RIP Wood's Glass Longwave UV Bandpass Filters

Several weeks ago there was a lot of discussion within our Facebook Group about the availability, and where to find, Kokomo 400 LW filter glass. Don Newsome, UV Systems - head of the Fluorescent Mineral Society research chapter (FMS - http://www.uvminerals.org) just sent out a heads up about this glass and asked me to post it here.  I added some of my comments at the end of this post.  ZWB2 filter glass is an excellent replacement for a Woods' Glass filter (used in the Convoy S2+ flashlight).

 
Long Wave Filters are Now More Expensive,
#400 LW Filter Glass is no Longer Made

 

Kokomo Opalescent Glass http://www.kog.com/ (KOG) in Kokomo, IN is America’s oldest art glass company.  Because they have been losing money trying to make acceptable #400 LW filter glass, the management has decided to discontinue the production of that #400 glass.  

 

The History of #400 LW Filter Glass
KOG has been making #400 LW filter glass for maybe 50 years (they do not even know how long) as one of their 22,000 variations of sheet glass. Their rolled sheet glass can be as large as 35” x 45” (see their website for how they roll their sheets).  Recently for whatever reason they have not been able to produce the #400 filter glass even though they have put considerable amount of time and money into that effort. So they decided that they will no longer attempt to make it.  In the past the #400 LW filter glass was reasonably inexpensive because it was made as a raw rolled art glass about 2.8 to 3.2 mm thick.  It was always much less expensive than LW or SW filters from other sources.  To my knowledge KOG was the only company in the US that made LW filters in larger sizes (up to about 45” long).

 

Art Glass vs Scientific Glass LW Filters
The other companies in the US, Japan, and Germany that make LW filters specialize in technical optic filters for the scientific community.  Therefore their filters are much more expensive than filters made by an art glass company.  Also, those other companies have standardized their glass production so the largest standard flat filter they make is 6 ½” square (165 mm square) at 5 mm thick.  Also most of the scientific optic filter glass is polished after molding.  This makes their glass even more expensive.  Molded as-poured flat filters are available for some filters but they are thicker (7 mm to 11 mm) and therefore have less UV transmission. 

 

Advantage vs. Disadvantage of Scientific LW Filters
The advantage of using these scientific LW filters is they transmit almost none of the 404.7 nm and 435.8 nm mercury emission lines that are generated in almost all UV light assemblies.  Therefore, when looking at your LW fluorescent minerals you do not see any addition visible light that can reflect off of dim or non-fluorescent specimens (you have to see that to appreciate that advantage).  Also, much more accurate spectral transmission curves are available with the scientific LW filters.  The disadvantage is that their LW filters are normally made much thicker than 3 mm and therefore with less UV transmission.  Because the scientific LW filters are thicker you do not get as much LW UV light on your fluorescent specimens as you would with the #400 LW filters.  Also, the scientific LW filters are in some cases more than100 times more expensive than the #400 filters were.

 

Availability of the #400 LW Filters
UV SYSTEMS, Inc. has a limited stock of the old #400 LW filters in stock in four sizes.  These are the FL-01 at 3” x 17”, the FL-02 at 6” x 18”, the FL-20 at 2” x 5.5” and the FL-60 at 2.65” x 9.25”.  We have enough of the #400 filters for the foreseeable future for the production of our LW350 or LW370 SuperBright 3, TripleBright II, and Dual TripleBright II UV lights.  Because we had to purchase some #400 stock from a glass vendor instead of directly from KOG the price for the FL-60 size filter is considerably more per square inch than the other #400 filter stock we have.

 

Scientific LW Filters
If UV SYSTEMS, Inc. is required to obtain the more expensive scientific LW filters our prices will have to be adjusted accordingly. 

 

Some History about LW Filters
LW Roundels for high pressure mercury arc lights have been made for over 75 years and are still being made.  Those roundels are usually 5 5/8” or 6.3” round for those mercury arc lights.  Since those round filters are convex on the front and concave on the back they are usually not suited for most hand-held or display UV lights.  Those LW filter roundels use a special glass formulation to make then less susceptible to the high IR and heat from those 100W to 175 W lamps; therefore they are not inexpensive.  They are also much thicker then the #400 with a different formation to not transmit the powerful visible light generated by those 100W to 175W lamps.

 

Flat sheet LW filters were made by a few companies back in the early 1950’s since LW fluorescence was popular back then.  Road side billboards were made back then that had LW UV fixtures that would fluoresce the whole billboard at night (very dramatic).  Those required large flat filters for the many 4 foot long lamps used in those fixtures.  Also in large cities like Chicago they had taxicabs with LW signs on the back of their cabs with portable powered LW UV lights that lit up their fluorescent signs at night.  My second LW UV light that I built in 1952 still has one of those flat polished filters in it.  Back then I also used to have a flat LW filter with molded ridges in it to defuse the UV light.  At the time I was told that the filter was made in France.  I am sure that those specific LW filter companies are no longer making those filters or are out of business just as those companies that made those UV light assemblies for billboards are no longer in business.

 

By Don Newsome
UV SYSTEMS, Inc.
16605 127th Ave. SE
Renton, WA  98058

Website: www.uvsystems.com

 

Mark Cole There are alternatives for Wood's Glass filters, and we use one (ZWB2) as a filter in the new Convoy 365nm flashlight - excellent results. A manufacturer in China makes ZWB2 filter glass: Chinese glass is available https://www.alibaba.com/.../Hot-Sale-UItraviolet-Glass... which is just as good for LW, and probably close in price https://www.alibaba.com/.../UV-pass-filter-ZWB1-ZWB2-ZWB3.... Hoya also makes U330, U340 and U360 which have similar transmission characteristic I believe, but I don't know the price. These glasses will also be less spotty than Kokomo with a more consistent transmission.

 

 Transmission Graph for Chinese ZWBx Filter Glass

 

A key point to make here is that it will soon be time for light manufacturers to get serious about lamps with high-power LW LEDs. High quality LEDs (such as the ones I show in my DIY pages on the MinerShop web site) generate as much, if not more, UV than fluorescent bulbs, and the output is directional - all comes out the front in a pinpoint source. The result is a very pure LW source with a 365nm peak, very little visible light, and very high power - far beyond what fluorescent bulbs put out. And the good news, a filter for the LEDs could be as small as 1/4" round - we could even use Hoya 325c for the best LW lamp around. (BTW - small "rounds" of the Chinese glass is sold on Ebay pretty cheap - would work perfectly with an LED)
 

 

 

 

 

 

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