PrimaLuceLab U filter

[enricosavazzi]

Something we had not seen for a while: a new (at least for me, and apparently also for Ultraviolet Photography) filter for digital UV imaging.

 

First tests and thoughts:

http://savazzi.net/p...alucelab_u.html

 

This page is not yet linked from the main index of the site, because some things may still change.

[colinbm]

Thanks Enrico for this excellent comparison.

I do wonder what the comparisons would look like with individual custom white balances ?

I not too your mention of "or no replacement window at all". Has there been any discussion of the pros & cons of not having the 'window' or cover glass on the sensor ?

As I understand it, the sensor has the cover glass / window to seal the sensor from the atmosphere, with an inert gas inside.

So with replacing the IRC filters with quartz of the equivilent thickness, it is 'only' the atmosphere inside the camera that the sensor is getting ?

Cheers

Col

[enricosavazzi]

Thanks Enrico for this excellent comparison.

I do wonder what the comparisons would look like with individual custom white balances ?

I not too your mention of "or no replacement window at all". Has there been any discussion of the pros & cons of not having the 'window' or cover glass on the sensor ?

As I understand it, the sensor has the cover glass / window to seal the sensor from the atmosphere, with an inert gas inside.

So with replacing the IRC filters with quartz of the equivilent thickness, it is 'only' the atmosphere inside the camera that the sensor is getting ?

Cheers

Col

Hi Col,

 

With custom white balance different for each filter, results are extremely variable and comparisons between the filters become dependent of the WB, and partly subjective. This is why I prefer to present test images shot with the same custom WB. No doubt, many viewers will not agree with my choice of a WB, but at least the images shot with different filters can be compared.

 

I was referring to the window that replaces the UV- and IR-cut filter. The thin protecting window that seals the chip package is not ordinarily removed, and with Bayer sensors it makes little sense to remove it in any case. The microlenses and Bayer filters block UV to a higher degree than most package windows. I converted my G3 by removing the UV- and IR-cut filter without replacing it with a quartz window, so only the window on the chip package lies between lens and sensor chip. When necessay to focus at infinity, I use either a lens adapter slightly shorter than the standard registration distance, or a lens adapter in which I mounted a 4 mm thick fused silica window with UV-optimized AR coatings. I generally leave out the fused silica window for close-up photography and demanding tests, so that there are fewer things that can skew the results.

 

Deep-UV videocameras (down to about 100-150 nm) indeed use naked sensors without protecting window, but they also use monochrome sensors without microlenses and Bayer filters. The general recommendation with these videocameras is to never, ever, remove the videocamera lens for any reason, even for a second. Even so, the useful life of these videocameras is limited if used outside controlled environments like clean-rooms.

 

Added: There are the so-called de-bayered cameras available for astronomy, which had their microlenses and Bayer filters ground away. A new chip package window is then added. This sounds very risky to me, and I have no idea about the long-term effects on the sensor and the quality of the new seal around the window. Some commercial solutions are available, see for example http://www.jtwastronomy.com/products/monochrome.html , but my best bet would be an astronomical camera designed from the start around a monochrome sensor without microlenses.

[OlDoinyo]

Has PrimaLuce published a transmittance spectrum for this filter? I gather that it is a bit more transmissive at shorter wavelengths than the Baader, though that would matter little with many lenses.

[enricosavazzi]

Not numerical data, as far as I know. Only a diagram at http://www.primalucelab.com/astronomy/filters/ultraviolet-uv-filter.html .

 

It does indeed transmit more UVA at shorter wavelengths than the Baader U, and is similar in this respect to the Asahi Spectra XRR0340. The XRR0340 uses a U340 substrate, and I believe that the PrimaLuceLab U also does. This produces the abundant false-color yellow of both filters. However, the PrimaLuceLab obviously transmits also more of the longish UVA wavelengths (false-color blue) than the XRR0340, and perhaps just a little VIS in the violet and blue wavelengths, although this is not shown in the transmission diagram. Given that Bayer sensors are more sensitive to VIS blue than UV, and that there is more VIS blue than UV in sunlight, a low VIS blue transmission (OD somewhere between 2 and 3) would not be noticeable in a linear-scale transmission diagram (i.e., a diagram with % transmission on the Y axis), but would be picked up by camera sensors. A logarithmic-scale diagram would show this.

 

I have not tested other UV lenses with the PrimaLuceLab U, but I suspect the predominant false-color yellow turns up with other lenses as well, although probably not with those that display only a marginal UV transmission.

[JCDowdy]

The color of this filter looks a lot like this one: Sky's the Limit 1.25" Venus U Premium High Transmission filter

[enricosavazzi]

It might be the same or a similar filter material, if the opposite side is black (we don't see it in the eBay picture). Price is far from the same, though.

 

I half-remember reading something, somewhere, about this type of dielectric filter coating easily separating from its UV-pass glass substrate. In fact, I once bought a surplus Omega filter on eBay that was useless for imaging because the dielectric coating on one surface had lifted off the glass for about one-quarter of the filter surface. This particular Omega filter was very expensive even in a small size and as a surplus (far more than the PrimaLuceLab U) because it had the same dielectric coatings on both surfaces, as opposed to just one side. Perhaps the coating process on both surfaces made the filter more vulnerable to separation on the surface that was coated first, because the substrate was then (probably) heated again while coating the second surface.