Minimize Stray Light Correction

The signal of a given sample is polluted by stray light coming into the instrument from other samples by means of either specular reflections (ghost images) or scatter. Stray light may be an significant contributor to the measured signal, particularly in the infra-red for ocean pixels close to clouds or land covered by vegetation.

The OLCI stray light correction algorithm is defined below. It uses characterisation of stray light contamination to estimate the degradation and correct it.

Stray light contribution to signal is evaluated from the already contaminated signal on the assumption that since it is a small contribution, the fundamental signal structure is preserved and it can be considered as Epsilon (?) in the approximation:

Once estimated it can be corrected for. A two step process is used:

  1. A first contamination taking place in the ground imager, i.e. the imaging part of the instrument optics, with mixing of energy from the whole field-of-view, including both spatial dimensions, but without spectral mixing.
  2. A second contamination occurring inside the spectrometer, with one spatial dimension (the along track direction) filtered out by the spectrometer entrance slit but including spectral mixing through scattering and reflections during or after spectral dispersion.

Therefore, stray light correction is implemented in two steps, following the instrument signal generation but in the backward direction. Since the output from radiometric scaling corresponds to the signal sensed at CCD surface, it includes both stray light contributions. It must therefore be corrected, firstly for the spectrometer contribution, and secondly for the ground imager contribution.

The spectrometer stray light term can be expressed as a two-dimensional convolution of the two-dimensional weighted radiance field. Note that the spectral dimension of the CCD shall be reconstructed from the 21 available samples (the OLCI channels) using linear interpolation on normalised radiance, avoiding the use of saturated samples.

The ground imager stray light contribution term can also be expressed as a two-dimensional convolution of the incoming radiance field, independently for each OLCI channel. It can be estimated from the radiance field corrected for the spectrometer contribution.

APO