Ocean Optics OOINLCorrect - Manual

OOINLCorrect
Loading Non-Linearity Correction
Coefficients Instructions
All CCD detectors exhibit a non-linearity in response to light. If left uncorrected, non-linearity will
result in slight, but detectable errors in the calculation of normalized values (absorbance, transmission
or irradiance). The pattern of non-linearity is different for the various detector models used in our
spectrometers. The magnitude varies from detector to detector, but is the same for all pixels in the
detector. Therefore, it is possible to measure linearity and correct for the errors in software.
The process of measuring a number (counts) that represent the amount of light shinning on the CCD is
not linear. Plotting a curve of counts versus light does not appear as a straight line. A polynomial
curve is used in the spectrometer’s driver to correct the count value to its theoretical relative value.
The Non-Linearity Correction (OOINLCorrect) software is designed so it can be used as a simple
intensity curve monitoring application as well as generating the coefficients.
We correct for the nonlinearity of the detector by running an experiment where we vary the amount of
light the detector receives; we keep the intensity of the light source constant but vary the integration
time. When we analyze this data, we have a number of points that are counts/sec vs. counts. We look
at 9 pixels across the detector (the nonlinearity of each pixel is identical) and normalize each pixel's
counts/sec to 1. When we combine the data from all 9 pixels, they overlap on a plot of normalized
counts/sec vs. counts. We fit this smooth function to a 7th order polynomial. This polynomial
produces a correction factor for each intensity. If we observe 2000 counts, we plug 2000 into the
resulting polynomial and get a number less than 1 (typically around 0.9). We divide the number of
counts by this correction factor.
After correcting for the nonlinearity of one of our spectrometers, the detector response is linear to
>99.8%. Before completing the nonlinearity correction, the detector response is linear to 93%.
A non-linearity correction feature is built into our application software (both OOIBase32 and
SpectraSuite) using a checkbox to activate the non-linearity correction function and text boxes for 8
coefficients. These coefficients describe a polynomial fit to the observed deviations from linearity as a
function of signal strength.
The nonlinearity can be characterized at the factory, or you can do it yourself using OOINLCorrect
software. Our USB spectrometers have an EEPROM; the factory evaluated linearity corrections are
stored on this EEPROM and loaded to software when you plug in the device.
OOINLCorrect software, when used with SpectraSuite or OOIBase32 application software, enables
you to correct for non-linearity yourself and store these 8 coefficients to the EEPROM. Contact an
Ocean Optics Application Scientist to determine if OOINLCorrect is appropriate for your application.
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