Atelier Bonryu(E)
ultraviolet photography
Laboratory: Ultraviolet Photography
Taking Ultraviolet Photographs - Filters
Filters for the ultraviolet photography: Usually filters for the UV photography do not hit store shelves in Japan but one can buy them easily by mail order. There are various filters transparent to the UV light with different frequency ranges and different transmission efficiency. However, when one would take a UV photograph by using a commercially available digital camera the wavelength range is about 300 - 400 nm and, therefore, a filter transparent to this range is necessary. To choose a filter of this kind it should be remarked that such filters are often transparent to the IR light as well as to the UV light even if it is opaque to the visible light. When one use this kind of filter which is transparent to both the UV and the IR lights it is necessary to combine an appropriate color compensating filter for blue or cyan colors with it. Transmission efficiency of such blue or cyan color compensating filter is rather high for the lights of violet to cyan color and rather low for red light to the infrared light. However, the ratio of the transmission efficiency of the UV to the IR light is at most only twice. Therefore, the IR light is reduced to only a half by this combination. As for commercially available glasses for observation of solar eclipse the main purpose of them is to reduce the visible and the IR lights and it seems usable for this purpose. However, most of them reduce the penetration of the UV light too, and they cannot be used as filters for the UV photography.
Filters opaque to visible and transparent to UV lights and almost opaque to the IR light are commercially also available (Baader Venus II, Hoya U-340, etc.). Though the maximum transmission efficiency of these filters for the UV light is about 75 %, that for the IR light is only 0.15 % (Baader Venus II) and less than 3 % (Hoya U-340). But, in general, these filters are more expensive than filters which transmit the IR light.
I began the UV photography by using a combination of the visible opaque and UV transparent filter "B+W 403" and the color compensating filter "Wratten CC20Cyan (Kodak)". The B+W 403 filter transmits the UV light with the wavelength of 300 - 400 nm but also the IR light with the wavelength of more than 700 nm. The transmission efficiency of the filter for the UV light (360 nm) is 60 % and that for the IR light (746 nm) is 17 %. Whereas the CC20Cyan filter transmits 75 % of the UV light and 60 % of the IR light. Therefore, the combination of these filters transmits 45 % of the UV light and 10 % of the IR light, which means that the ratio of the IR light to the UV light is reduced from 28 % to 22 % (see above figure). As shown later a photograph of a nectar guide has been taken. Therefore, it may be safe to conclud that I could take a UV photograph by using the combination of the filters but it worries me that the influence of the IR light might be still large. And now I use mainly the Hoya U-340 filter for taking the ultraviolet photograph.
Robert Wood and Ultraviolet transparent filter: As described in the page of the IR photography Robert William Wood (1863 - 1955) is called a father of the infrared and the ultraviolet photography. He was an American physicist specialized in the optics and was famous as an inventor of the wood glass. He was interested in many things in various fields even outside the optics research and concerning the optics he is the first who took a zone plate photograph in addition to the research and invention concerned with the IR and the UV photography. From these points he is very important person relating the content of this web pages. I am thinking to prepare a separate page on him in more detail in this site later.
Whereas Wood made a research of the absorption of light through glass where various atoms or chemical compounds are added to realize visible opaque and UV transparent glass, and in February of 1903 he published papers on the visible opaque and ultraviolet transparent filter which uses nitroso-dimethyl-anilin in three academic journals (Philosophical Magazine, Philosophical Zeitschrift, Astrophysical Journal). The first filter he made transmits blue light considerably but it was improved by and by. As for photographing by using this filter a landscape photograph was taken on test emulsion in 1910. By the way as an IR film became commercially available in 1930 Robert W. Wood is the first person who took an infrared photograph in the world. As for the UV photographs he took photographs of landscapes and the lunar surface in 1910. In 1919 at the French physical society he published details of the glass filter used for the UV photography, which is called as the Wood's glass later. The filter is made of BaNaSi glass where 9 % of nickel oxide is added. In these days there were a lot of relating papers published, for example, applications of the ultraviolet photography such as observation of human skin, a secret communication by using a invisible light, and so on. In the world war I secret messages were exchanged between an aircraft and battlefield by using the UV light (during night) and the IR light (during daytime) and researches on the UR and the IR lights became active. Robert W. Wood is also a pioneer of the ultraviolet photography of a human skin and there remain the ultraviolet photographs of human skins which he took. These are summarized in the web site Pioneer of Invisible Radiation Photography. Also, as for Wood's glass andWood's lamp the brief description of Wikipedia will be helpful.
We consider on "some difficult problems concerned with the ultraviolet (UV) photography" in more detailed manner. As for the fewness of the quantity of the UV light using auxiliary light sources may be effective. By using a UV dedicated lens absorption of the UV light by the lens is reduced. However, these measures may cost you considerably. Especially there is a small choice of commercially available UV dedicated lenses and they are very expensive, and, moreover, they are not necessarily indispensable. Unlike with these a visible opaque and UV transparent filter is a basic need for taking ultraviolet photographs and without it a UV photograph cannot be taken. By contrast this kind of filters is not so expensive as the UV dedicated lens. However, a visible opaque and UV transparent filter may cost you much because there is no such filter among the commercially available gelatin filters as in the case of the IR photography. Consequently there are many ideas to make the UV photography inexpensive. Though I have not experienced them the following ideas are introduced at various sites on the internet; (1) to use a broken fragment of a black light as the filter, (2) to use a sheet-type band-pass filter (“gelatin” filter), Fujifilm BPB-42 as a substitution of a visible opaque and UV transparent filter. Though BPB-42 is not a UV transparent filter it transmits a light with the wavelength around 420 nm which is near the UV wavelength region. Though any of them is not an ideal UV transparent filter they are inexpensive and one can take a photograph which looks like a UV photograph. For example, the BPB-42 filter transmits visible lights with the wavelength of 380 - 500 nm with the maximum transmission efficiency of 54 % and under the visible light it looks black as a neutral density filter. In the following we describe on a standard visible opaque and ultraviolet transparent filter.
Transmission properties of filters used for the UV photography
Dependence of the transmission efficiency on the wavelength for various filters used for the UV photography. Data were scanned from brochures and reconstructed. Details of the figure are described in the main text. Though I use a color compensating filter CC20C of Wratten (Kodak) for reducing transmission efficiency of the infrared light the curve in the figure is for the CC20C filter of Fujifilm.
