Apogee Instruments S2-111 User manual

APOGEE INSTRUMENTS, INC. | 721 WEST 1800 NORTH, LOGAN, UTAH 84321, USA
TEL: [phone] | FAX: [phone] | WEB: APOGEEINSTRUMENTS.COM
Copyright © 2021 Apogee Instruments, Inc.
OWNER’S MANUAL
NDVI SENSORS
Models S2-111 and S2-112
Rev: 15-Oct-2021
Upward (model S2-111)
Downward (model S2-112)

TABLE OF CONTENTS
Owner’s Manual ............................................................................................................................................................................... 1
Certificate of Compliance......................................................................................................................................................... 3
Introduction ............................................................................................................................................................................. 4
Sensor Models ......................................................................................................................................................................... 5
Specifications ........................................................................................................................................................................... 6
Deployment and Installation.................................................................................................................................................... 9
Cable Connectors................................................................................................................................................................... 11
Operation and Measurement ................................................................................................................................................ 10
Maintenance and Recalibration............................................................................................................................................. 14
Troubleshooting and Customer Support................................................................................................................................ 15
Return and Warranty Policy................................................................................................................................................... 16

CERTIFICATE OF COMPLIANCE
EU Declaration of Conformity
This declaration of conformity is issued under the sole responsibility of the manufacturer:
Apogee Instruments, Inc.
721 W 1800 N
Logan, Utah 84321
USA
for the following product(s):
Models: S2-111, S2-112
Type: NDVI Sensors
The object of the declaration described above is in conformity with the relevant Union harmonization legislation:
2014/30/EU Electromagnetic Compatibility (EMC) Directive
2011/65/EU Restriction of Hazardous Substances (RoHS 2) Directive
2015/863/EU Amending Annex II to Directive 2011/65/EU (RoHS 3)
Standards referenced during compliance assessment:
EN 61326-1:2013 Electrical equipment for measurement, control, and laboratory use –EMC requirements
EN 50581:2012 Technical documentation for the assessment of electrical and electronic products with respect to
the restriction of hazardous substances
Please be advised that based on the information available to us from our raw material suppliers, the products
manufactured by us do not contain, as intentional additives, any of the restricted materials including lead (see
note below), mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), polybrominated
diphenyls (PBDE), bis (2-ethylhexyl) phthalate (DEHP), butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), and
diisobutyl phthalate (DIBP). However, please note that articles containing greater than 0.1% lead concentration are
RoHS 3 compliant using exemption 6c.
Further note that Apogee Instruments does not specifically run any analysis on our raw materials or end products
for the presence of these substances, but we rely on the information provided to us by our material suppliers.
Signed for and on behalf of:
Apogee Instruments, October 2021
Bruce Bugbee
President
Apogee Instruments, Inc.

INTRODUCTION
Radiation reflected from surfaces (e.g., plant canopies, soil) provides information about the state of the surface.
Reflectance is the ratio of radiation reflected by the surface to radiation incident on the surface.
A common index calculated from reflectance measurements is normalized difference vegetation index (NDVI).
NDVI is calculated from red and near infrared (NIR) reflectance and provides a measure of surface greenness.
The typical application of NDVI sensors is monitoring plant canopies. NDVI is related to leaf area of the plant
canopy and canopy chlorophyll content of leaves and is often used to monitor green up in the spring and
senescence in the fall.
Apogee Instruments S2 series two-band sensors consist of a cast acrylic diffuser (upward-looking sensor) or acrylic
window (downward-looking sensor), pair of photodiodes that measure specific wavelength ranges, and signal
processing circuitry mounted in an anodized aluminum housing. A cable to connect the sensor to a measurement
device is also included. S2 series sensors are designed for continuous irradiance (upward-looking sensor) or
radiance (downward-looking sensor) measurements in indoor and outdoor environments. Reflectance derived
from paired upward-looking and downward-looking sensors can be used to calculate NDVI. Apogee NDVI sensors
are offered with digital SDI-12 output or with analog voltage output.

SENSOR MODELS
This manual covers the analog output models S2-111 and S2-112 (in bold below). Digital models are covered in
their respective manuals.
Model
Signal
Description
S2-111
Voltage
Measures red and NIR irradiance (upward-looking) for
NDVI
S2-112
Voltage
Measures red and NIR radiance (downward-looking) for
NDVI
S2-411
SDI-12
Measures red and NIR irradiance (upward-looking) for NDVI
S2-412
SDI-12
Measures red and NIR radiance (downward-looking) for
NDVI
An upward-looking (S2-111) sensor’s model number
and serial number are located on the bottom of the
sensor. If the manufacturing date of a specific
sensor is required, please contact Apogee
Instruments with the serial number of the sensor.
A downward-looking (S2-112) sensor’s model
number and serial number are located near the
connector on the sensor cable. If the manufacturing
date of a specific sensor is required, please contact
Apogee Instruments with the serial number of the
sensor.

SPECIFICATIONS
Calibration Traceability
Apogee S2 series NDVI sensors are calibrated through side-by-side comparison to the mean of three transfer
standard sensors under sunlight. The transfer standard NDVI sensors are calibrated through side-by-side
comparison to the mean of six replicate direct and diffuse solar spectra collected in Logan, Utah, using an Apogee
PS-300 spectroradiometer. The Apogee PS-300 spectroradiometer is calibrated with a quartz halogen lamp
traceable to the National Institute of Standards and Technology (NIST).
NDVI
S2-111
(upward-looking)
S2-112
(downward-looking)
Power Supply
Self-powered
Output (sensitivity)**
14 mV per W m⁻² (Red)
20 mV per W m⁻² (NIR)
12.5 mV per W m⁻² nm-1 sr-1 (Red)
25 mV per W m⁻² nm-1 sr-1 (NIR)
Calibration Factor
(reciprocal of
sensitivity)**
0.07 W m⁻² nm-1 per mV (Red)
0.05 W m⁻² nm-1 per mV (NIR)
0.08 W m⁻² nm-1 sr-1 per mV (Red)
0.04 W m⁻² nm-1 sr-1 per mV (NIR)
Calibration Uncertainty
± 5 %
Output Range**
40 mV (Red)
40 mV (NIR)
15 mV (Red)
15 mV (NIR)
Wavelength Ranges
Red detector = 650 nm ± 5 nm with 65 nm FWHM*
NIR detector = 810 nm ± 5 nm with 65 nm FWHM*
Measurement Range
2x full sunlight
Measurement
Repeatability
Less than 1 %
Long-term Drift
(Non-stability)
Less than 2 % per year
Response Time
Less than 1 ms
Field of View
180°
30°
Directional (Cosine)
Response
± 2 % at 45°; ± 5 % at 75° zenith angle
Temperature Response
Less than 0.1 % per C
Housing
Anodized aluminum body with acrylic diffuser
IP Rating
IP68
Operating Environment
-40 to 70 C; 0 to 100 % relative humidity
Dimensions
30.5 mm diameter,
37 mm height
30.5 mm diameter,
34.5 mm height
Mass (with 5 m of cable)
140 g
Cable
5 m of shielded, twisted-pair wire; TPR jacket (high water resistance, high UV stability, flexibility in cold conditions);
pigtail lead wires; stainless steel (316), M8 connector
*FWHM = full-width half-maximum
** The Output (sensitivity), Calibration Factor (reciprocal of sensitivity), and Output Range are all approximations and variable from sensor to
sensor

Cosine Response
Upward- and Downward-looking Two-band Sensors
Directional, or cosine, response is defined as
the measurement error at a specific angle of
radiation incidence. Error for Apogee S2
series upward-looking sensors is
approximately ± 2 % and ± 5 % at solar zenith
angles of 45° and 75°, respectively.
Upward (model S2-111)
Downward (model S2-112)

Spectral Response Graph of NDVI
The spectral range of the NDVI
sensors is defined by a center
wavelength of 650 nm ± 5 nm with 65
nm full-width half-maximum (Red)
and 810 nm ± 5 nm with 65 nm full-
width half-maximum (NIR). Measured
spectral responses are plotted in the
graph.

DEPLOYMENT AND INSTALLATION
Mount the upward-looking sensor to a solid surface with the nylon mounting screw provided to prevent galvanic
corrosion. To accurately measure irradiance incident on a horizontal surface, the sensor must be level. An Apogee
Instruments model AL-100 Leveling Plate is recommended to level the sensor when used on a flat surface or being
mounted to surfaces such as wood. To facilitate mounting on a mast or pipe, the Apogee Instruments model AL-
120 Solar Mounting Bracket with AL-100 Leveling Plate is recommended.
To minimize azimuth error, the sensor should be mounted with the cable pointing toward true north in the
northern hemisphere or true south in the southern hemisphere. Azimuth error is typically less than 1 %, but it is
easy to minimize by proper cable orientation.
In addition to orienting the cable to point toward the nearest pole, the sensor should also be mounted such that
obstructions (e.g., weather station tripod/tower or other instrumentation) do not shade the sensor. Once
mounted, the blue cap should be removed from the sensor. The green cap can be used as a protective covering
for the sensor when it is not in use.
Nylon Screw: 10-32x3/8
Nylon Screw: 10-32x3/8
Model AL-120
Model AL-100

Downward-looking sensor mounting
Mount the sensor to a solid surface with the nylon mounting screw provided to prevent galvanic corrosion. To
facilitate mounting on a mast or pipe, the Apogee Instruments model SM-400 Two Band Radiometer Downward-
looking Mounting Bracket is recommended. The bracket can be adjusted to any angle between 0° (sensor pointed
straight down) and 90° (sensor horizontal).