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Image Curvature Correction and Cosmic Removal for High-Throughput Dispersive Raman Spectroscopy
Volume 57, Number 11 (Nov. 2003) Page 1368-1375
A key factor determining the sensitivity of a Raman spectrometer is the usable detection area, which is the product of the usable slit width and the height. For the majority of process Raman samples, the larger the sampling area is, the more the scattered Raman signal can be gathered. On a multi-channel-detector-based dispersive spectrometer, a given spectral resolution limits the slit width. Extending the slit height using a straight slit usually causes the image to be curved on the detector due to optical effects. If left untreated, the curved slit image will degrade the peak shape and spectral resolution; therefore, the slit height must also be kept small if this negative effect is to be avoided. The mechanism of the curvature formation was analyzed for an on-axis-lens-based spectrograph, and a correction technique was developed to generate a straight slit image on the charge-coupled device (CCD). This allowed a large portion of the CCD height to be used without degrading the spectral resolution. A large fiber bundle was usable instead of a single small core fiber, generating significant increase in collected signal strength in clear or translucent samples. The straight image also enabled a new cosmic spike removal method, wherein the CCD image was divided into multiple strips, and a comparison among them allowed the identification and removal of cosmic spikes in a single CCD integration. On the contrary, many existing cosmic removal methods rely on comparison of multiple sequentially acquired spectra, potentially introducing artifacts, particularly when the spectral features are changing.