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Stray light correction

Spectral stray-light correction

Spectral stray light in a spectroradiometer can be described by the instrument’s spectral line spread function (SLSF). An example of a SLSF of a CCD array spectroradiometer is shown in the figure at the top right.

We have developed a correction method to obtain spectral stray-light corrected signal, YIB, by using a simple matrix multiplication of the raw measured signals, Ymeas, by a spectral stray-light correction matrix, Cspec, (Equation 1)

YIB = CspecYmeas   (1)

Only a one-time characterization of the instrument for a set of SLSFs is required to derive the correction matrix, Cspec. A correction, which can be done in real time, can reduce errors due to stray light by more than one order of magnitude. Thus, the application of this method could lead to significant reductions in the overall measurement uncertainties in applications where spectral components of the source have a large dynamic range. An example of stray-light correction results for a CCD-array spectroradiometer is shown in Figure 1, where the light source is an incandescent lamp with a green bandpass filter. The residues of the stray light signals are at least one order smaller than the original stray-light signals.

 Raw signals and stray-light corrected signals of CCD array
Figure 1. Plot of raw signals and stray-light corrected signals of a CCD-array spectroradiometers. The peak raw signal is normalized to 1.


For more technical information, see Simple spectral stray light correction method for array spectroradiometers and New NIST method improves accuracy of spectrometers.

Spatial stray-light correction

Spatial stray light in an imaging instrument can be described by the instrument’s point spread function (PSF). An example of a PSF of a CCD imaging radiometer is shown in the figure at the bottom right.

We have also developed a correction method to acquire spatial stray-light corrected signals (transposed to a column vector (CV)), YIR,cv, involving simple matrix multiplication of the raw measured signals (transposed to a CV), Ymeas,cv, by a spatial stray-light correction matrix, Cspat, (Equation 2).

YIR,cv = Cspat Ymeas,cv                 (2)

Only a one-time characterization of the instrument for a set of PSFs is required to derive the correction matrix, Cspat. A correction, which minimally impacts data acquisition time, can reduce errors due to stray light by more than one order of magnitude. Thus, application of this method could lead to significant reductions in the overall measurement uncertainties in applications where images have high contrast ratios. An example of stray-light correction results for a CCD imaging radiometer is shown in Figure 2, where the light source is an integrating sphere with the center of exit port covered by an opaque black sheet, The plot is a 1 dimensional image signals along the center line across the sphere port. The residues of the stray-light signals are at least one order of magnitude smaller than the original stray light signals.

 Raw signals and stray-light corrected signals of imaging radiometer
Figure 2. Plot of raw signals and stray-light corrected signals of an imaging radiometer. The peak raw signal is normalized to 1.

For more technical information, see Characterization and correction of stray light in optical instruments.

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Spread functions for stray light 

Examples of spread functions.  Top, a plot of a spectral line spread function (SLSF) of a CCD-array spectroradiometer.  Bottom, a plot of a point spread function (PSF) of a CCD imaging radiometer.

Contact

Applied spectroradiometry and imaging metrology:
Yuqin Zong, Project Leader
301-975-2332 Telephone
301-840-8551 Facsimile

100 Bureau Drive, M/S 8442
Gaithersburg, MD 20899-8442