Baader U-filter transmission (problematic IR leak?)

[Lars Harrekilde-Petersen]

Technical question:

As seen in the attached image, the rather new Baader U SLOAN/SDSS filter has some low level IR leak in the 1000 to 1200nm range.
Since I have a de-Bayer'ed (i.e. monochrome) CMOS camera, I wonder whether that leak may be troublesome.
I expect a typical full spectrum camera wouldn't have problems with it, since the CFA will cut there, but a bare silicon sensor may be too sensitive.

I am specifically hoping Klaus Schmitt will be of help here!

I also wonder whether the SLOAN/SDSS version may cut some important wavelengths near 390nm that the "usual" Baader U Venus (350nm) or particularly the UBVRI Bessel version include.
See the next attached graph (which Baader Planetarium was kind enough to provide me).

 

[Dr Klaus Schmitt]

Hi Lars,
unfortunately I have never used the SLOAN nor the UBVRI filter from company Baader, but since many years I do use the Baader-U "Venus" filter in its 2" size (only use that one) on my "full spectrum" converted cameras with best of success and no IR-leakage - well if you try hard and shoot into a strong frontal light from and angle you might be "lucky" to produce some "leakage". BUT since any serious photographer would ALWAYS use a narrow sun shade, this will not happen...

I did post 3x about that on my BLOG, so you might like to have a look at that here: https://photographyoftheinvisibleworld.blogspot.com/search?q=leakage

Be reminded that every good UV pass filter should have an out-of-band (esp. NIR) blocking of at least OD3 (1E-3) which the Baader-U has, better would be OD4 (but very tough to get this done, especially if one wants high UV transmission...)

And indeed that SLOAN filter is made for shorter UV wavelengths, but actually below 350nm not much happens biologically, so a filter centered around 350-360nm indeed brings the best results - based on the few images I took within the last 10 years or so

PS: have a look for seller rapidspectra-0 on ebay (it is OMEGA OPTICS surplus actually!!) who has rather good filters (my secret source for years) - ask for larger sizes! They have at times... https://www.ebay.com/itm/145041475326

 

[Lars Harrekilde-Petersen]

Hi Lars,
unfortunately I have never used the SLOAN nor the UBVRI filter from company Baader, but since many years I do use the Baader-U "Venus" filter in its 2" size (only use that one) on my "full spectrum" converted cameras with best of success and no IR-leakage - well if you try hard and shoot into a strong frontal light from and angle you might be "lucky" to produce some "leakage". BUT since any serious photographer would ALWAYS use a narrow sun shade, this will not happen...

I did post 3x about that on my BLOG, so you might like to have a look at that here: https://photographyoftheinvisibleworld.blogspot.com/search?q=leakage

Be reminded that every good UV pass filter should have an out-of-band (esp. NIR) blocking of at least OD3 (1E-3) which the Baader-U has, better would be OD4 (but very tough to get this done, especially if one wants high UV transmission...)

Hi Klaus,

Thanks for your input.

I am on the fence regarding what high transmission UV pass filter to get, and while the SLOAN version seems to be an improved version of the well-known Baader U Venus filter, I find no tests published of any kind of this newer, purely dielectric, filter.

The graph of the SLOAN U-filter, as you probably saw, has a leak from about 1000nm to 1200nm, which may or may not be problematic, given the sensitivity of digital camera sensors is low there. Since you haven't tried that filter, I am inclined to just ask Baader Planetarium whether they can guarantee that the leak is of no practical consequence even in sun-lit situations.

Another filter I have found of interest is the U-Bessel filter from Chroma. It has extremely high transmission and sharply cut bandwith. However, it also has a leak, this time at the extreme opposite end of the silicon light sensitivity window, as you can see in the attached graph (50% transmission short of 1200nm). Again, this may be outside the spectrum where the sensor has any sensitivity, but it bothers me not knowing before buying. Regarding the latter, that filter is silly expensive: $700 for a mounted two-inch filter.

Would you advise against getting a filter that lets through a substantial amount of light in the 380 to 400nm range?
That is the borderline of visible/ultraviolet, but does it contain valuable information from a biological viewpoint?

Best regards,
Lars

 

[Dr Klaus Schmitt]

Sorry, missed your answer!

I do recommend not to use a filter which passes 385-400nm as a UV converted camera is so much more sensitive in this UV-band that it basically overlays (and hence destroys) valuable info in deeper UV regions! Such I notiecd when doing many years of experiments with filters and lenses for UV recording.

Bees for instance are sensitive down to 300nm, but the most important flower info may already be seen around 350-360nm - hence why the baader-U filter is a great one as it has its max. transmission there!