Apogee Instruments S2-111 User manual

APOGEE INSTRUMENTS, INC. | 721 WEST 1800 NORTH, LOGAN, UTAH 84321, USA

TEL: (435) 792-4700 | FAX: (435) 787-8268 | WEB: APOGEEINSTRUMENTS.COM

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

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).

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