NIR-pass filters

All the near-infrared (NIR) long-pass filters described in this post are absorptive glass filters and sold for special photography effects. All modern photography digital cameras have internal UVIR-cut filters. Use of these filters requires special film, special digital cameras or modified DSLR or DML cameras. Some filters can be used with unmodified digital cameras with very long exposure times. Few normal camera objectives are designed to work well in NIR. Two problems are common: 1) focus shift which needs special attention when focusing is done in visible light (e.g. not using an EVF or live-view from the camera sensor, 2) reflections causing “hot spots”. Lately some anti-reflection coated (AC) and multicoated (MC) NIR long-pass filters have been advertised as helping reduce the problem of hot spots (sold by Kolari Vision). I have not tested these, as I do not even have one of them. However, some other NIR filters seem to have some type of antireflection coating based on their spectrum. See the post on UV-cut filters for other examples of spectra for filters with different anti-reflection coatings. Taking into account the reflection on both surfaces of the filter, uncoated optical glass will loose about 10% of transmittance on near normal (at 90 degrees to the surface) illumination. The exact value can be computed from the difference in refractive index between air and glass, while the refractive index of the glass depends to some extent on its composition.

The precision of the optical polishing and lapping and anti-reflection coating are the properties that will determine whether the filters will degrade image quality or not. Imperfections in the optical polishing are most visible when panning while recording video. With respect to mechanical protection the hardness of the glass and possibly also of the metal in the filter frame (brass vs. aluminum) may be relevant. The thickness of optical glass filters used in photography is usually between 1 and 3 mm, except for stacks. Some filters have an additional “nano” coating that helps repel dirt making them easier to keep clean.

The most common cut-on wavelength for “IR” filters used for photography is 720 nm, so we start be comparing filters with this characteristic, at least nominally. The add a red filter known to multi coated, the Hoya 25A HMC for comparison. The Haida IR720 is the only one of the NIR filters in this figure to be advertised as antireflection coated. It seems to use a coating optimized for visible light, very similar to that used in Hoya’s 25A HMC. As transmittance is well above 95% for the Hoya R72 we must conclude that this filter has an AR coating, possibly a single coating optimized for NIR as the high transmittance can be observed all the way to the range of measurement at 1100 nm.

Red and near infrared (NIR) long-pass filters with cut-on wavelengths between 600 and 730 nm.

Of the filters measured, only the ones from Heliopan have brass rings. The Orange and Yellow filters are multi-coated and reflect negligible amounts of visible light with a normal beam. The RG665 is AR coated, possibly even MC, although not advertised as such. The RG695 and RG780 are not AR coated. The coatings of the Orange and Yellow filters seems to be optimized for visible light, while the Heliopan RG665 has a coating optimized for NIR.

Long-pass filters from Heliopan with cut-on wavelengths between 450 and 800 nm.

 

A collection of cheaper filters from Zomei, none of them AR coated and with rather large variation in cut-on wavelength between batches, here visible comparing filters of the same specification except for size:

NIR long-pass filters from Zomei with cut-on wavelengths between 650 and 950 nm.

The KolariVision Gen II filters are missing from this article as I do not have any of them. According to the data provided by Kolari Vision, they have a very effective AR coating optimized for NIR.

As for cost, taking as example the filters with cut-on near 720 nm and a size of 58 mm, the Zomei is the cheapest at  less than 15 € (on never-ending sale), the Haida is at 30 €, the Hoya at 65 € and the Heliopan at 75 €, while the Kolari Vision at 85 USD, would cost over 100 € after delivery to Europe and VAT. In AliExpress one can find even cheaper filters, at around 10 €.


The spectra have been measured all with the same Agilent diode array spectrophotometer. The data are available in R package ‘photobiologyFilters’. The figures were produced with R packages ‘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.


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