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Short-Wave Infrared (SWIR)
Communicating with the Instrument Controller (Computer)
ASD Document 600540 Rev. J
The 512-channel array permits the entire VNIR spectrum to be scanned in
parallel at 1.4 nm wavelength intervals. A single sample can be acquired in as
little as 17 ms.
The Near-Infrared (NIR), also called Short-Wave Infrared (SWIR), portion of
the spectrum is acquired with two scanning spectrometers:
•
SWIR1 for the wavelength range of 1000 nm to 1830 nm
•
SWIR2 for the wavelength range of 1830 nm to 2500 nm
The SWIR scanning spectrometers have one detector for SWIR1 and another
for SWIR2. This is different from the VNIR spectrometer, which has an array
of 512 detectors. Thus, SWIR spectrometers collect wavelength information
sequentially rather than in parallel.
Each SWIR spectrometer consists of a concave holographic grating and a
single thermo-electrically cooled Indium Gallium Arsenide (InGaAs)
detector. The gratings are mounted around a common shaft that oscillates
back and forth through a 15 degree swing. As the grating moves, it exposes
the SWIR1 and SWIR2 detectors to different wavelengths of optical energy.
Each SWIR spectrometer has ~600 channels, or ~2 nm sampling interval per
SWIR channel. The spectroradiometer firmware automatically compensates
for the overlap in wavelength intervals.
Like the VNIR detectors, the SWIR1 and SWIR2 detectors convert incident
photons into electrons. This photo current is continually converted to a
voltage and is then periodically digitized by a 16-bit analog-to-digital (A/D)
converter. This digitized spectral data is then transmitted to the instrument
controller for further processing and analysis by the controlling software.
The grating is physically oscillating with a period of 200 ms. It performs a
forward scan and a backward scan, resulting in 100 ms per scan. This is the
minimum time required for any SWIR samples, or full-range samples.
The FieldSpec spectroradiometer communicates with the instrument
controller using a cross-over 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
create smaller data packets (adding the packet sizes as above.)
8
Chapter 1 Introduction
FieldSpec® 3 User Manual
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