Fluorescence of glass filters

Until very recently I was not aware that optical glass filters can fluoresce when exposed to ultraviolet radiation. In this short post I use two filters from Heliopan that I own as examples of this. However, many other filters, and even the glass elements in some camera lenses can also fluoresce. The photographs below illustrate this, using a non-fluorescing filter from Formatt-Hitech as reference. The three photographs were taken with a UV blocking filter on the camera objective and ultraviolet-A illumination from LEDs with peak of emission at 365 nm (Led engin LZ1-10UV00-0000).

Unknowingly using a filter that fluoresces can make us think that the object being photographed itself fluoresces. Photographs with default white balance are shown below. A photograph taken using the same exposure and set up but with the UV-A LEDs switched off is shown as reference.

Same images with white balance set in Capture One 10 to the Novoflex Zebra reference grey card used as background. As reference we use a “normal” photograph taken under white light.

Conclusion: If you intend to image UV-excited auto-fluorescence be aware that many other objects may also fluoresce, including the filters you may be using to block the UV radiation, or the camera objective itself.

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

(c) 2017 Pedro J. Aphalo


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