UV-cut filters

Differences in the transmission spectrum

equipment
filters
Author

Pedro J. Aphalo

Published

2020-06-12

Modified

2023-04-22

Keywords

filters, ultraviolet, spectra

Note

I have substantially updated this page when transferring it to Quarto. I added data I have collected in the meantime, and changed the plots to be built in R within the R markdown source file. As a result the page now also includes the listing of the R code used to create all plots.

Introduction

As described in the post UVIR-cut filters both absorptive filters and interference filters are sold as UV-blocking filters. All modern photography digital cameras have internal UVIR-cut filters and most modern objectives transmit little UV-radiation. There are some exceptions, most if not all Olympus cameras are sensitive to long-wave UV-A radiation and a few modern objectives also transmit long UV-A radiation.

“UV” filters

Many filters sold as UV-filters do not differ from those sold as clear protection filters enough to matter. However, a few UV-filters do absorb in the whole UV-band and even into the visible violet band. Their effect might make a difference in high UV environments such as high elevation mountains and snow in sunny weather, and for special techniques. On the other hand, most photographers use UV-filters to mechanically protect their objectives. For such use the most important considerations are well polished and parallel surfaces and control of reflections to prevent flare and ghosting. Only photography using film and older objectives benefits significantly from the haze-cutting effect of UV and skylight filters. Possibly because of this, there is a lot of variation in how well UV filters intended for photography block UV-A radiation (Figure 1), with transmission of UV-A ranging from less than 0.06% to 80%.

Code
fig1.filters <- c("Zeiss_UV_Tstar_2.0mm_52mm",
                  "Firecrest_UV400_1.2mm_52mm",
                  "Hoya_UV0_HMC_Super_2.0mm_52mm",
                  "Thingyfy_MC_UV_0.7mm_58mm",
                  "KnightX_HD_MCUV_1.1mm_72mm",
                  "Unbranded_UV_Aliexpress_1.1mm_72mm",
                  "Kenko_UV_fake_1.9mm_30.5mm")
autoplot(filters.mspct[fig1.filters], facets = 1,
         annotations = list(c("+", "boundaries", "wls"),
                            c("-", "peaks")))
Figure 1: Spectral transmittance of “UV” filters of different types and brands. The first number after the in the labels gives the measured thickness of the filter, and the second one the thread size of the filter ring in which the filter is mounted (e.g. 52mm means M52 thread).

Based on their transmittance we can guess that the cheap unbranded filter has no anti-reflection coating while the others are all multi coated, with only slight differences in reflectance. The very steep cut-on slope in the top three plots suggests that these filters rely in part on interference filtering to block UV while the bottom three rely on the absorption of UV radiation by the glass itself.

Protector filters

Of the two filters sold as high-quality protector filters the B&W one cuts more UV than three of the UV-cut filters described above (Figure 1 and Figure 2), while the Hoya, is similar to some UV-filters from other brands. The anti-reflection coating in the B&W filter makes this filter almost perfectly transparent to visible light.

Code
fig2.filters <- c("BW_007_Clear_MRC_nano_1.2mm_46mm",
                  "Hoya_Protector_HD_2.1mm_72mm",
                  "Hoya_Protector_HD_2.0mm_52mm")
autoplot(filters.mspct[fig2.filters], facets = 1,
         annotations = list(c("+", "boundaries", "wls"),
                            c("-", "peaks")))
Figure 2: Spectral transmittance of “protector” filters of different types and brands. The first number after the in the labels gives the measured thickness of the filter, and the second one the thread size of the filter ring in which the filter is mounted (e.g. 52mm means M52 thread).

Haze and Skylight filters

Haze filters are nowadays rather difficult to find, although they are the only ones that one can expect to have some haze-removing effect on modern digital cameras. They are slightly yellow in colour, especially the stronger types. These are absorptive filters, of which the Haze 2A from Tiffen is a good example (Figure 3 upper panel). A filter type formerly popular for colour slide film is the Hoya Skylight 1B which does not cut UV radiation effectively, but instead absorbs some blue and green light (Figure 3 lower panel). With slide film this would correct the bluish cast introduced by UV radiation without actually blocking it. From the plots we can infer that the Tiffen filter is not AR coated while the Hoya is. Currently, I do not have any yellow glass filter comparable to 2A or 2E, such as Schott GG405 or GG415. I include in the figure a pale yellow glass filter with a nominal half maximum cut-in at 450 nm.

Code
fig3.filters <- c("Tangsinuo_JB450_2.0mm_52mm",
                  "Tiffen_Haze_2A_2.6mm_52mm",
                  "Hoya_Skylight_1B_3.0mm_67mm")
autoplot(filters.mspct[fig3.filters], facets = 1,
         annotations = list(c("+", "boundaries", "wls"),
                            c("-", "peaks")))
Figure 3: Spectral transmittance of “haze” filters of different types and brands. The first number after the in the labels gives the measured thickness of the filter, and the second one the thread size of the filter ring in which the filter is mounted (e.g. 52mm means M52 thread). Although the filters are not from Kodak, the denominations 2A and 1B originate in Kodak’s Wratten filter codes, which they approximate.

I haven’t tested any of the gelatine filters from Kodak as they are expensive a quite fragile. Optically, they perform very well as they are much thinner than glass filters. Wratten filters are still available from Kodak but only as gelatine filters. For example Haze 2E filters seems to be currently available only in this form.


Based on a recent post at the KolariVision blog showing the spectral transmittance of the sensor filters of various digital cameras, we can conclude that for general digital photography the UV-blocking ability of UV-cut filters is rarely relevant. However, my experience with Olympus cameras suggests they might be an exception if combined with specific objectives that transmit UV-A radiation.

The optical properties and anti-reflection multi-coating are the properties that will determine whether the filters will degrade image quality or not. With respect to mechanical protection the hardness of the glass and possibly also of the metal in the filter frame (brass vs. aluminium) may be relevant. Some filters have an additional coating that helps repel dirt making them easier to keep clean.

When UV radiation is very intense compared to visible radiation, UV-blocking filters may still be needed, as yellow, orange and red glass filters and the glass in some lenses fluoresce when exposed to UV radiation. Possibly, being a source of visible stray light reaching the sensor or film.

When using full-spectrum-converted cameras for visible photography, total blocking of UV radiation, as well as of NIR radiation, is needed. It is almost impossible to fully restore the colour rendition of an unmodified camera solely by means of filters, but filters are needed to make editing of the obtained files possible.


Variation among “identical” filters

Firecrest UV400 filters

I own seven Firecrest UV400 filters in different sizes, bought over several years, directly from the manufacturer, Format-Hitech in the U.K. These filters most likely rely on the thin-film coating to create an interference filter that reflects UV radiation. Whether in addition the substrate absorbs UV is difficult to know. At least, no absorptive glass of a thickness of about 1 mm can yield such a steep cut-in near 400 nm. The variation among these filters in the cut-in half maximum wavelength is within \(\pm 2 nm\), except for one filter, which cuts at 410.9 nm (Figure 4). These filters are AR MC and reflect very little visible light.

Code
fig4.filters <- c("Firecrest_UV400_1.0mm_72mm",
                  "Firecrest_UV400_1.1mm_46mm",
                  "Firecrest_UV400_1.1mm_49mm",
                  "Firecrest_UV400_1.2mm_52mm",
                  "Firecrest_UV400_1.2mm_62mm",              
                  "Firecrest_UV400_1.2mm_72mm",
                  "Firecrest_UV400_1.2mm_77mm")

fig4a <- autoplot(filters.mspct[fig4.filters],
                  annotations = list(c("+", "boundaries"),
                            c("-", "peaks")))
fig4b <- autoplot(filters.mspct[fig4.filters], range = c(380, 430),
                  annotations = list(c("+", "boundaries", "wls.labels"),
                                     c("-", "peaks")))
(fig4a / fig4b) * theme(legend.position = "none")
Figure 4: Spectral transmittance of seven filters of the same type and brand. Formatt-Hitech sells under the Firecrest brand three types of “UV” filters, a generic UV filter, UV360 and UV400, with cut-in at increasingly long wavelengths. The first number after the in the labels gives the measured thickness of the filter, and the second one the thread size of the filter ring in which the filter is mounted (e.g. 52mm means M52 thread). The lower plot zooms-into a range of 50 nm centred of the half maximum transmittance.

Tiffen Haze 2A filters

I own three Tiffen Haze 2A filters bought rather recently (i.e., they are not vintage filters). Tiffen seems to currently be the only remaining maker of Haze filters that allow no UV radiation pass through and do not fluoresce. Like most other colour filters from Tiffen the absorption is not based on absorptive glass, but instead by a substance encased between two layers of clear optical glass. This is important because most glass filters that absorb UV and violet radiation strongly fluoresce in the visible, compromising image quality and making them unsuitable for blocking UV radiation when photographing UV-induced fluorescence. The three filters I measured, have a more gradual slope than the Firecrest UV400 filters, and very little if any difference in the half-maximum wavelengths. On the other hand, one of the three filters seems to have lower transmittance than the other two across the visible region (Figure 5).

Code
fig5.filters <- c("Tiffen_Haze_2A_2.6mm_49mm",
                  "Tiffen_Haze_2A_2.6mm_52mm",
                  "Tiffen_Haze_2A_2.1mm_62mm")
fig5a <- autoplot(filters.mspct[fig5.filters],
         annotations = list(c("+", "boundaries"),
                            c("-", "peaks")))
fig5b <- autoplot(filters.mspct[fig5.filters], range = c(400, 440),
         annotations = list(c("+", "boundaries", "wls.labels"),
                            c("-", "peaks")))
(fig5a / fig5b) * theme(legend.position = "none")
Figure 5: Spectral transmittance of three filters of the same type and brand.

Sources of variation

Absorptive glass used for filters has specifications that are subject to manufacturing tolerances. Different melts of a given type of glass differ in the absorption spectrum. Most types of glass change their transmittance when exposed to UV-radiation or strong light. This is called solarization and depends on the composition of the glass, including accidental impurities like iron. The surface of some types of filter glass can oxidise in contact with air.

In the case of interference filters, the films deposited on the glass are extremely thin and their deposition is difficult, so different batches of coated glass can differ to some extent.

Filters with an absorbing medium encased between two plates of clear optical glass are much less common nowadays than in the past. In some cases the medium used was gelatin while Tiffen current ColorCore technology’s description does not tell what is the matrix used. There is, however, a publication that tells that Haze 2E filters need regular replacement as the dye or pigment used in them fades as a result of exposure to UV radiation, shifting the cut-in wavelength into the UV region.

Tip

All the spectral data shown above in figures have been measured by the author with the same Agilent/Hewlett-Packard diode array spectrophotometer (HP 8453). The specifications from Schott are from 2015. The data are available in R package ‘photobiologyFilters’. The figures were produced in R with R package ‘ggspectra’, which is an extension to package ‘ggplot2’. All these packages are open-source and available through CRAN. See also the R for photobiology web site.

The code is included in this page, but “folded”. To display the code used to produce the figures click on the word Code or the triangle next to it.