In this first installment I compare the current cost of UV-B LEDs to that five years ago. I describe in detail two type of UV-A flashlights. I describe new types of LEDs emitting light with a "sun-like" spectrum. For the last two items original spectral data from my tests is presented.Non peer reviewe

Aphalo, Pedro J.

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https://helda.helsinki.fiPractical Hints and Tips [on use of light-emitting diodes in research]Aphalo, Pedro J.2019-01Aphalo , P J 2019 , ' Practical Hints and Tips [on use of light-emitting diodes in research] ' ,UV4Plants Bulletin , no. 2 , pp. 9-14 . https://doi.org/10.19232/uv4pb.2019.1.30http://hdl.handle.net/10138/325851https://doi.org/10.19232/uv4pb.2019.1.30cc_by_sapublishedVersionDownloaded from Helda, University of Helsinki institutional repository.This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Please cite the original version.UV4Plants Bulletin, 2019, no. 1 MethodsPractical hints and tipsPedro J. Aphalo, ORCID: 0000-0003-3385-972XViPS, Organismal and Evolutionary Biology, University of Helsinki, Helsinki, FinlandDOI: 10.19232/uv4pb.2019.1.30 © 2019 The Author, licensed underNote: The opinions given and recommenda-tions of products are those of the author(s),and do not imply endorsement or not by theUV4Plants Association.The cost of UV LEDs continues todecreaseFinally, “high-power” UV-B LEDs are beco-ming affordable. They are not yet a com-modity and are mostly available through spe-cialized distributors like Roithner Lasertech-nik although a few medium power LED typeshave appeared also in the catalogues of ma-jor electronic component distributors suchas Mouser and Digikey. There are also a few(re)sellers in China, selling UV-B LEDs evencheaper. However, there are a few things tokeep in mind when buying LEDs: 1) within agiven part designation there are usually binsor sub-types available. These are not equiva-lent in performance, so a cheaper LED withthe same designation, is likely to be one witha lower radiation output. Reliable sellers willprovide the part designation in whole, ratherthan only the first part of it. 2) When LEDs areproduced there is variation in the wavelengthof the radiation emitted by individual LEDs.This means both that there are sometimesbroad tolerances (e.g.±5nm or even±10nm)in the specifications, and that LEDs are alsoclassified or binned by manufacturers basedon wavelength of emission. Bins are usuallyalso denoted by some digits or letters in thetrailing part of the part code.How much have prices changed? I will dothe comparison based on UV radiation out-put, not on electrical power. Part of the re-duction in cost is due to increased efficiencyin the conversion of electrical power into op-tical radiation. This also reduces ancillarycosts, and the improved life further reducesthe effective cost of using UV-B LEDs. If wetake LEDs available from Roithner Lasertech-nik as example, in December 2014 I boughtLEDs emitting 15mW at 1180€ per piece,while this October 2019 I bought LEDs emit-ting 58mW at 310nm for 178€ per piece (cur-rent price 146€). The cost per watt of emit-ted UV-B has decreased from 76500€ downto 3070€, or 25 times in 5 years! The LEDsI bought this year were expensive ones, in-corporating a quartz lens. Cheapest availa-ble from a, reliable in my experience, Chi-nese supplier (https://www.leds-global.com/) emit 18mW at 310nm and cost 16.90€,which makes their cost 940€ per watt, or1.2% of what I paid in 2014!UV flashlightsDuring the past few months I have been as-ked about UV-A flashlights and I have beenusing one of them for some time and recentlyacquired a second one. I also borrowed athird one for testing. I give here some hintson which flashlights are worthwhile buyingand which ones are not. Good-quality UV-flashlights can be useful as radiation sourcesfor photography and short term experiments©2019 by the authors 94 P. J. Aphalo: Practical hints and tips367.7 393.7● ●UVA0.000.250.500.751.00300 350 400 450 500Wavelength (nm)Spectral  photon  exposure  Q(λ)  (relative  units)Flashlight LEDs Nichia 365 nm Cree 'UV'Figure 4.1: Comparison of two different “UV” flashlights. The output of both flashlights was normali-zed for this figure, in reality the radiation output of the flashlight with the Cree LED was only a fractionof that of the one with the Nichia LED. The flashlight with the Nichia LED had an unspecified UV-passand VIS-blocking filter, while the one with the Cree LED had a clear glass. The spectrum for the Jax-man U1c flood is not shown as it overlaps the one for the Convoy 2+. (Emission spectra measured bythe author. Data to be included in the next release of R package ‘photobiologyLamps’.)Figure 4.2: A Convoy 2+ flashlight next to a Jax-man U1c flood flashlight, both equipped withUV-pass/VIS-block filters.but many of those being sold are unsuitable.These flashlights are rather widely availableas they have various uses: checking the fluo-rescent safety threads and patterns in banknotes, identifying minerals in stones, look-ing for various types of contaminants bothFigure 4.3: The Convoy 2+ flashlight and theJaxman U1c flood flashlight have similar bodies,and the same generic “remote cable switch”works with both.in households and commercial settings andeven finding scorpions. Most if not all theseflashlights use UV-A or violet LEDs, and al-most all of them are sold with a clear glasswindow. First thing to be aware of is thatmany of these UV flashlights are based onLEDs with peak of emission at 385 to 410nm.The ones that are useful as UV-A sources arethose with LEDs with peak of emission at365nm as the others emit too much visiblelight (Fig. 4.1). Look for those using LEDs ma-nufactured by the Japanese company Nichia,10 ©2019 by the authorsUV4Plants Bulletin, 2019, no. 1Figure 4.4: Light fields of a Convoy 2+ flashlight(top) and a Jaxman U1c flood flashlight. The pho-tographs are of the UV-induced fluorescence of awhite sheet of paper with rulings 10mm apart.as these have strong output and less VIS con-tamination. In all cases the clear glass shouldbe replaced with a good quality UV-pass VIS-blocking filter.At this time, my recommendation is to buya Convoy 2+ flashlight with a Nichia LEDat 365nm (ca. 25€) or a Jaxman U1c floodflashlight, also with a Nichia LED at 365nm(ca. 60€) (Fig. 4.2, 4.3). As we will see be-low these flashlights are different and suita-ble for different purposes. Both are readilyavailable through many different sellers ineBay and Aliexpress. As always there is arisk when buying from such on-line sellers,so it is important to check that the flashlightyou have received matches the descriptionand works as expected before the time forFigure 4.5: A granite boulder at night. Top:illuminated with a white LED light source, ex-posure time 0.2 s; bottom illuminated with theConvoy 2+ flashlight by “light painting” and pho-tographed through UV-blocking filters, exposuretime 55s. The yellow-fluorescing lichen on theboulder and the red fluorescence from the mossat the base of the stone are both clearly visible.complaints and refunds is over. Buying fil-ters is trickier than buying the flashlights un-less you are willing to pay good money forthem. Safest is to buy a filter made of Hoyaglass already cut to size, buy this may costyou more than the flashlight. Filters 20.5mmin diameter and 2mm-thick, made from gene-ric filter glass under the denomination ZWB2can be good enough, but quality seems tovary a lot (they are sold for anything between0.45 to 5€). It is also important to considerboth type, quality, and thickness of the glassfilters in addition to their diameter. OneeBay seller (uviroptics) sells these Convoy 2+flashlights with a Hoya U-340 filter already in-stalled at a price of ca. 90€ and the filter by©2019 by the authors 114 P. J. Aphalo: Practical hints and tipsitself for ca. 30 euro. The quality of the filterspre-installed by other sellers can vary greatlyand will be type ZWB2 or inferior.The Convoy 2+ Nichia 365nm flashlightsemit UV-A radiation in a concentrated beam,which can be a drawback for some applicati-ons. At close distance, at the centre of thebeam UV-A irradiance is a few times morethan in full sunlight at midday on a summerday (e.g. at 200mm distance I have measuredin the hot-spot of the UV-A illuminated fielda photon irradiance of 1370 μmolm−2 s−1).However, the area irradiated is mall and ir-radiance uneven (Fig. 4.4, top). The Convoy2+ uses a LED rated at about 3W of electricalpower. The Convoy 2+ is good for spottingfluorescent object at night at distance. Forphotography of UV-A-induced visible fluores-cence one usually needs to “paint” the areabeen photographed as is rarely possible to il-luminate the whole area photographed simul-taneously with such a narrow beam (Fig. 4.5).Recently similar flashlights have appea-red under a different brand name: Jaxman.These are readily available with pre-installedfilters. Price and quality wise the Convoy2+ is a good choice if one needs a concen-trated UV-A radiation beam. Jaxman hasa variant under the denomination of “U1Cflood” which provides a much broader andeven UV-A radiation field (Fig. 4.4, bottom).Even though the Jaxman is of the same smallsize as the Convoy, it has a LED rated at 6Wof electrical power. Still as the UV-A radi-ation is projected evenly over the illumina-ted area, the maximum irradiance is lowerthan for the Convoy 2+ (e.g. at 200mm dis-tance I have measured in the hot-spot of theUV-A illuminated field a photon irradianceof 117 μmolm−2 s−1). I have used the Jax-man very little but spectral measurementsindicate that both have an almost identicalemission spectrum and leakage of VIS light.While the Convoy 2+ has only and on-offswitch, the Jaxman, has low and high powersettings. Although one could expect the Jax-man to heat up quickly because of the higherpower rating, the metal body seems to dissi-pate heat well enough.Both flashlights are externally very similar,with the bodies so similar as to be interchan-geable. Theymeasure 25mm in diameter and117mm in length. The heads are different inthe electronics, LED and optics. They bothuse one rechargeable Li-ion battery of type18650. These batteries are fairly common,but not something you will get in the nearestsupermarket. You need a charger or a batterywith a USB charger circuit built in.The Jaxman U1c flashlight comes in multi-ple versions, the “flood” version I have and anormal or non-flood version which I haven’ttested. This version with normal optics canbe expected to be similar to the Convoy 2+in its 3W version, but I have no proof of this.This non-flood version at the moment sellsfor around 40€ for the 3W version and 55€for the 6W version. They also have the twostep power switch that the Convoy 2+ lacks,as well as the pre-installed UV-pass filter.Although these flashlights emit in the UV-A, given the very strong and concentratedradiation output, it is advisable to use eyeprotection especially when using them in dar-kness or in weak visible light.LEDs for horticulture andmuseumsNichia (nichia.jp) has recently releasedLEDs chips designed for use in horticultureproduction. What is special about them? Pri-marily two features: very high energy conver-sion efficiency (About 2 or 2.5 times as effi-cient as Valoya B50 luminaires) and an emis-sion spectrum unusual for grow lamps as itcontains a good deal of radiation emission inthe green range (Fig. 4.6 top). According tomy tests photon irradiance can reach nearly2500 in a growth chamber (Aphalo and Bele-vich, unpublished).Both Nichia and Toshiba/Seoul Semicon-ductor have released new series of LEDs12 ©2019 by the authorsUV4Plants Bulletin, 2019, no. 1451.8530.4622.3452.7622.3lllllUVB UVA PAR0.02.55.07.5300 400 500 600 700 800Wavelength (nm)Q(λ)  (µmol s−1 m−2 nm−1)5000 K3000 KRescaled to PAR = 1000 µmol m−2 s−1Nichia horticulture419.6624.5531lllUVB UVA PAR024300 400 500 600 700 800Wavelength (nm)Q(λ)  (µmol s−1 m−2 nm−1)5000 KsunlightRescaled to PAR = 1000 µmol m−2 s−1Nichia OptiSolis 5000 K and sunlight407.5540.5622.3531llllUVB UVA PAR0246300 400 500 600 700 800Wavelength (nm)Q(λ)  (µmol s−1 m−2 nm−1)5000 KsunlightRescaled to PAR = 1000 µmol m−2 s−1Toshiba/Seoul Semiconductor Sunlike 5000 K and sunlightFigure 4.6: Emission spectra of white LEDs from Nichia and Seoul Semiconductor (LEDs’ emissionspectra measured by the author. Data to be included in the next release of R package ‘photobiology-LEDs’.)©2019 by the authors 134 P. J. Aphalo: Practical hints and tipsadvertised as having an emission spectrumsimilar to that of sunlight (Fig. 4.6 middle).These LEDs are mainly aimed at achievingextremely good colour reproduction. Thosefrom Nichia, sold under the name of Op-tisolis, do not emit UV and are advertisedas ideal for use in museums. Those fromToshiba/Seoul Semiconductor are sold un-der the name Sunlike and emit to someextent at 390 nm (Fig. 4.6 bottom) (moreinformation at https://www.nichia.co.jp/en/product/led_sp_optisolis.htmland http://www.seoulsemicon.com/en/technology/SunLike/). Both series includeversions emitting warmer and cooler whitelight. Energy conversion efficiency is slightlylower than that of the LEDs described in theprevious paragraph. These LEDs emittingsun-like light would still need to be combi-ned with UV-B, UV-A and far-red-emittingLEDs to achieve an spectrum that couldconsidered a simulation of sunlight relevantto plants. Ready-assembled modules usingthese different LED chips are available fromLumitronix (https://www.leds.de).14 ©2019 by the authors

Practical Hints and Tips [on use of light-emitting diodes in research]

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