  Back-illuminated charged-coupled device (BI-CCD) arrays increase quantum
efficiency but also amplify etaloning, a multiplicative, wavelength-dependent
fixed-pattern effect. When spectral data from hundreds of BI-CCD rows are
combined, the averaged spectrum will generally appear etalon-free. This can
mask substantial etaloning at the row level, even if the BI-CCD has been
treated to suppress the effect. This Note compares two methods of etalon
correction, one with simple averaging and one with row-by-row calibration using
a fluorescence standard. Two BI-CCD arrays, both roughened by the supplier to
reduce etaloning, were used to acquire Raman spectra of murine bone specimens.
For one array, etaloning was the dominant source of noise under the exposure
conditions chosen, even for the averaged spectrum across all rows;
near-infrared-excited Raman peaks were noticeably affected. In this case,
row-by-row calibration improved the spectral quality of the average spectrum.
The other CCD's performance was shot-noise limited and therefore received no
benefit from the extra calibration. The different results highlight the
importance of checking for and correcting row-level fixed pattern when
measuring weak Raman signals in the presence of a large fluorescence
background.
