ASD Document 600541 Rev. G 7 TerraSpec® User Manual
www.asdi.com Chapter 1 Introduction
Short-Wave Infrared (SWIR)
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 1800 nm.
• SWIR2 for the wavelength range of 1800 nm to 2500 nm.
The SWIR scanning spectrometers only 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 about a common shaft which 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 spectrometer firmware automatically compensates for the
overlap in wavelength intervals.
Like the VNIR detectors, the SWIR1 and SWIR2 detectors convert incident
photons into electrons. This photocurrent 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.
Communicating with the Instrument Controller (Computer)
The TerraSpec spectrometer communicates with the instrument controller
using a cross-over Ethernet cable or optional Wireless Ethernet (WiFi)
interface. The amount of data that is sent depends on the configuration of the
spectrometer.
• 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 TerraSpec spectrometer can create packet data sizes over 5 KB.
Other single or dual range configurations of the spectrometer create smaller
data packets (adding the packet sizes as above.)
When features for spectrum averaging (or sample count) are turned on within
the application software on the instrument controller, the averaging is
performed at the spectrometer.
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