Transmittance of spectral irradiance by climate screens and nets used in horticulture and agriculture

Solar spectral photon irradiance (μmol m^-2 s^-1) transmitted by shade screens and nets from several manufacturers was measured with an array spectroradiometer, which had been calibrated for measurements of UV and visible solar radiation (Maya2000 Pro Ocean Optics, Dunedin, FL, USA; D7-H-SMA cosine diffuser, Bentham Instruments Ltd, Reading, UK - see Hartikainen et al., 2018 for details of the calibration). A protocol of dark measurements and measurements excluding UV radiation was followed to quantify and account for the dark noise and stray light in the UV waveband. Both a correction for the shape of the slit function and for stray light were included in the post-processing of the spectra (Aphalo et al., 2016).

The measurements of spectral irradiance under climate screens, and shade and insect nets, were done on clear days in sunny conditions close to solar noon (between 10 a.m. to 2 p.m local time) at NC State University campus (35.78°N, -78.67°W) in late July and early August 2017, and in Viikki Field Plots at the University of Helsinki (60.22°N, 25.01°E, 55 m asl) in July and August 2018. The methods for measurements in North Carolina follow the same protocol described below and published in Kotilainen et al., (2018).

The measurements were done in an open field with no surrounding structures or buildings within ~20 m. Repeated measurements of each different sample were made in a randomised order, thus ensuring comparability among measurements. Measurements were made on a tripod 0.7 m above the ground and the sample was secured to a wooden plate 5 cm above the diffusor. A test comparing four larger (1 x 1 m) samples against those of the standard dimensions that we used, found that the area of screen/net measured did not affect the results. Thus, there was no evidence that the relatively small dimensions of the sample, allowed unfiltered diffuse or scattered radiation to be measured.

Measurements under each screen/net sample (Svensson 13 x 19 cm, Mallas Textiles 8 x 10 cm) were made twice to account for any possible effect of sample placement over the cosine diffuser and change in the sun angle during a set of measurements. In our measurements and analyses we focused on the differences in spectral quality, created when employing these screens and nets, in order to address the lack of detailed studies of these light environments.

Measurements of solar spectral irradiance in the wavelength range from 290 nm to 900 nm were processed in R, using the photobiology packages developed for spectral analysis (Aphalo, 2015). We present spectral photon irradiance (μmol m^-2 s^-1) which is more relevant than energy irradiance (W m^-2) when studying plants. A plant absorbs photons producing a chemical change (Grotthus Law). Nevertheless, essentially the patterns of spectral attenuation by different screens and nets will be consistent, irrespective of whether spectra are expressed as photon or energy irradiance.

Utilizing predefined functions available in the photobiology packages, we calculated the integrals and ratios (of these integrals) as follows:: UVB:PAR 280–315 nm/400-700 nm, UVA:PAR 315–400 nm/400-700 nm, blue:green (B:G) 420–490 nm/500-570 nm, blue:red (B:R) 420–490 nm/620-680 nm. Red and far-red for the calculation of R:FR ratio are 655–665 nm and 725–735 nm, respectively. UVB radiation and UVA radiation are defined according to ISO, blue, green and red according to Sellaro et al. (2010), and R:FR according to Smith(1982). A common approach used in horticulture to compare light sources and experiments is to assess the relative contributions of different wavebands between 400 nm to 900 nm by dividing them into 100-nm increments (Both et al., 2017). We also used this approach giving, blue100 = 400–500 nm, green100 = 500–600 nm, red100 = 600–700 nm, far-red100 = 700–800 nm and near-infrared100 = 800–900 nm. The same definitions of the UV-waveband are maintained for both spectral integrals and their ratios throughout, i.e. according to ISO. This is because the UVB and UVA wavebands of solar radiation follow distinct daily patterns of variation; UVB irradiance is highest during the four hours around solar noon, whereas the UVA waveband of solar radiation remains a similar proportion of total irradiance throughout the day. These differences also imply that UVA and UVB radiation follow different diurnal and seasonal patterns of variation (Seckmeyer et al., 2007). A nominal shading factor (SF) was used in the analysis of the results and figures to demonstrate that reduction in total irradiance due to the depth of shade does not, in itself, affect the spectral quality under the screens and nets.

Data Files

DataBaseScreensNets.zip

Graphs (.jpg files) of actual measured (1) Energy Irradiance, (2) Photon Irradiance, and (3) proportion transmittance, for each screen and net.  (1) Energy Irradiance figures (suffix _EI.) and (2) Photon Irradiance figures (suffix _PI.) are the mean of measured values to irradiance under the filter and corresponding measurements without the screen or net (“open” measurement) for comparison (290-898 nm wavelength range).  The proportion transmittance under each screen or net is calculated from the open and measured spectrum (suffix _Trans). The low wavelength end of the spectrum is trimmed (<310 nm) since % transmittance are inflated by small measurement noise in the UV-B region where irradiance values are very low.

The database screens and net are identified by the name of the company “_” name of the screen/net for all 197 materials.

ImagesScreensNets.zip

Image files (.jpg files) from photos and scans of each of the measured screens and nets. One image from each of the 197 materials measured is stored in folders arranged according to the company for each filter type. The companies are: Criado y Lopez; HowiTech; Huanchang yarns; Jiangsu Huachang Yarns & Fabrics; Mallas_Textiles and Svensson.

ProcessNetFilesNovAll.txt

The measured spectral irradiance for each material (screen and net) from 290 nm – 898 nm. Data are in columns giving: (A) Company – the Company name; (B) FilterName – the filter name; (C) a serial number, effectively equivalent to the order in which the materials were measured; (D) wavelength – at intervals recorded by the array spectrometer; (D) FilterEI - energy irradiance of transmitted solar radiation measured 3-5 cm beneath the materials (screens & nets); (E) FilterPI – photon irradiance equivalent to the energy irradiance; (F) OpenEI – energy irradiance of solar radiation at the same location without the filter material (screen / net) (G) OpenPI – photon irradiance equivalent to the energy irradiance; (H) FilterFactor – the proportion of irradiance transmittance by the filter material (screen / net) at each wavelength measured, a value between 0.0 and 1.0 (value out of range at low wavelengths in the UV-B region are replaced with 0.0 or 0.1).

Processing of raw spectra was done with Photobiology packages in R. Full spectra were recorded with an integration time set manually to give maximum counts of just less than 60 000 are the wavelength corresponding to peak spectral irradiance. Bracketing was performed by recording a second spectrum (long spectrum) with ten-times longer integration time to achieve greater accuracy in measurements in the UV region (< 400 nm). These two spectra were spliced together. Each filter measurement was accompanied by a dark measurement (to estimate dark noise) and a measurement under a polycarbonate filter (PC) to correct for stray light. These two readings were performed immediately after the filter material (screen/net) was measured; both within 10 s total of the filter material measurement for both the full spectrum, and long spectrum.

ProcessNetFilesNovAllAve.xlsx

This excel file contains the same information in columns as the file ProcessNetFilesNovAll.txt but with a second worksheet showing the trimming calculations for out-of-range readings at low UV-B wavelength and with an addition final column, the irradiance spectrum open29_irrad (described below)

Open29_irrad.txt

In order to obtain standardised BSWF files to comparison with each other, the calculated % spectral transmittance results for each filter material (screen/net) were applied to a “standard” solar noon open spectrum from Helsinki close to midsummer (Open29_irrad.txt).

This spectrum was measured at Viikki Fields, Helsinki on Wed June 27^th 2018 at 13:15:33 EEST (Integration Time, 110000 μsec ) in a completely open area.

For utility of the database users should substitute the spectrum from their own location to obtain spectral irradiance data for the effects of the filter materials (screens/net) at their site.

ProcessNetFilesNovAllBSWFcorrected.txt

This matrix of spectral integrals and ratios calculated with the Photobiology packages in R to each of the spectra presented in ProcessNetFilesNovAll.txt.  Column headings are the filter material ID, made up from the Company name _ filter name. The first column contains row names identifying spectral integrals and ratios – first as energy irradiance then as photon irradiance and finally as photon ratios. Calculations are made using the BSWF (Biological Spectral Weighting Function): PAR_e; UVB_e; UVA_e; UVb350_e; UVa350_e; Blue_e; Green_e; Red_e; Far_red_e; GEN_G_e; GEN_T_e; PG_e; DNA_N_e; CIE_e; FLAV_e; Infra_red_e; PAR_q; UVB_q; UVA_q; UVb350_q; UVa350_q; Blue_q; Green_q; Red_q; Far_red_q; GEN_G_q; GEN_T_q; PG_q; DNA_N_q; CIE_q; FLAV_q; Infra_red_q; UVB_UVA; UVB_PAR; UVA_PAR; R_FR_Sellaro; R_FR_Smith10; R_FR_Smith20; B_G; B_R; PhyEqi.

ProcessNetFilesNovAllBSWF.xlsx

This files contains the same data as ProcessNetFilesNovAllBSWFcorrected.txt and shows on individual worksheets, processing of original, smoothed, and corrected data, comparisons of the Original vs. Corrected, and Original vs. Smoothed data. The same BSWF calculations for the example open spectrum open29_irrad (used for standardisation) are given on their own worksheet, as is the corresponding “FilterFactor” for each spectral integral and spectral photon ratio. This information could be useful for situations where a full spectrum is unavailable for a location and comparisons among filters need to be made from only partially data (e.g. PAR PPDF).  The final worksheet “Type” lists the filters and their function (i.e. shade, pest net, hale net, ground cover etc.).