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UltravioletPhotography

Filter Thickness


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I have read several posts on this forum about filter thickness. Could I clarify please - am I right in thinking that the thickness of a filter (1, 1.5 2mm) affects the transmittance (i.e. density), and quality of the image, but the wavelength transmitted stays the same? Am I right to assume that a 1mm filter will give better "quality" than a 2mm for example?

Adrian

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Andy Perrin

The intensity of the transmitted light decreases exponentially with the thickness. There is a coefficient called the absorption coefficient, alpha, which is a function of wavelength, and that tells you how much light is absorbed per unit distance in the direction the ray is traveling in. The transmitted intensity is then the initial intensity times exp(-alpha*s), where s is the thickness in the direction of travel. This is ignoring all reflections on the front and back surfaces of the filter, to keep things simple, but the reflectance doesn't depend on the thickness except for thin layers (like in the coatings), and it's fairly small for uncoated glass filters anyway, about 4% of the light on each surface.

 

At each wavelength, there will be a different value of alpha. So light at that wavelength will be attenuated by a different amount if the thickness changes. This means that the transmittance curves that we see on the forum here are specific to a given thickness, which is often 1mm. Curves at other thicknesses can be obtained by using the value at one mm to calculate alpha, then working backward from exp(alpha*s) to get the new value at some other thickness s.

 

As for quality, there are issues of the sensitivity of the camera itself to different wavelengths, and how much makes it through the lens.

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Filter thickness does not shift the wavelength.

Filter thickness does affect the amount of the wavelength which is transmitted.

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The thickness does shift the wavelength (higher nm / lower nm) for stacks and longpass filters.

The thickness effects the amplitude of the transmission along the entire transmission curve of any filter glass.

When bandpass filters and dual bandpass filters are used alone, the thickness will not change the peak transmission wavelength/nm point(s), however the amplitude (%) will be changed along the entire transmission curve, and the bandwidth will be changed also.

With longpass filters, the thickness will change the cutoff point wavelength, as well as the amplitude (%).

With stacked filters, the thickness of each filter glass in the stack will shift the peak transmission wavelength higher or lower (nm), and the amplitude will also be higher or lower, not only for the peak transmission, but also for the entire bandwidth, and the transmission bandwidth will be changed, regardless of the type of filter glass used.

You should see no issues between 1mm and 2mm glass. Most camera filters are from 2mm to 2.5mm thick, but that doesn't mean filters can't be thinner or thicker,

and often times stacks are thicker than 3mm even.

If you have some specific filter glass or stacks in mind, I will be happy to plot any curves for you to compare.

These graphs illustrate each of my points.

 

post-87-0-04061100-1469760883.jpg

 

post-87-0-96765200-1469760892.jpg

 

I often warn people against using Hoya U-340 1mm or Chinese 'equivalent' versions of UG11 or U-340 that are 1mm thick, because those have a leak in the 500nm range.

I would also warn that not all glass sold as Schott and Hoya are true. Chinese glass is dirt cheap, it does not perform as well as Schott or Hoya bands.

A glass formula calculated for Schott glass (for example) will not work the same when using an off brand filter glass.

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