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    Filter glass
    From: John Rae
    Date: 2005 Nov 10, 17:51 -0800
    Our fabricating shop (real boilermakers, for you sports fans) uses mostly density 11 welding filters.  But almost nothing can be seen through this grade with normal light.  So I have tried a grade 5 density filter, that I can actually see through with outside light.  They cost about $1.18 each in bulk.  At this price, and with the optical quality they seem to exhibit, this is a bargain.
    These filter glasses are 2 inches by 4.25 inches  by 3 mm thick.  ( a mixture of imperial and metric dimensions!)  The specified light transmission for grade 5 is stated to be nominal 1.93%.  In addition to protecting against light and heat, they are also designed to protect against impact from flying chips, etc.  As a result of this, I have not yet learned how to cut these filter glasses using an normal glass cutter, as they are hard to scratch, and fracture at odd angles.  Next move to try a spectacle grinding shop.  Has anyone suggestions about cutting this glass?
    So my next project is to make a Bris device from three full size filters (two clear and one number 5).  This (50 x 108mm) size will certainly fully cover my 7 x 35 objective lens. These binoculars claim to have an 8 degree field of view.  So while looking at the horizon, as long as the sun is higher than about 5 or 6 degrees, it is extremely easy not to even glimpse the direct sun.  Therefore, I was happy enough to use my sunglasses while looking at the reflected images of the weak sun through clear glass.  (But I wouldn't do this in summer, or concentrate long enough to see tangency.)  Even so, after testing the grade 5 density, I might change to another density filter.
    Regrettably, I won't see a noon sun elevation of even 40 degrees until about the Ides of March.  So the Bris device should be OK for those of us in northern latitudes.
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