MNL-1024 Rev A
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Detector Overview
Detector Description
The detector used for the Flame-NIR+ is an InGaAs photodiode array with integrated CMOS readout circuit.
During each integration period, the current through each photodiode charges an integrated capacitor. After the integration period
completes, the charge in each capacitor is measured and then reset. When the capacitor is fully charged, the detector is considered
saturated and provides the maximum output level.
Detector Charge Capacity
Ocean Insight products aim for a large signal-to-noise ratio (SNR) in optical measurements so that small signal variations can be
observed and a large dynamic range is available. The maximum single scan SNR on most CCD based devices is dominated by the
well depth. This is because the maximum possible SNR for an integrating photodetector is limited by photon shot noise, which is the
square root of the number of electrons collected during the integration period.
The Flame-NIR+, however, has a very large charge storage capacity compared to most CCDs. As a result, the maximum single scan
SNR is significantly higher. In fact, photon shot noise is not typically the dominant source of noise in Flame-NIR+ measurements.
Instead, SNR is dominated by other sources of noise such as readout noise or thermal noise, depending on the integration time.
Signal Averaging
Signal averaging is an important tool in the measurement of spectral structures. It increases the S:N and the amplitude resolution of
a set of samples. The types of signal averaging available in our software are time-based and spatial-based.
When using the time-base type of signal averaging, the S:N increases by the square root of the number of samples. Signal averaging
by summing is used when spectra are stable over the sample period. Thus, even though the maximum single scan SNR is 6000:1 on
the Flame-NIR, a S:N of 60,000:1 should be readily achieved by averaging 100 spectra.
Spatial averaging or pixel boxcar averaging can be used to improve S:N when observed spectral structures are broad. The traditional
boxcar algorithm averages
n
pixel values on each side of a given pixel.
Time-based and spatial-based algorithms are not correlated, so the improvement in S:N is the product of the two processes.
Internal Operation
Pixel Definition
The Flame-NIR+ has 128 pixels, all of which are optically active and not masked. This is slightly different from other Ocean Insight
spectrometers,
which have “dummy” and optical black pixels. Therefore, all pixels read from the Flame
-NIR+ in OceanView, OmniDriver,
Содержание FLAME-NIR+
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