UV-B lamps emit visible light. Does it matter?

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Most UV-B lamps emit UV-A and visible light in addition to UV-B radiation, but we usually only worry about the contribution of wavelength shorter than those intended (UV-C during UV-B irradiation).

  1. Can longer wavelengths emitted by UV-B light sources be safely ignored in the design of all photobiological experiments?
  2. How can we study the effect of UV-B using lamps that in addition emit UV-A and visible radiation?
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If irradiation is done under weak or no background visible light, visible light emitted by UV-B lamps can be the main cause of the observed responses. My own direct experience is with q-panel lamps emit enough visible light to induce germination, most likely through phytochromes. In this example the apparent effect of UV-B irradiation remained almost unchanged when lamps were filtered with a filter blocking all UV radiation and blue light.

If the irradiation is done under broad band white light, e.g. sunlight, it may be safe to assume that the contribution from the UV-B lamps is irrelevant in the visible, but not on the UV-A.

What solutions there are to this problem?

  1. If working with small samples, one may be able to afford a filter that blocks UV-C, UV-A, visible and infra-red radiation but transmits UV-B. Most likely one would need to use a stack of multiple expensive filters as used for imaging.
  2. With whole plants or in the field, all we can do is add an additional treatment, a treatment that contains only the unwanted uninteresting disturbances from the lamps, and compare this against the combination of desired UV-B treatment plus uninteresting disturbances. It is important to also include a real control to assess the side effects of our lamp installation. We need of course to remember, that this set up assumes that effect of the UV-B treatment in the absence of the side effects of lamps is the same as when they are present.

Take home message: only lasers emit truly monochromatic light, any other source will in addition emit some other wavelengths than what we expect from their names or even published spectra. Plants under the right circumstances can respond to extremely weak irradiances and fluences. In particular the phytochrome mode of action for “very-low-fluence responses” (VLFR) can trigger germination when seeds are exposed to 3 μmol m-2 s-1 for 1 s, equivalent to full sunlight for 2 ms or 0.002 μmol s-1 m-2 for 30 min.

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