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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
examined and tested for application and compared with
information of other spectrometers and photometers. By this, a
combination and connection of different spectral data is
achievable.
For monitoring purposes in phyto-remediation applications
especially the following questions came up:
With which test setup and by what means can the
sensors be examined ?
e Which standard must be reached in order to compare
different sensors ?
e Is the spectral sensitivity and the data of the sensor
stable over a longer range of time ?
Currently, in the monitoring process a sensor of "the
manufacturer TriOS used having the following specifications.
Spectral range: 350 nm — 950 nm
Detector type: Channel silicon photodiode array
Spectral sampling: 3,3 nm / pixel
Usable channels: 190
Accuracy : Better than 6 — 10 %
This VIS-Scanner is an independent and compact hyper-spectral
sensor in the range of visible light. It has been designed for
outdoor use under unfavorable conditions. A ZEISS
spectrometer has been integrated as OEM-component in the
system. Grid and sensor are rigidly connected while the
standard SMA-connector of the light-wave conductor is
separated from the system in order to avoid damage of grid and
sensor due to external influences on the connector. Data is
transferred using an external interface which also accomplishes
the electrical connection with a voltage from 12 V DC to 240 V
AC. When connecting the sensor to a computer with wire
lengths of up to 50 m the connection either to a serial port or by
using a serial to USB converter the connection to USB 1.0 or
USB 2.0 standard is easily possible.
Figure 1: Mobile hyper-spectral sensor TriOS
1.3 Spectral data and calibration configuration
If directional interdependencies can be neglected the result of
any measurement with a spectrometer is intensity in a defined
band of wavelengths within a period of time. As long as the
measurement device does not acquire or consider the incident
light on a surface from which reflected or transmitted light is to
be measured, the data is not comparable to other measurements
which are taken with different devices or under different
conditions. This is due to the fact that reflection or transmission
of light depends on the incident radiation, cf. Weisensee 1992.
Thus, spectral measurements must be standardized, e.g.
according to DIN 5036. For the standardization of transmittance
measurements either the spectral transmittance factor or the
spectral optical density can be applied.
Don
r(4) 2 —— (1)
Den
Z spectral transmittance
Por p transmitted spectral radiant power
$,, incident spectral radiant power
1
D(A) - lg (2)
P(A) |
D spectral optical Density
The above formulae show that the value that is independent of
incident radiation is the logarithm of the ratio of incident and
transmitted radiation. Thus, for every field measurement of an
object surface also a measurement of the incident light is
required. In laboratory work a calibrated light source is
sufficient for a larger measurement campaign if it can be
assumed that besides the sensor also the light source remains
constant.
In order to avoid environmental influences on the sensor a
closed system consisting of the following parts has been build
up:
e electric and thermal stabilized tungsten-halogen light
source,
e shielded cuvette container with SMA-adapter,
e sensor and
e fiber optic cable.
CUVETTE LIGHT SOURCE
SENSOR
89
Figure 2: Concept of intensity measurement in a closed system.
Four different cuvettes with defined properties are currently in
use for testing and calibration purposes of the system, cf. ISO
9000-9004. One filter (in the following referred to as Fl, see
table 1) consists of holmium oxide glass (Ho303) which has
several narrow absorption peaks in the ultra-violet, visible and
near infra-red band. Furthermore, three neutral glass filters (F2
— F4, cf. table 2) which are characterized by high evenness and
stability and a constant transmittance in the visible range are
used.
1 2 3 4 3
279,30 360,90 453,60 536,40 638,00
Table 1: Calibrated wavelengths (Peaks) of the holmium
oxide cuvette F1.
No. 440nm | 465nm 546nm 590nm | 635nm
F2 0,268 0,237 0,238 0,255 0,255
F3 0,499 0,457 0,469 0,500 0,486
F4 0,964 0,899 0,926 0,960 0,915
Table 2: Spectral optical density of the calibrated cuvettes
(F2 — F4).
