Monday, September 10, 2012

Filter leakage in reflected UV ultraviolet photography - DIY filter vs Baader-U II

Today again about leakage in reflected UV photography and how to deal with it. I have posted about that before here.

I wanted to show here some examples using SCHOTT UG11 in comparison with using IR suppressing SCHOTT S8612 filter glass of different thickness and to show how such IR leakage can be efficiently cured - at least for most normal shooting conditions. In early morning or late evening sunlight, where IR is predominant or when shooting against the sun or when reflective elements are present in an image, that won't work then, sometimes not even when using the otherwise excellent Baader-U filter.

[click on image to see a larger one]

So here an example for strong IR leakage using only 2.5mm SCHOTT UG11:

So how does that picture should look like? Here what the Baader-U filtered result looks like:

And here the result of my shorter wave, IR leak proof Jupiter-U filter:

It is pretty obvious, that IR leakeage virtually destroys the importatnt aspect of that image, as well as it softens the image, i.e. a strong lack of contrast is caused.
So let's see if stacking 1mm of SCHOTT S8612 filter onto the UG11 filter solves the problem:

Nice, but was it enough? Let's see what 2mm of SCHOTT S8612 will bring us:

Much better now, as the contrast is strongly enhanced, the UV pattern on petals is quite nicely present. So what I can recommend is stacking 2mm of SCHOTT S8612 filter glass onto SCHOTT UG1, UG11 UV transmissive filter glass (also works with HOYA U-340), but unfortunately if total cost are taken into account, this is not really far from getting a Baader-U filter...

And there is another BUT, the lower overall transmission caused by stacking such filters. Have a look at my measurements here, which clearly shows, that controlling leakage using ionic colored filter glass comes at a considerable transmission loss:

In that case, one of about 1.7 stops mathematically; in real use due to the sensor sensitivity curve, it is less than that, but still a significant one. And I haven't even talked about ghost images when stacking uncoated filters or the reflectance loss of about 8% per filter glass. So company Baader obviously made her homework very well, if you compare the results of the graph above (Baader-U is denoted by the violet line, the SCHOTT UG11 stacked with 2mm SCHOTT S8612 is denoted by the blue line), showing the very high reached UV transmission, rather flat broadband. It does not reach very deep into UV, but most UV sensitive cameras will anyway not be able to reach that deep and/or when using normal glass based lenses, where even the best of those barely reach beyond 330nm.

CAVEAT: on 2nd thought there seems to be something wrong with these S8612 filters I got from a US filter making company, as the resulting UV transmission is much lower than expected when compared to results of the SCHOTT filter calculation software. In principle only the UV transmission seems to be affected, not the leakage suppressing part, so the made statements are all valid, except the possible 1.7 stop loss. I will update about that here, as soon as I have found out about it. Btw. SCHOTT BG40 may be used as a replacement for S8612, but needs about double the thickness.
UPDATE as of 09-2012: As it turns out, these S8612 filters were cladded on both sides using some optical glass (B270) to prevent them from graying (an oxidation process this S8612 filter glass type shows) and this cladding has been attached to the S8612 using some UV cured adhesive. Most likely that B270 glass used and/or the used adhesive absorb UV more than expected, hence leading to the results measured above. So if you would like to use S8612 filter glass, make sure you get uncladded filters (and have to accept the fact that it may over the years need repolishing).

There is a newer part III about IR leakage in UV photography here.

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site