 1970s with the advent of the CCD, image capture, image processing and image display have all gone digital. Now we are closing the circle with the introduction of digital illumination. This technology, developed in the Cancer Imaging Department of the British Columbia Cancer Research Centre is about to make the transition from the laboratory to commercialization.

DIGITAL LIGHT

Current illumination technologies rely on the use of glass or gel filters and filter changers to control the color or spectral composition of illumination light, electronic controls to control the intensity, and mechanical or electromechanical shutters and chopper wheels to control the exposure time. The new digital illumination technology uses a digital light processor, or DLP , to shape the spectral composition, the intensity distribution, and the temporal behavior of the light source's output.

The concept is intriguingly simple. A DLP is a micro-mirror device with a million or more microscopic movable mirrors. Each of the tiny mirrors is individually addressable by the control electronics and can be switched from an ON position to an OFF position at 20 kHz rates. Projecting a ?rainbow? spectrum from a white lamp onto the DLP and then selecting which mirrors are turned on and which are turned off gives complete and precise digital control over wavelength and intensity, as well as the temporal exposure sequence. The reflected light from the selected mirrors is put through a mixing element and output through the projection optics.

The entire apparatus, packaged into a compact module, comprises a filterless, digitally controlled, spectrally programmable light engine, or SPLE, that produces in 50 microseconds any desired spectral output. Of course it cannot create light that isn't there to begin with but it can shape the output spectrum of the original source to enable a range of advanced imaging solutions. One SPLE module can replace multiple lamps, filter sets, filter changers, shutters, irises and chopper wheels. It also simplifies calibration, as there are no filters that can bleach and it can automatically compensate for lamp aging.

SPECTRAL FLATTENING

Arc lamps and metal halide lamps are used predominantly to illuminate the field of view in endoscopic imaging. Their spectra are very choppy; metal halides in particular have numerous large intensity spikes at certain wavelengths. As a result, their white light can appear harsh and may not always render the tissue or object under view in the most aesthetically pleasing or even diagnostically useful way. A SPLE can take out these spikes and flatten the spectrum to give even illumination across the spectrum or can mimic more familiar light sources, such as daylight or a tungsten lamp, to provide a familiar reference and thus facilitate diagnosis (Figure 1).

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