Typical for modern digital photography is the protection of the image sensor against UV-A and UV-B radiation in the wavelength range of 280 – 380 nm1. For this purpose UV blocking filters are used which are nothing else than longpass filters with a minimum transmission in the UV range and a maximum transmission in the VIS range. In modern camera lenses, this UV protection is already provided by the lenses used. An additional UV filter therefore makes no sense for optical reasons. Instead, they are used as mechanical protection for the front lens of the camera lens.
In this experiment, a longpass filter working in exactly the opposite way is used (Fig 1). This filter is only transmissive for wavelengths smaller than 400 nm and blocks the VIS range. This vividly enables the study of UV radiation-absorbing materials such as sunscreen.
The UV filter originates from an old fluorescence microscope and was used for the excitation of fluophores in the ultraviolet range, in particular from the DNA fluorescent dye 4′,6-diamidine-2-phenylindole (DAPI)2.
To illustrate the optical spectrum, I made several measurements with my self-developed and built spectrometer. The wavelength is plotted against the relative intensity (dimensionless unit). A commercially available halogen lamp serves as a control. Its spectrum is shown in blue and covers the whole VIS, as well as near UV and infrared range. If the UV longpass filter is placed in front of it, it becomes clear that it is practically opaque for wavelengths between 400 and 680 nm (red curve). Only in the near infrared range the transmission is increased again. If a UV laser in combination with the UV filter is used as the light source instead, its high transmission in the UV range becomes apparent (grey curve).
I have designed and 3d printed a filter holder for the old Leitz or Leica microscopy filters. The holder fits directly onto the Canon EF 50 1.4 lens. This way all light is directed through the filter. In addition, all filters in the series can be replaced quickly and easily.
The older filters are made of solid colored glass, typical of the 1960s. This has the disadvantage of generally lower light transmission, regardless of wavelength. Each of these filters also acts as a signal attenuating filter over the entire spectral range
Due to the generally low transmission of the filter, photography is difficult. I used a Canon EOS 6D in combination with a Canon EF 50mm f1.8. The camera was firmly mounted on a tripod. For the shot I took off the filter, focused, switched to manual focus and then put the filter back on. The photo was taken with an aperture of f1.8, 6 s exposure time and ISO 800. During the recording it was very cloudy accordingly low was the UV radiation.
- PETG filament, black
- 1.FULTON JE Jr. Utilizing the Ultraviolet (UV Detect) Camera to Enhance the Appearance of Photodamage and Other Skin Conditions. Dermatologic Surgery. Published online March 1997:163-169. doi:10.1111/j.1524-4725.1997.tb00013.x
- 2.Kapuscinski J. DAPI: a DNA-Specific Fluorescent Probe. Biotechnic & Histochemistry. Published online January 1995:220-233. doi:10.3109/10520299509108199