CHSOS just tested  an interferential UV/IR cut filter for Ultraviolet Fluorescence Photography (UVF). This filter is the solution to the auto fluorescence of other UV filters

[1]. It is recommended for UVF close up photography. The CHSOS filter set for art examination comprises two filters for UVF photography: XNite CC1 coupled with B+W 420. The first is transparent to the UV (360-400 nm) and it is necessary to block any infrared light that can be produced by the UV lamp or being present in the scene from other sources. If UV LEDs lamps are used such as the CR-UV Viewer or the Xenopus High flux lamp the infrared component is much less than that leaking from wood lamps.

The B+W 420, a UV cut-off filter, is necessary to stop the UV reflected from the subject. A modified full spectrum camera becomes even more sensitive to UV light and therefore UV reflected must be shielded otherwise the images will be dominated by the purple color due to UV light. So far the B+W 420 filter works fine and there would be no need to replace it. It has been used by CHSOS in a number of art examination studies [2, 3] and for the workflow method for pigments identification [4] and it works fine in the panoramic photography set up [5]. The limits of the B+W 420 filter become obvious when it is necessary to take a close up UVF image of the subject.  B+W 420 has a comparable transmittance curve as the interferential UV/IR filter. The difference is that the B+W 420 is fluorescent itself under UV light. If the picture is taken from a distance and the UV lamp are at a close angle, the reflected UV light that hits the filter is a fraction of the UV fluorescence and the excited fluorescence of the filter doesn’t affect the image. On the other hand, if a close up UVF photo is taken the amount of UV hitting the filter could affect considerable the image quality.


pigments checker v2 vsPigments Checker is for photographers, conservators and scientists interested in technical documentation of paintingss. It has 54 swatches of historical pigments designed for infrared photography, ultraviolet photography and other technical photographic methods for art examination. Check it out!

Pigments Checker is a collection of 54 swatches of his­tor­i­cal pig­ments that have been applied using gum ara­bic as a binder on a cel­lu­lose and cot­ton water­color paper, acids and lignin free. This paper is not treated with opti­cal bright­en­ers, it’s slightly UV flu­o­res­cent, and it reflects IR. Two cross-hair lines, 0,2 mm (ver­ti­cal) and 0.4 mm (hor­i­zon­tal) are printed on each swatch of paper before the appli­ca­tion of paint, in order to have a means to eval­u­ate the pig­ments’ trans­parency in the IR and IRR imag­ing. Among all the pig­ments and their vari­eties ever used in art these pig­ments col­lec­tion select the most used ones from antiquity to early 1950’.


Ultraviolet photography filters

Under UV light the B+W 420 filter emits an orange-yellowish fluorescence. For close up photo the interferential filter is strictly recommended since has no fluorescence at all.

Ultraviolet photography filters pigments

The top and the bottom rows images were taken respectively with a 200 mm (f/8) and a 50 mm (f/8) lens at 2 m and 0.7 m distance. The lamp and the subject remained in the same relative position and just the camera was moved forward. the CC1 images were taken just with the XNite CC1 filter mounted. They are out of focus because the UV reflected dominates the images and the lens was focused on the VIS range. They also shows the characteristic purple color due to UV reflected hitting the sensor. The images at 0.7 m are slightly affected by the yellow fluorescence of the BW 420 filter, as shown in the detail of the zinc white swatch.

 

[1] Luis Bravo Pereira “UV Fluorescence photography of works of art: replacing the traditional UV cut filters with interference filters” International Journal of Conservation Science, 1, 3, 2010, pp. 161-166.

[2] A. Cosentino, S. Stout, R di Mauro, C. Per­ondi “The Cru­ci­fix Chapel of Aci Sant’Antonio: Newly Dis­cov­ered Fres­coes” Archeo­mat­ica, 2, 36–42, 2014.

[3] A. Cosentino, M.C. Caggiani, G. Rug­giero, F. Salvem­ini “Panoramic Mul­ti­spec­tral Imag­ing: Train­ing and Case stud­ies” Bel­gian Asso­ci­a­tion of con­ser­va­tors Bul­letin, 2nd Trimester, pp 7–11, 2014

[4] A. Cosentino “Iden­ti­fi­ca­tion of pig­ments by mul­ti­spec­tral imag­ing a flow­chart method” Her­itage Sci­ence, 2:8, 2014.

[5] A. Cosentino “A prac­ti­cal guide to Panoramic Mul­ti­spec­tral Imag­ing” e-conservation Mag­a­zine, 25, pp 64–73, 2013.

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