Spectroscopy: Binary coded aperture trumps slit for signal

Users of conventional slit-based dispersive spectrometers face a dilemma.

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Users of conventional slit-based dispersive spectrometers face a dilemma. A spectrometer designed with narrow slits for a high resolution has a low optical signal; widening the slit to increase the signal lowers the resolution. For some applications, the result is a compromise instrument that combines a barely acceptable resolution with a low signal-to-noise ratio. Engineers at Centice Corp. (Durham, NC) have produced an alternative that sidesteps this problem by replacing the slit with a binary coded aperture that, when used with computer processing, boosts light throughput by a factor of 12 over a slit-based system. Dubbed multimodal multiplex spectroscopy (MMS) by Centice, the approach was debuted at this year’s Pittcon conference (March 12-17; Orlando, FL).

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A binary coded aperture (top left) replaces a slit in a dispersive spectrometer. Combined with a computer algorithm, the coded aperture results in a 12× greater light throughput than a slit. In a schematic representation (top, right; schematic does not show actual binary coding), three widely separated spectral features are imaged, creating three nonoverlapping patterns. When three closely spaced spectral features are imaged (top, center), the patterns overlap, but the algorithm separates the signals. The result is a large improvement in throughput compared to slit- and pinhole-based spectrometers (bottom).
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The coded aperture consists of many adjacent narrow, slit-width stripes, each of which has a different binary one-dimensional (1-D) spatial coding. Placed in an otherwise conventional dispersive spectrometer, the coded aperture results in the spreading of light of each wavelength across a broad swath at the detector plane where the aperture is imaged.

Algorithm a key part

In a conventional instrument, this spreading would degrade resolution, but in the MMS version, the binary coding allows a 2-D image sensor combined with an image-processing algorithm to separate each wavelength to the resolution that would be attained by a slit the width of an individual coded stripe. For an MMS aperture with 24 stripes, the increase in light throughput over a single slit is 12 (the binary coding passes, on average, half the light). The advantage of MMS over spectrometers that use an optical fiber (which is a pinhole source) instead of a slit is much greater yet-for example, a factor of 864, based on one calculation for a typical instrument with a fiber input.

The technology was originally developed at Duke University under grants from the National Institutes of Health and from DARPA for tissue spectroscopy and spectroscopic imaging, explained Mike Fuller, Centice’s director of product marketing, at Pittcon. As a spinout formed in 2004, Centice is the exclusive licensee and, in fact, revealed the technology to a certain extent before its official debut; at Photonics West 2006 (Jan. 21-26; San Jose, CA), Centice announced partnerships with Newport (Irvine, CA) and Ocean Optics (Dunedin, FL) under which its partners will launch spectrometers incorporating the MMS technology.

John Wallace

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