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Communicating with the Instrument Controller
Dark Current Measurement
Note:
ASD.Document 600540 Rev. C
The spectroradiometer firmware automatically accounts for the overlap in
wavelength intervals by using a preset wavelength within the common subset
at which to place a "splice".
The FieldSpec spectroradiometer communicates with the instrument
controller using an Ethernet cable or Wireless Ethernet (WiFi) interface. The
amount of data that is sent depends on the configuration of the
spectroradiometer.
•
A single sample of VNIR is ~1024 bytes.
•
A single sample of SWIR1 is ~2400 bytes.
•
A single sample of SWIR2 is ~2400 bytes.
A full-range FieldSpec spectroradiometer can create packet data sizes over
5 KB. Other single or dual range configurations of the spectroradiometer
could create smaller data packets (adding the packet sizes as above.)
When spectrum averaging features are turned on at the instrument controller
within the application software, the averaging and consolidation of samples
happens within the spectroradiometer.
Dark Current (DC) refers to current generated within a detector in the absence
of any external photons. DC is the amount of electrical current that is inherent
in the spectrometer's detectors and other electrical components and is additive
to the signal generated by the measured external optical radiation.
This manual uses DC to refer to all systematic contributions to the detector's
signal. DC is a property of the detector and the associated electronics (not the
light source). DC varies with temperature. In the VNIR region, DC can also
vary with integration time.
Whenever DC is taken, a mechanical shutter is used to block off the entrance
slit of the VNIR spectrometer so the signal can be measured. This signal is
subtracted from each subsequent spectrum until another DC is taken. The
SWIR spectrometers take a DC on every scan.
The DC measurement can be updated at any time, but should be updated more
frequently in the beginning of a given session while the instrument warms up.
Noise is the uncertainty in a given measurement, one channel at a time. Noise
can be reduced by using more samples and averaging the signal.
Dark Current is different from Noise. Noise by definition is radom and its
source is indeterminate. Dark Current is relatively stable and it can be
characterized.
The DC shutter is an electro-mechanical device, so there is another level of
DC created by the simple electronics that hold it closed. ASD measures this
offset and stores it in the DCC (Dark Current Correction) entry in the
8
Chapter 1 Introduction
FieldSpec® 3 User Manual
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