Decagon Devices SRS Operator's Manual
SRS
Spectral Reflectance Sensor
Operator’s Manual
Decagon Devices, Inc.
Version: January 15, 2014 — 12:16:45
SRS Sensors
Decagon Devices, Inc.
2365 NE Hopkins Court
Pullman WA 99163
Phone: 509-332-5600
Fax: 509-332-5158
Website: www.decagon.com
Email: support@decagon.com or sales@decagon.com
Trademarks
c
2007-2013 Decagon Devices, Inc.
All Rights Reserved
ii
SRS Sensors CONTENTS
Contents
1 Introduction 1
1.1 Customer Support . . . . . . . . . . . . . . . . . . . . 1
1.2 About This Manual . . . . . . . . . . . . . . . . . . . 2
1.3 Warranty . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.4 Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . 2
2 About SRS 3
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Specifications . . . . . . . . . . . . . . . . . . . . . . . 4
3 Theory 6
3.1 Normalized Difference Vegetation Index (NDVI) . . . 6
3.2 Fractional Interception of Photosynthetically Active
Radiation . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Canopy Phenology . . . . . . . . . . . . . . . . . . . . 10
3.4 Photochemical Reflectance Index (PRI) . . . . . . . . 11
3.5 Sun-Sensor-Surface Geometry Considerations . . . . . 12
3.6 Calculating Percent Reflectance from Paired Up and
Down Looking Sensors . . . . . . . . . . . . . . . . . . 14
4 Connecting the SRS 19
4.1 Connecting to Decagon Data Logger . . . . . . . . . . 19
4.2 3.5 mm Stereo Plug Wiring . . . . . . . . . . . . . . . 20
4.3 Connecting to a Non-Decagon Logger . . . . . . . . . 20
4.4 Pigtail End Wiring . . . . . . . . . . . . . . . . . . . . 21
5 Communication 23
5.1 SDI-12 Communication . . . . . . . . . . . . . . . . . 23
6 Understanding Data Outputs 25
6.1 Using Decagon’s Em50 series data loggers . . . . . . . 25
6.1.1 Up Looking Sensor Outputs . . . . . . . . . . . 25
6.1.2 Down Looking Sensor Outputs . . . . . . . . . 25
6.2 Using other data loggers . . . . . . . . . . . . . . . . . 26
7 Installing the SRS 27
7.1 Attaching and Leveling . . . . . . . . . . . . . . . . . 27
7.2 Cleaning and Maintenance . . . . . . . . . . . . . . . . 27
iii
CONTENTS SRS Sensors
8 Troubleshooting 28
8.1 Data Logger . . . . . . . . . . . . . . . . . . . . . . . . 28
8.2 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . 28
9 Declaration of Conformity 29
iv
SRS Sensors 1 INTRODUCTION
1 Introduction
Thank you for choosing Decagon’s Spectral Reflectance Sensor (SRS).
We designed the SRS for continuous monitoring of Normalized Difference Vegetation Index (NDVI) and/or the Photochemical Reflectance
Index (PRI) of plant canopies. We intend it to be low cost, easily
and quickly deployable, and capable of reliable operation over years.
Deploy the sensors over plant canopies to record first appearance of
green canopy, canopy closure, canopy senescence, light use efficiency,
and other variables. Customers can use these measurements to determine light capture, water use, phenology and biomass production.
This manual will help you understand the sensor features and how
to use this device successfully.
1.1 Customer Support
If you ever need assistance with your sensor, have any questions or
feedback, there are several ways to contact us. Decagon has Customer Service Representatives available to speak with you Monday
through Friday, between 7am and 5pm Pacific time.
Note: If you purchased your sensor through a distributor, please contact them for assistance.
Email:
support@decagon.com or sales@decagon.com
Phone:
509-332-5600
Fax:
509-332-5158
If contacting us by email or fax, please include as part of your message your instrument serial number, your name, address, phone, fax
number, and a description of your problem or question.
1
1 INTRODUCTION SRS Sensors
1.2 About This Manual
Please read these instructions before operating your sensor to ensure
that it performs to its full potential.
1.3 Warranty
The sensor has a 30-day satisfaction guarantee and a one-year warranty on parts and labor. Your warranty is automatically validated
upon receipt of the instrument.
1.4 Seller’s Liability
Seller warrants new equipment of its own manufacture against defective workmanship and materials for a period of one year from the
date of receipt of equipment.
Note: We do not consider the results of ordinary wear and tear,
neglect, misuse, or accident as defects.
The Seller’s liability for defective parts shall in no event exceed the
furnishing of replacement parts “freight on board” the factory where
originally manufactured. Material and equipment covered hereby
which is not manufactured by Seller shall be covered only by the
warranty of its manufacturer. Seller shall not be liable to Buyer for
loss, damage or injuries to persons (including death), or to property
or things of whatsoever kind (including, but not without limitation,
loss of anticipated profits), occasioned by or arising out of the installation, operation, use, misuse, nonuse, repair, or replacement of said
material and equipment, or out of the use of any method or process
for which the same may be employed. The use of this equipment constitutes Buyer’s acceptance of the terms set forth in this warranty.
There are no understandings, representations, or warranties of any
kind, express, implied, statutory or otherwise (including, but without limitation, the implied warranties of merchantability and fitness
for a particular purpose), not expressly set forth herein.
2
SRS Sensors 2 ABOUT SRS
2 About SRS
2.1 Overview
The SRS are two-band radiometers that measure either incident or
reflected radiation in wavelengths appropriate for calculating the
Normalized Difference Vegetation Index (NDVI) or the Photochemical Reflectance Index (PRI). Sensors are manufactured in four different versions: NDVI-hemispherical, NDVI-field stop, PRI-hemispherical
and PRI-field stop. The hemispherical versions (Figure 1) are built
with Teflon diffusers for making cosine-corrected measurements, and
are primarily designed for up looking measurements of incident radiation. The field stop versions (Figure 2) have a field of view restricted
to 20◦and are designed for pointing downward to measure canopy
reflected radiation in NDVI and PRI wavelengths.
The reflected radiation from a vegetated surface is highly variable,
depending on the amount and type of vegetation cover. This variability requires a relatively large number of sensors to properly characterize this surface. The field stop and hemispherical versions can
both be used to quantify canopy reflected radiation. The correct
choice of sensor will depend on the objectives of the study. The
hemispherical sensor will do a better job of averaging reflected radiation over a broad area, but if it is not installed normal to the canopy
surface it will also average sky, leading to measurement error. The
field stop sensor can be aimed at a particular spot or have a particular orientation giving the user more control over what portion of the
canopy is being measured. When using the field stop sensor in an
off-nadir orientation the user should be careful that the sensor is not
pointed above the horizon.
Calculating NDVI or PRI requires knowing both the incoming and
reflected radiation. Unlike the reflected radiation, the incoming radiation is spatially uniform above the canopy. So, you only need one up
facing radiometer to compute the vegetation indices for many down
facing radiometers. The up looking radiometer should be leveled and
have a hemispherical field of view.
3
2 ABOUT SRS SRS Sensors
The SRS is a digital sensor. Its outputs follow the SDI-12 standard. The SRS is best suited for use with Decagon’s Em50, Em50R,
and Em50G data loggers. However, customers can use the SRS with
other loggers, such as those from Campbell Scientific.
Figure 1: Hemispherical Version
Figure 2: Field Stop Version
2.2 Specifications
Accuracy: 10% or better for spectral irradiance and radiance values
Measurement Time: < 300 ms
NDVI Wavebands: 630 and 800 nm central wavelengths, with 50
and 40 nm full width half maximum band widths
PRI Wavebands: 531 and 571 nm central wavelengths, with 10 nm
full width half maximum band widths
Dimensions: 43 x 40 x 27 mm
Power Requirements: 3.6 to 15 V DC, 4 mA (reading, 300 ms) 30
µA (quiescent)
Operating Temperature: −40 to 50◦C
Connector Types: 3.5 mm (stereo) plug or stripped & tinned lead
wires (Pigtail)
Cable Length: 5 m standard; custom cable length available upon
request.
4
SRS Sensors 2 ABOUT SRS
Other Features:
• SDI-12 digital sensor, compatible with Decagon’s EM50
family and CSI loggers
• In-sensor storage of calibration values
• Four versions
NDVI-hemispherical
NDVI-field stop
PRI-hemispherical
PRI-field stop
• NDVI or PRI sensors with Teflon cosine correcting heads
• NDVI or PRI sensors with 20 degree field stops sealed
with clear acrylic
• NIST traceable calibration to known spectral radiance or
irradiance values
• Sensors can be mounted facing up or down, singly or in
tandem, leveled or aimed
• Fully sealed from the elements and UV resistant to minimize drift over time
5
3 THEORY SRS Sensors
3 Theory
Decagon designed the SRS instruments to measure the NDVI and
PRI vegetation indices from plant canopies. We caution users that
NDVI and PRI are measurements of electromagnetic radiation reflected from canopy surfaces, and therefore provide indirect or correlative associations with several canopy variables of interest and
should not be treated as direct measurements of these variables.
NDVI has a well-established and long history of use in remote sensing research and ecological applications related to canopy structure.
PRI, while showing great promise for quantifying canopy physiological function, is far more experimental with new uses and caveats
continually being uncovered. While NDVI and PRI can be powerful
tools for inferring structure and function of plant canopies, you must
take into account their limitations when interpreting the data. Section 3 provides an overview of the theory and discusses some of the
uses and limitations of each vegetation index.
3.1 Normalized Difference Vegetation Index (NDVI)
A number of nondestructive methods exist for remotely monitoring
and quantifying certain canopy characteristics. Some of those characteristics are: foliar biochemistry and pigment content, leaf area
index (LAI, Nguy-Robinson et al., 2012), phenology, and canopy
photosynthesis (Ryu et al., 2010). One of the most common nondestructive techniques involves measuring the NDVI. The NDVI is one
of a large number of vegetation indices. The principle derives from
a well known concept that vegetation reflects light differently in the
visible spectrum (400 to 700 nm) compared to the near infrared (>
700 nm).
Green leaves absorb light most strongly in the visible spectrum,
but are highly reflective in the near infrared region (Figure 3). Because bare soil, detritus, stems, trunks, branches, and other nonphotosynthetic elements show relatively little difference in reflectance
between the visible and near infrared, measuring the difference between reflectance in these two bands can be related to the amount
6
SRS Sensors 3 THEORY
of photosynthetic vegetation in the field of view of a radiometer.
See Royo and Dolors (2011) for an extensive introduction to using
spectral indices for plant canopy measurements.
Figure 3: Reflectance spectra for bare soil (Soil) and a healthy
wheat crop at various stages of development: heading (H), anthesis
(A), milk-grain stage (M), and post maturity (PM). Consider two
things about this figure: First, the considerable difference between
reflectance spectra from the soil and all stages of plant
development. Second, the changes in the visible spectra as the
canopy matures and senesces. We reproduced this figure with
permission from Royo and Dolors (2011).
Calculate NDVI with equation 1.
NDV I =
ρ
NIR
− ρ
red
ρ
NIR
+ ρ
red
(1)
Where, ρ
red
and ρ
NIR
are percent reflectances in the red and near
infrared (NIR). We assume percent reflectance to be the ratio of reflected to incident radiation in the specified waveband. A detailed
description of how to calculate reflectances from measured radiation
values is provided in equation number 4. NDVI has been shown to
7
Loading...