UV short-pass filters

Differences in the transmission spectrum

equipment
filters
Author

Pedro J. Aphalo

Published

2023-04-14

Modified

2024-02-20

Keywords

filters, ultraviolet, spectra

Filters

I have been buying special filters from Tangsinuo through AliExpress or eBay for some years now. Chinese glass is cheaper than that from the best known brands Hoya (Japan) and Schott (Germany), specially when bought as ready-assembled filters for photography from Heliopan or specialized suppliers. Hoya and Schott glass are the “gold standard”. Chinese glass filters on the other hand have a reputation of having inconsistent quality. So while much cheaper, they tend to be seen as cheap and low quality substitutes for Hoya and Schott glass. There are two types of quality problems: 1) the out-of-specifications transmission spectra and 2) striations, bubbles and other optical defects. In most cases, the later problems are irrelevant when filters are used on light sources but important when used on a camera lens. Off-specification spectral characteristics are important in both cases, and can affect both cut-off or cut-in wavelengths and how well off-band wavelengths are blocked.

In general Chinese glass is described as more variable, with specifications that seem to have more relaxed tolerances for a given type. For example, the infrared long-pass filters from Zomei can have cut-in wavelengths significantly different to those expected from their type, and can vary among filters having the same type description. Whether this is important depends on how the filters are used. If one has a single filter, a difference of a few tens of nanometres in the infrared will be difficult to notice in the resulting image without a side by side comparison, but if one would like consistent results with equivalent filters of different sizes or if ones aims are not just pictorial but scientific illustrations, such differences can matter. Poor blocking of off-band wavelengths can be a major problem in UV photography.

That Chinese glass is more variable means that a seller that selects and sells good melts of glass will sell much better goods than one who does no selection or even may sell the rejects. The difficulty is that good quality and poor quality or out-of-specifications glass melts are all sold using the same codes to identify them. Frequently, in AliExpress and eBay filters are described using codes for Schott glass. However, such glasses cant differ significantly from the corresponding Schott glasses. In other words, one needs to trust that the seller sells filters that are close enough to the description provided. If one lacks a spectrometer to measure the transmission of the filters that one buys, one cannot be sure that the filter that has been delivered agrees with the description.

Spectral Measurements

I am able to measure the spectral transmittance of the filters I buy as well as their thickness. The typical spectral transmittance for Schott and Hoya glass are available as part of the specifications provided by the manufacturers. So, as I have measured the spectral transmittance of filters bought from Tangsinuo, I can compare these measured spectra to the specifications of Schott glass they are supposed to be equivalent to.

Some types of filters I have bought in two or three sizes from Tangsinuo, and comparing them can give some rough idea of how much variability there in the spectral transmittance of filters from a given supplier. It is however, important to keep in mind, that there is variation from melt to melt also for Hoya and Schott glass and that the spectra published by Schott are described as typical and subject to variation.

I have measured total spectral transmittance, that is including the effect of reflection. Except two, the filters I have are 2 mm-thick. The specifications from Schott are for internal transmittance plus a reflection factor, and not always for a thickness of 2 mm. As a first step we need to compute the total transmittance for a thickness of 2 mm. Figure 1 shows the resulting spectra that we will use as reference.

Code
Schott_UG.mspct <- 
  filters.mspct[c("Schott_UG1", "Schott_UG11", "Schott_UG5", "Schott_BG3")]
Schott_UG.mspct <- 
  convertThickness(Schott_UG.mspct, thickness = 2e-3)
Schott_UG.mspct <- 
  convertTfrType(Schott_UG.mspct, Tfr.type = "total")
Schott_UG.mspct <- 
  trim_wl(Schott_UG.mspct, range = c(200, NA), fill = 0) 
Schott_UG.mspct <- 
  trim_wl(Schott_UG.mspct, range = c(NA, 1050), fill = NULL)

autoplot(Schott_UG.mspct, annotations = c("-", "peaks"), 
         w.band = c(UV_bands("CIE"), IR_bands("CIE"))) +
  labs(linetype = "")
Figure 1: Total transmittance spectra for a thickness of 2 mm computed from Schott’s typical internal transmittance spectra provided in their documentation.

We then compare the specifications to actual Schott glass of the same type (Figure 2). The agreement is not perfect. This is expected, as the specifications are for a typical glass melt and subject to variation.

Code
autoplot(c(Schott_UG.mspct["Schott_UG1"],
           filters.mspct["UQG_UG1_2mm"]),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(c(Schott_UG.mspct["Schott_UG1"],
           filters.mspct["UQG_UG1_2mm"])),
         annotations = list(c("+", "boundaries"), c("-", "peaks"))) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 2: Ultraviolet band-pass filters. Schott UG1 specifications compared to original Schott glass supplied by UQG. Measured total spectral transmittance or computed from Schott specifications.

Next a comparison between Schott UG1 and generic ZWB2 at the same thickness of 2 mm (Figure 3). Transmittance of the ZWB2 is higher both in the UV region and in the NIR region. Normalization shows that the shape of the spectrum is very similar to the measured UG1 but a little less dense at the same thickness.

Code
autoplot(c(Schott_UG.mspct["Schott_UG1"],
           filters.mspct[c("Tangsinuo_ZWB2_2.0mm_52mm",
                         "Tangsinuo_ZWB2b_2.0mm_52mm")]),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(c(Schott_UG.mspct["Schott_UG1"],
           filters.mspct[c("Tangsinuo_ZWB2_2.0mm_52mm",
                         "Tangsinuo_ZWB2b_2.0mm_52mm")])),
         annotations = list(c("+", "boundaries"), c("-", "peaks"))) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 3: Ultraviolet band-pass filters. Schott UG1 specifications compared to ZWB2 glass supplied by Tangsinuo. Measured total spectral transmittance or computed from Schott specifications. Measured total spectral transmittance or computed from Schott specifications.

Next a comparison between Schott UG11 specifications and generic ZWB1 (Figure 4) at the same thickness of 2 mm, gives a good match although the ZWB1 transmits slightly more in the NIR. Transmittance of the ZWB1 is higher both in the UV region and in the NIR region. Normalization shows that the shape of the spectrum is very similar to the measured UG11 with slightly higher transmittance in the NIR. Overall a little less dense at the same thickness. The second ZWB1 was sold very cheaply and not advertised as for use in photography, it is thinner at 1.6 mm and also has a few optical imperfections. This second ZWB1 filter transmits almost twice as much in the NIR as the UG11 or the ZWB1 sold for use in photography.

Code
autoplot(c(Schott_UG.mspct["Schott_UG11"],
           filters.mspct[c("Tangsinuo_ZWB1_1.6mm_67mm",
                         "Tangsinuo_ZWB1_2.1mm_52mm")]),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(c(Schott_UG.mspct["Schott_UG11"],
                     filters.mspct[c("Tangsinuo_ZWB1_1.6mm_67mm",
                                     "Tangsinuo_ZWB1_2.1mm_52mm")])),
         annotations = list(c("+", "boundaries"), c("-", "peaks"))) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 4: Ultraviolet band-pass filters. Schott UG11 specifications compared to ZWB1 glass supplied by Tangsinuo (two filters bought on different occasions). Measured total spectral transmittance or computed from Schott specifications.

The Schott UG5 filter has relative high transmittance in the NIR region (Figure 5). The ZWB3 filter it is usually described as a UV+NIR pass filter, or a VIS-blocking absorptive filter.

Code
autoplot(c(Schott_UG.mspct["Schott_UG5"],
                     filters.mspct["Tangsinuo_ZWB3_2.2mm_52mm"]),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(c(Schott_UG.mspct["Schott_UG5"],
                     filters.mspct["Tangsinuo_ZWB3_2.2mm_52mm"])),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         range = c(200, 1000)) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 5: Ultraviolet band-pass filters. Schott UG5 specifications compared to ZWB1 glass supplied by Tangsinuo (two filters bought on different occasions). Measured total spectral transmittance or computed from Schott specifications.

The Hoya B390 (Schott BG12 no longer in production) is a UV-A+violet+blue pass filter with some transmission in the NIR region (Figure 6).

Code
autoplot(filters.mspct["Tangsinuo_ZB1_2.0mm_52mm"],
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(filters.mspct["Tangsinuo_ZB1_2.0mm_52mm"]),
         annotations = list(c("+", "boundaries"), c("-", "peaks"))) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 6: Ultraviolet-violet-blue band-pass filter. ZB1 glass supplied by Tangsinuo (one filter) describes as similar to Hoya B390. Measured total spectral transmittance.

Schott BG3 is an absorptive filter that blocks green, yellow, and orange-red light (Figure 7). In this case the ZB2 filter has a cut-in wavelength in the NIR at a wavelength that is about 25 nm longer. If the aim is to block these wavelength by stacking the ZB2 with a NIR blocking filter this can be a significant advantage compared to the BG3.

Code
autoplot(c(Schott_UG.mspct["Schott_BG3"],
                     filters.mspct["Tangsinuo_ZB2_2.0mm_52mm"]),
         annotations = list(c("+", "boundaries"), c("-", "peaks")),
         pc.out = TRUE) +
  labs(linetype = "")

autoplot(normalize(c(Schott_UG.mspct["Schott_BG3"],
                     filters.mspct["Tangsinuo_ZB2_2.0mm_52mm"])),
         annotations = list(c("+", "boundaries"), c("-", "peaks"))) +
  labs(linetype = "")
(a) Actual
(b) Normalized
Figure 7: Ultraviolet+violet+blue+NIR band-pass filters. Schott BG3 specifications compared to ZB2 glass supplied by Tangsinuo (one filters). Measured total spectral transmittance or computed from Schott specifications.
Tip

All the spectral data shown above in the 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 and compute reflectance from the refractive index click on the word Code or the triangle before it.