S
acebourne/ ground
ing spectral indices
>rrain surfaces.
jectral bands in the
>1d instrument.
n the radiometric
T-p).
corrections and for
rmalized difference
en in Table 3.
te any two bands in
400 nm to 1800 nm.
ands as follows
Band width
40
155
g outputs go to data
allel.
ftware is designed to
on and temperature
| indices viz. Ratio
transferred from Data
npatible machine.
adiance
-Sr-)
ted with software)
IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002
9.1 Optical head +18 Voltsat5 mA
92 Display unit/
Data Logger: +9 V, 20 mA
10. Size: 100 mm * 100 mm * 180
11. Weight (kg): 3:5
12. Environmental conditions:
12.1 Operating
temperature (°C): Oto 50
122 Storage
temperature (°C): Oto 70
12.3 Humidity RH (%): 30 to 90 at 40 °C
Table 3. Specifications of Ratio Radiometer
5. DESCRIPTION
Functional block diagram of the ratio radiometer is shown in
Figure 1. The instrument is a combination two modules for two
individual spectral bands. Both the modules are mounted in a
common frame structure. The instrument and modules are easy
for mechanical fabrication. Each module has the components: a
mechanical field-of-view (FOV) assembly, interference filter,
and detector and associated processing electronics. Each field-
of-view assembly essentially consists of a set of apertures that
forms the desired field of view of 15 degree. The overall
dimensions of the field-of-view assemblies are kept optimal to
raiViisible channel
| I -eardndrared channel
absorb all the scattered radiations within the field of view and
absorb unwanted radiation outside the field of view.
Interference filters have been incorporated to select any desired
spectral bands. Silicon and germanium photodiodes have been
incorporated to detect the input optical power in two bands. The
spectral response for silicon detector is from 350 nm to 1100
nm while for germanium detector is from 800 nm to 1800 nm.
The detectors have been shielded properly to avoid any
background radiations by mounting them inside the cavities.
Detectors, field-of-view assemblies, and interference filters are
optically aligned. Both the modules are FOV registered, so that
they look the same target areas at a distance of one meter. The
analog signals produced by the detectors are processed with the
processing electronics. The analogue outputs from these two
bands go to a standard data logger where data is stored along
with house keeping information. The data logger is auto
ranging that covers four decades of voltage range from 10 mv
to 10 v with the resolution of 14 bits. It has linearity of +
0.05%, accuracy of + 0.03%, and temperature coefficient of +
0.01% of reading per degree centigrade. The data logger also
stores the date and time stamps along with every logged data.
The data and house keeping information stores in a sequential
manner. The data stored in data logger would be transferred to
PC with RS 232 interface and processed subsequently with
specially designed software. Execution of software program
generates an output file containing corrected outputs with
respect to offsets and temperatures, and spectral indexes: ratio
index and normalized difference index and other house keeping
/ field information.
Electronic
Mm Processing PCB
Detector |
Fig 1. Functional block diagram of Ratio Radiometer
6. RADIOMETRIC CALIBRATION
The instrument was calibrated in a radiometric quantity of
spectral radiance with a standard source. The calibration
standard is "OL Series - Integrating Sphere Calibration
Standard”, S/N 95201250. Optronic Laboratories, Inc. USA
(Optronic Laboratories, Inc., 1996). The standard source has
been supplied along with the calibration certificate (Optronic
Laboratories, Inc., 1996). The standard is traceable to National
Institute of Standards and Technology (NIST), USA. The
standard consists of a 150 watt quartz tungsten halogen lamp
driven by a constant current power supply and an integrating
sphere to get Lambertian output spectral radiance. The source
has capability to produce continuous radiant flux in the
wavelength range, from 350 to 2500 nm. The aperture setting of
the standard produces an output in 6 decades of dynamic range.