On the concepts and correct use of radiometric quantities for assessing the light environment and their application to plant research

Abstract

Light is one of the most important factors for photosynthetic organisms to grow. Historically, the amount of light in plant sciences has been referred to as light intensity, irradiance, photosynthetic active radiation, photon flux, photon flux density, etc. On occasion, all these terms are used interchangeably, yet they refer to different physical units and each metric offers distinct information. Even for experts in the fields of plant photobiology, the use of these terms is confusing, and there is a loose implementation of each concept. This makes the use of radiometric units even more confusing to non-experts when looking for ways to measure light, since they could easily feel overwhelmed by the specialized literature. The use of scientific concepts must be accurate, as ambiguity in the use of radiometric quantities can lead to inconsistencies in analysis, thus decreasing the comparability between experiments and to the formulation of incorrect experimental designs. In this review, we provide a simple yet comprehensive view of the use of radiometric quantities in an effort to clarify their meaning and applications. To facilitate understanding, we adopt a minimum amount of mathematical expressions and provide a historical summary of the use of radiometry (with emphasis on plant sciences), examples of uses, and a review of the available instrumentation for radiometric measurements.

Keywords: Irradiance; Light meter; Photon flux density; Photosynthetically active radiation; Spectroradiometer.

Fig. 1

Basics in the terminology of light intensity. A Comparison of the most common terms that refer to the quantitative aspects of light in relation to photosynthesis. A search was performed on the 17th of January 2020 that covers all years in the database published in research articles and reviews (English only) using Scopus: the number alongside each label is the total number of publications. All terms were explored using the following searches: [“Irradiance” AND “photosynthesis”], [“Light Intensity” AND “photosynthesis”], [“Photon Flux Density” OR “PFD” AND “photosynthesis”], [“Photosynthetic Photon Flux Density” OR “PPFD” AND “photosynthesis”], and [[“Photosynthetically Active Radiation” AND “Photosynthetic Active Radiation”] OR “PAR” AND “photosynthesis”]. B The diagram that describes the concept of light intensity using a point source as an example. If the light intensity is measured over the integrated area of the sphere, it is called optical intensity. If the radiant power is measured only in a small area of the sphere determined by the solid angle (Ω), it is called radiant intensity. If the light power is measured in an integrated area, it is called irradiance. R represents the sphere. Figure 1B was modeled after Andy Anderson (Amherst College, Amherst MA, USA)

Fig. 2

Comparison of ground-level sun spectra in different atmospheric conditions or radiometric quantities. A The effect of turbidity (concentration of aerosols) in the sun spectrum illustrates how atmospheric conditions affect the sunlight spectrum. Spectra simulated using SOLCORE (Alonso-Álvarez et al. 2018) and turbidity values from 0 (clear sky; top) to 0.35 (hazed; bottom) are shown. B Comparison of spectral irradiance and PFD of the sun at turbidity of 0.15. Both panels were simulated using the SPECTRL2 model under the following conditions: latitude: 52° 23′ 24″ N; longitude: 1° 33′ 36″; date and time 2011/6/21 at 12:16 PM; aerosol optical depth model: “rural,” pressure: 103,000.0 Pa; relative humidity: 30%; precipitable water: 0.00142 cm; ozone thickness: 34 mm

Fig. 3

Spectral responses of different photoreceptors involved in morphogenetic effects. Absorption spectra of different photoreceptor proteins: UVR8 (purple), cryptochrome (blue), phototropins and zeitluples (green), phytochrome in red form (P_r in red), and far-red form (P_fr in black). All spectra formalized to their highest peak. Bars above the spectra indicate reported wavebands of morphogenetically active radiation for the respective photoreceptors. Spectra re-digitized from (Galvão and Fankhauser 2015)

Fig. 4

Types of light meters and detector elements. A Diagram of a thermopile-based pyranometer. The instrument is composed of two glass domes, a thermopile detector, a desiccant to protect the detector element and the electronics, and a solar shield to protect the electronics against UV. B Comparison of the spectral response of commonly used detector elements: a thermopile (TD2X, Thorlabs Inc.), silicon diode A (FDS100, Thorlabs Inc.), and silicon diode B (FD11A long-range, Thorlabs Inc.). Note that not all silicon diodes have the same spectral response and thermopiles have a steadily even response. C Comparison of the spectral response of commercial light meters. Three examples were chosen to compare contrasting sensors: LI-190R (LI-COR Inc.), SQ-110 (Apogee Instruments Inc.), SKP215 (Skye Instruments Ltd.), plus an ideal PAR detector (LI-COR-Biosciences 2018). If the response is above the ideal curve, it overestimates photons on those wavelengths and, if the contrary, it underestimates them. D Diagram of the basic components and function of a spectroradiometer. Light is captured at the entry port and diffused through a cosine corrector. Light crosses a slit to then be projected into a diffraction grating to obtain monochromatic light. The slit size determines the spectral resolution, the narrower the slot, the higher the resolution. Light is then corrected and projected onto a diode array to assess the PFD at each wavelength. Note that this comparison does not intend to favor one brand over another but just to illustrate the spectral response differences. Readers are encouraged to consult two technical reports of different companies: LI-COR Biosciences (2018) and Blonquist & Johns (2019)

Fig. 5

Quantum meters and cosine correctors. A Quantum meter anatomy. B Lambert’s cosine law, light collection, and function of the cosine corrector. C Light captured by a quantum meter equipped with either a 2π (hemispherical) cosine corrector or a 4π (spherical) cosine corrector

Fig. 6

Application example of how to assess the PFR using either 2π or 4π sensors in the field. In orange, the emitted sunlight is displayed as an arrow traveling at a quasi-right angle towards the sensors. In green, the backscattered (scattered and reflected light) reaches the sensors