3.2 Radiometric calibration
Radiation calibration is to determine
the corresponding relationship between the
response value of each channel of the
imaging spectrometer and incident radiation.
The key point for a radiation calibration
system is to establish a set of standard
radiation sources with high accuracy and
high stability. High and low temperature
blackbodies covering the entire field of
view were adopted for the calibration in
thermal infrared band. For the band of 0.45
µm to
2.5
mm a large aperture integrating
sphere radiation source with good area
uniformity. The integrating sphere has a
diameter of 900mm with a hole of 240mm;
inside the sphere there are eight quartz
halogen tungsten lamps of 250 W with a
peak radiance of 2.46E-5w/cm
2 Sr nm
( 960-970nm). The interior of the
integrating sphere is coated with BaSO4 and
F4mixed diffuse reflection materials. The
output radiance of the integrating sphere
gives 8level absolution spectral radiation
via accurate radiation standard transfer.
The establishment of radiometric
calibration system is the transfer process of
radiometric standards. It involves the
recurrence of standards, contrast
measurement, reference medium etc. High
precision of the calibration system can not be
guaranteed unless the error source is
controlled. Calibration of the imaging
spectrometer by the radiometric calibration
system can determine the radiation sensitivity
of various bands of the instrument, linearity
of response, dark current. the influence of
stray radiation etc.
4. Onboard calibration
An on-board calibrator is used to calibrate
the imaging spectrometer during flight. Its
role is to provide radiation reference data for
checking the relative stability of spectral and
radiometric response of each channel of the
instrument during operation. The OMIS
system is equipped with a calibration lamp
for visible and short wave infrared channels
and two radiometric calibration blackbodies
for middle infrared and thermal infrared .
Three calibrators are distributed uniformly on
both sides of the zenith and scanning window
of the imaging spectrometer (Fig. 4.1 ). In
every scanning the instrument observes and
records earth object and each calibrator once.
The difference between the current level of
the blackbody and the signal of the
calibration lamp can be used as relative
radiation reference for visible and far IR and
short wave IR.
Figure 4.1 Layout of the Onboard Calibration System
4.1 Blackbody
A carefully made blackbody can be
used as firstlevel calibration source of
thermal radiance. A temperature control
unit adjusts the temperature of blackbodies
BB1 and BB2 to make it stable at its set
temperature. The blackbody temperature
controlled by the first level thermal-electric
unit that spans from –15to 50can meet the
need. of operation. The precision of the
blackbody temperature is controlled at about
±0.2and display indicates ±0.1. Ideal
requirement for the blackbody as radiation
standard surface is that the spectral emissivity
of the blackbody is 1.0, but it is rather
difficult to make such a perfect blackbody. A
simple way is to paint a layer of black
painting which has a high spectral emissivity
of 0.94. With this emissivity the reflectivity
will be 0.06 which indicates that during
calibration, the blackbody also contains 6%
reflective energy of environment radiation
beside its own radiation. So in quantitative
analysis of remote sensing data, we must
consider the influence of environment
introduced by the 6% reflective energy. The
spectral emissivity variance of 0.1 is equal to
the blackbody temperature variance of 0.6K.
