Introduction 

Apogee's new research-grade PAR-FAR sensors are excellent, cost-effective tools for measuring the output of plant lights for photobiology and plant morphogenic studies.

The PAR-FAR Sensor has a unique two-band sensor for measuring both the PAR (PPFD) and Far-red outputs of electric grow lights.

The new research-grade Apogee PAR-FAR sensor is for measuring both the traditional PPFD photosynthetic photon flux and separately quantifying the photon flux of far-red photons (700-760 nm). The outputs include the traditional quantum flux, the far-red photon flux, and the far-red fraction (far-red photon flux density / sum of PPFD and far-red photon flux density).

This sensor quantifies far-red photons, and for many applications it reduces the need for a more complex measurement with a spectroradiometer.

Why are Far-red measurements important?

Specific wavelengths of radiation elicit distinct plant responses. Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total radiation across a range of 400 to 700 nm. PAR is often expressed as photosynthetic photon flux density (PPFD): photon flux in units of micromoles per square meter per second (µmol m^-2 s^-1, equal to microEinsteins per square meter per second) summed from 400 to 700 nm (total number of photons from 400 to 700 nm).

In addition to PPFD, red and far-red wavelengths are of particular interest because they influence photosynthetic and morphogenic activity. Phytochrome pigments sensitive to varying ratios of red and far-red light provide information to the plant about its light environment and, therefore, its optimal growth patterns. Increasing the fraction of red light indicates less shading and generally results in more conservative vertical growth patterns; increasing the far-red fraction indicates more shading and results in more aggressive vertical growth patterns. Sensors that measure PAR and red/far-red radiation are often called PAR-FAR sensors. These sensors quantify both PPFD and far-red radiation incident on the sensor’s detecting surface.

Features

Apogee Instruments S2 series PAR-FAR sensors consist of a cast acrylic diffuser, pair of photodetectors that measure specific wavelength ranges, signal processing circuitry mounted in an anodized aluminum housing, and a cable connected to the radiometer measurement device. Sensors are potted solid with no internal airspace and are designed for continuous measurement in indoor and outdoor environments. S2 sensors output an analog voltage for each photodetector that is directly proportional to red/far-red radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all angles of a hemisphere.