Filters for UV photography

A comparison between the Baader U filter and the StraightedgeU filter, both with sun and a modified flash as light sources. Examples of flowers from two species, which display different false-colours with the two filters.

The first two sets of three photographs were taken in sunlight under a clear sky. The third set indoors using a modified flash for illumination. A full-spectrum converted Olympus E-M1 with a Sigma DN 60 mm f:2.8 A MFT objective was used hand held, with image stabilization, autofocus and auto-exposure enabled in the first set. In the second and third sets a Sigma DN 30 mm f:2.8 A MFT was used instead.  In each set two different UV pass filters were used for UVA images and a UV+IR cut (visible band pass) filter for the reference image. White balance for UV images was obtained from an image of a white PTFE slab. Images were white balanced in Photo Ninja 1.3.5.

In the case of the  first set, depicting L. vulgare, in sunlight, the difference between the two UV pass filters are unexpected and striking. Even the background green vegetation is not grey in the case of the Baader filter when white balance is based on the PTFE reference.

The yellow colour cast with the Baader U in the first two sets might be the result of overexposure as there was no UV-reflective elements in the frame. Even if related to overexposure, where did the yellow cast originate? It could simply be the difference in transmittance between the two filters combined with the spectral sensitivity of the sensor of the camera, on the other hand the Sigma DN 60 mm f:2.8 A objective seems to emit yellow fluorescence when illuminated by UV radiation, as well as the Sigma DN 30 mm f:2.8 A, although the later’s fluorescence seems a bit weaker at sight. This is not totally unexpected, as some other objectives are known to fluoresce when under UV. I need to investigate this in more detail…

All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2018 Pedro J. Aphalo

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