Summary
The basis for putting airborne imaging
spectrometer technology into practical
application is to quantize the data of the
imaging spectrometer and establish the
quantified relationship between digital output
and spectral radiance. High accurate
calibrated system on the ground and on-board
calibrator are the necessary conditions for
conducting the research on quantization of
imaging spectrometer data. This paper mainly
introduces the calibration system for an
OMIS.Operative Modular Imaging
Spectrometer.developed by Shanghai
Institute of Technical Physics, Chinese
Academy of Sciences
1. Introduction
Today when remote sensing technique
has been developing towards practical
application , interpretation of remote sensing
images just from relatively strong or weak
intensity of targets radiation cannot meet
the need of practical application.. The
requirement for quantization of data is
becoming more and more important. The
successful development of imaging
spectrometer enables simultaneous imaging
of the same ground objects at hundred of
continuous spectral bands. The spectral
resolution of OMIS is of NM level. Since the
remote sensing data contains both images and
spectra of a target it is possible to obtain,
from remote sensing,, spectral characteristics
of the target which are similar to those in a
lab. The relative radiation information
obtained from the remote sensing imaging
data itself basically meets the need for remote
sensing target extraction and classification.
Quantified parameters of the instrument must
be provided for further target recognition and
image interpretation.
The response of an OMIS can be divided
into spectral response and radiation response.
The influence of the environment.
temperature.impact and the variation in
optics, mechanics of the instrument itself and
in the performances of detectors will make
the system response vary . The major reason
for further progress of OMIS is that it is hard
to effectively compare the information
acquired at different time by the same
instrument or the information of the same
target acquired by different instruments. In
order to establish quantified relationship
between the digital output of the OMIS and
the radiance of a target it is important to
conduct absolute calibration on the spectral
radiance of the spectrometer. Normally
accurate calibration of the instrument is
carried out in a laboratory, at the same time
the variation in the system response is
monitored. Therefore, setting up a ground
calibration system and on-board calibrator for
visible and short-wave IR bands are one of
the important contents in the development of
new generation of OMIS.
2. Requirement for Calibration of OMIS
The detecting spectral range of OMIS
(Operative Modular Imaging
Spectrometer) is from 0.46
mm to 12.5
mm
with 128 bands consisting of five
Spectrometers .For application purpose
requirements for calibration accuracy for
various bands of the OMIS are given in
table 1:
Table 1. Calibration Requirements of the OMIS
3. Laboratory Calibration System of OMIS
3.1 Spectral calibration
The role of laboratory calibration is to
determine the central wavelength and the
bandwidth of various channels of OMIS.
An ideal calibration light source should
fully fill the field of view and aperture, be
monochromatic and uniformly distributed.
However, the aperture of the imaging
spectrometer is large and its spectral
resolution is high, it is to realize an
ideal calibration light source. Therefore, the
spectral calibration process under nonideal
light was studied. Usually calibration light
source consists of a monochromator and a
collimator for spectral calibration in lab. In
order to obtain good monochrome the slit of
the monochromator must be small, e.g. if
the highest spectral resolution of the
imaging spectrometer is 10nm,the
resolution of the monochromator is at least
1nm or so. The slit of the monochromator
must be so small that it will not fully fill the
field of view. At that time the image of the
slit on the detector can be rotated by 90.
Since the slit usually has enough height and
if the height of the slit is changed into width,
the requirement of filling the fieldofview
of disperse direction is met with.
When the monochromator scans at certain scanning step length, corresponding spectral
response curves can be obtained for various
channels of the imaging spectrometer,
which can determene the central
wavelengths of various bands and
equivalent spectral nand width. Figs 3.1 and
3.2 show the structure of the calibration
system and the light path.
Fig 3.1 Structure of the Calibration System
Fig 3.2 Light path of the Calibration System
