Good question Brent, and the answer is why it is so important to block IR.
(I'm going to go slow here and lead you by the hand, this is nothing personal, just easier for the layman to understand, ok?)
First, IR or InfraRed, is heat energy. Everything around us puts out heat energy of some kind, there's no way around it. The only exception is in far intergalatic space where the temperature is absolute zero, where all atomic movement is frozen.
As things (people, dogs, pavement, glass) heat up, they begin to emit IR from the very far end of the spectrum, way out beyond 1 billionth of an inch wavelength. This is considered "cold" IR.
As the thing warms up further, and gets around 200 F or so, a slight red glow begins, this is the temperature at which you can begin to see the "effect" of heat. At this point, the item is radiating IR from the very far end of the spectrum into the visible red of the spectrum.
At this point, the more heat, the higher the temperature, the more the color of the item moves toward orange, yellow, green, and then finally blue, and beyond that UV. Once the items hits about 4,000 degrees or so, it is radiating across the entire spectrum.
Radiation always starts at the low end of the IR spectrum, furthest away from the visible portion of the spectrum.
To our eyes, the most hazardous portion of the IR spectrum is from about 2,500 nanometers to around 950 nanometers. Visible light starts at about 750 nanometers (very dark red) to 400 nanometers (near ultraviolet). The temperatures at 2,500 nanometers are approximately 100 F.
The problem is that this radiation varies in intensity and by the mass of the piece that is being worked. This is why, for the most part, soft glass lampworking doesn't require much in the way of IR protection. The temperatures are relatively low, the pieces are small, and there is very little extraneous "additional" radiation, such as may be found from fuming.
Borosilicate, on the other hand, requires much higher temperatures, for much longer time periods, the pieces tend to have greater mass, and there is a lot more "extraneous" radiation, not only from fuming, but also from the metals often found in the glass, added as colorants.
Fuming temperatures are relative low, but the visible light radiation is high due to the metals being evaporated and having visible light flares. This is why borosilicate eyewear has to have a two-pronged filtration, full IR filtration to protect the eye from the hazardous IR radiation, and strong visible light filtration to remove the various spectra from the metals burning off.
Tests have been done on the spectra put out by all sorts of glassworking, kiln work, hot shop (traditional glassblowing) and torch work. Every single test has indicated very high levels of IR radiation (of couse, it's hot!).
So, to answer your question, if you are working borosilicate glass of any kind, once the glass begins to glow in the visible range, you are being exposed to ALL the ranges in the IR spectrum.
This is why it is so important that the filter eyewear follow the ANSI standards, and filter out the maximum amount of IR radiation. When these standards were first developed, they were for metal workers, welders mainly, who were exposed to IR radiation on a regular basis. The size of the welding rod and amperage, or size of the cutting tip and thickness of the metal are the guidelines that were originally used to determine the proper shade. And, as the rod got bigger, or the metal got thicker, of course more heat was needed, and therefore more IR filtration.
Here's what ANSI says a shade 3 filter MUST do, in order to be called a shade 3:
Transmit Average visible light of 13.9%, average UV of 0.07%, and average IR of 9%.
A shade 5: Transmit 3.16% visible, 0.02% UV, and 2.5% IR.
This is where the problem is. By calling something a shade 5, the expectation is that the filter is blocking 97.5% of the IR radiation. The product in question does not. It blocks approximately 60%. Is this enough? It depends on a lot of factors. But here's the problem: the sunglass effect.
With a dark lens, the pupil opens up wide to allow more light to enter the eye. When the lens also transmits high levels of IR energy, more of this energy enters the eye because the pupil is wider open than normal. So, potentially, any in IR is offset by the wider opening of the pupil of the eye.
The darker the lens, the wider the pupil opens. It then becomes a vicious circle.
Hope this answered your question.
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