Filter Frequently Asked Questions
Most UV reflectance photography has been done with a Venus filter. How are your filters different?
Two types of photographic filters are used for ultraviolet photography - the Dichroic, also known as an Interference filter, and the Absorptive filter. Either of these general filter types may be structured as a UV-bandpass filter; that is, a filter that allows only ultraviolet wavelengths to pass through the camera lens to the film or sensor.
UVR Optics makes both dichroic and absorptive filters. Absorptive filters operate by attenuation of light through absorption of specific wavelengths. Among the advantages of our absorption filters are the relatively low cost, high stability, ease of cleaning, and freedom from bandshift.
A further consideration is the band of near-UV (UVA) wavelengths that UVR Optics filters pass. As you can see in this chart, the AndreaU MK II is optimized to provide a FWHM (full width half maximum) bandwidth from 346nm to 383nm. That span is significant, because terrestrial life-forms - trees, plants, flowers, insects, birds, etc. - use those wavelengths.
In order to get the maximum transmission of the higher wavelengths of UV, UVROptics decided to utilize the properties of the Dichroic filter. As you can see in the chart to the right, a dichroic filter can be made that permits a very sharp drop in the higher wavelengths and a broader peak. The graph is representative of the performance of our SEU Gen2 filter based upon independent testing by a firm in Sweden.
I can see a few pin-point size bright spots in the filter, what are those?
Those are bubbles within the glass, or infinitesimal remnants of bubbles trapped between glass layers and revealed by the optical cement. Our colored filter glass commonly has a very small number of bubbles. It is not entirely possible to avoid bubbles in the glass due to the nature of the glass composition and production processes. Colored glass producers have developed a method of describing the bubble content of a filter glass. UVR Optics chooses their glass with care, based upon a number of criteria, including bubble content. Be assured that any bubbles visible in our glass will not have a negative effect on the performance of the filter. See Schott documentation for more information - schott-bubbles-and-inclusions.