UVIR-cut filters

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Filters can block radiation either by reflection or absorption. Absorptive filters are usually made of glass containing various metal ions while cheaper plastic filters tend to be coloured with organic dyes. There is a third type of absorptive filter, which are rare nowadays that consist of a coloured gelatine layer in-between two glass sheets. Most high-quality absorptive filters sold for photographic use are made of solid glass and absorptive. In the case of square filters plastic is more common than for smaller circular filters. The current perfected version of the gelatine-between glass filters is Tiffen’s “core technology”. With absorptive filters the angle at which the light impings on them affects the path length so that absorbance increases when the angle of incidence is shallow. This effect is small enough to be rarely noticeable in photographs.

Some UVIR-cut filters are absorptive filters, these filters look blue as they also absorb a significant portion of the incident red light. Two examples with no anti reflection coating.

Interference filters block radiation by reflection, i.e., those that block visible light look like a mirror. They are based on coatings deposited on the surface(s) of clear glass. The wavelengths reflected depends on the coating materials’ refractive index and the thickness of the coats. In the case of interference filters, the angle of incidence also affects which wavelengths are reflected and which ones are transmitted. This effect is usually obvious with wide angle lenses, restricting the usefulness of these filters to normal and long focal lengths and narrow view angles.

Many UVIR-cut filters sold for photographic use are interference band-pass filters with sharp cut-in and cut-off (very steep slopes). The limits of the band of transmitted wavelengths varies among brands, types within brands and sometimes even between production batches. Another difference is that the more expensive UVIR-cut filters are multicoated (MC) to avoid reflections while others may have single coatings (ARC) or no coating other than those for reflecting out-of-band radiation.  Both MC or ARC reduce reflections, both at the front and back of the filters preventing or attenuating flare and ghosting and increasing the transmittance in the visible region. The number of layers used for MC can be in the tens, and MC is usually much more effective than ARC, reducing reflectance in many cases to between 0.2 and 0.1 % of the incident light.

UVIR-cut filters are needed very rarely with unmodified modern digital cameras as they already have built-in filters on their image sensors for the same purpose. The functioning of cameras that have been converted to full spectrum by removal of the built-in filter can be restored to close to the original with an external filter. With such a filter they can be used almost normally for visible-only photography. It is almost impossible to find a filter with the same exact optical properties as the removed one, so usually a custom colour profile will be also needed. The internal filters differ among cameras and consequently also the colour-related algorithms and calibrations used.

Camera objectives also differ in the range wavelengths they transmit and how well they are optically corrected at wavelengths near the boundaries of the visible region. In some cases UV-cut or UVIR-cut filters can improve image resolution and contrast, specially under illumination rich in UV (high mountains) or IR (incandescent light) radiation. It has been suggested that this is the case when using Panasonic or Leica lenses used on Olympus MFT cameras.

Other uses of UVIR-cut filters are for example blocking of excitation UV radiation and IR fluorescence when photographing VIS fluorescence induced by UV-radiation excitation.

Some example spectra are shown below. The most obvious differences are in the cut-on and cut-off wavelengths, they are given for the half maximum transmittance (HM). In addition we can see that the maximum transmittance is higher for the more expensive Firecrest and Heliopan filters which are multicoated. We can also see that in two of the Rocolax filters the transmittance varies with wavelength within the “pass-band”. All these spectra were measured using the same instrument and procedure.

Using wide angle objectives with interference filters results in differences between the effect of the filter in the centre and edges of the photograph, visible as  vignetting. As the cut-in and cut-off wavelengths depend on the angle of incidence, this type of vignetting not only results in darker image corners but also on colour casts than vary from the centre to the edges of a photograph.

Photograph taken with a full-spectrum converted Olympus E-M1 camera and a Sigma 19 mm f:2.8 DN objective (angle of view 59.3º according to specifications). One can see the color shift from reddish in the center to blue-green in the edges and corners which are also darker even in this lens that many photographers would consider a normal lens rather than a wide-angle lens.


We can conclude that when used with wide angle objectives front-mounted filters should preferably be of the absorptive type. Optionally, interference filters can be mounted at the back of the objective. Some UV-cut filters are based on interference coatings and others are absorptive, similarly to the UVIR-cut filters discussed here. For special uses like photography of UV-induced auto fluorescence the cut-off and cut-on wavelengths are critical as they determine if the excitation radiation enters the camera or not. In this case, how efficient is a filter at blocking out-of-band radiation is also important.


 


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