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INTRODUCTION

Specific wavelengths of radiation trigger distinct plant responses. Radiation that drives photosynthesis is called

photosynthetically active radiation (PAR) and is typically defined as total radiation across a wavelength range of

400 to 700 nm. PAR is almost universally quantified as photosynthetic photon flux density (PPFD), the sum of

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photons from 400 to 700 nm in units of micromoles per square meter per second (µmol m

, equal to

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-1

microEinsteins m

). While microEinsteins and micromoles are equal (one Einstein = one mole of photons), the

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Einstein is not an SI unit, so expressing PPFD as µmol m

is preferred. Daily total PPFD is typically reported in

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-1

units of moles of photons per square meter per day (mol m

) and is often called daily light integral (DLI).

The acronym PPF is also used and refers to the photosynthetic photon flux. The acronyms PPF and PPFD refer to

the same variable. Both terms are used because there is not a universal definition of the term flux. Flux is

sometimes defined as per unit area per unit time and sometimes defined as per unit time only. PPFD is used in this

manual.

In addition to wavelengths within the PAR range, far-red wavelengths (those just beyond 700 nm) are of particular

interest because they influence plant photosynthetic and morphogenic activity. Phytochrome pigments sensitive

to varying ratios of red and far-red light provide information to the plant about the light environment, and

therefore, influence growth patterns. Increasing the fraction of PAR, and specifically red radiation, relative to far-

red radiation indicates less shading and generally results in more conservative vertical growth patterns. Increasing

far-red radiation relative to PAR indicates more shading and results in more aggressive vertical growth patterns.

Sensors that measure PPFD are often called quantum sensors because they measure the number of incident

photosynthetic photons and one photon is a single quantum of radiation. Far-red sensors are similar in that they

measure incident photons, but the wavelength range is different. Far-red sensors can be thought of as quantum

sensors that measure radiation just beyond 700 nm. Sensors that pair detectors to measure both PPFD and far-red

photon flux density can be called PAR-FAR sensors.

The primary application of PAR-FAR sensors is monitoring plant light environments, including calculation of the far-

red fraction (far-red photon flux density / sum of PPFD and far-red photon flux density), in photobiology studies

(e.g., researching plant morphogenic activity).

Apogee Instruments S2 series PAR-FAR sensors consist of a cast acrylic diffuser, pair of photodetectors that

measure PAR and far red wavelength ranges (400-700 nm for PAR, 700-760 nm for far-red), and signal processing

circuitry mounted in an anodized aluminum housing. A cable to connect the radiometer measurement device is

also included. Sensors are designed for continuous measurement in indoor and outdoor environments. S2-100

series sensors output two voltage signals, one from each photodetector, that are directly proportional to the

radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all

angles of a hemisphere. The S2-442 model outputs a digital signal using Modbus RTU communication protocol over

RS-232 or RS-485.