NIOSH and OSHA have determined that the IR TLV (threshold limit value) to the unprotected eye is 10 mW/CM² for time periods greater than 16 minutes. (10 mW is .010 Watts)

There are two critical areas to take into account when trying to determine the exposure value of IR radiation. The first critical area is the heat source, be it a furnace, glory hole, or torch. These heat sources typically run at a higher temperature than the glass being worked, and thus are a higher hazard. The second critical area is the piece being worked. The piece will typically have two zones, the heated area, or that portion of the piece that is currently being worked, and the rest of the piece, which is radiating at a lower temperature than the worked area. All three of these areas contribute to the total radiation that the glassworker is being exposed to.

The basic forumula is: Total Exposure = A1 + A2 + A3, where Ax are the areas referred to above.

The basic formula for calculating exposure is: R * (A / (2 * 3.14.159 * r²) ) Where R = Radiance of the object in watts per square centimeter, A = the area of the radiation, in square centimeters, and r = the distance to the eye from the heat source, in centimeters.

The flame calculation is this: For a 3500 degree flame, Radiance is 75 Watts per square centimeter. If the flame is one inch wide at the base and (for example) averages 8” long, the area of the flame is 8 square inches, or 51.61 square centimeters. The distance of 16” equals about 40 centimeters.

Plugging into the formula above, E = 75 * ( 51.61 / ( 2 * 3.14159 * 40²) )

Which reduces to: E = 75 * ( 51.61 / 10053 )

Which further reduces to: E = 75 * .0051

Which results in: E = .375

The flame alone contributes 0.375 Watts per square centimeter at the eye.

For the working area, let’s assume that the temperature of the glass is 2200 degrees (which is a radiance of 25 Watts per square centimeter), and the area being worked is 5 centimeters by 10 centimeters (or 50 square centimeters). We’ll keep the same working distance of 40 centimeters.

E = 25 * ( 50 / 10053)

Which reduces to: E = 25 * .0049

Which results in: E = .123

The working area contributes 0.123 Watts per square centimeter at the eye.

For the nonworking area, let’s assume that the temperature of the glass is 1100 degrees (which is a radiance of 3 Watts per square centimeter), and the nonworking area equal to the working area, or 50 square centimeters). Remember that if you are building a large piece, the nonworking section will continue to grow, so keep that in mind. We will also keep the same working distance of 40 centimeters.

E = 3 * (50 / 10053)

Which reduces to: E = 3 * .0049

Which results in: E = .015

The nonworking area contributes 0.015 Watts per square centimeter to the eye.

Add these areas up, 0.375 + 0.123 + 0.015 and the result is 0.513 Watts per square centimeter to the unprotected eye.

Now, let’s figure the protection value of the various filters. Remember that the IR TLV is 0.010 Watts per square cm.

Phillips Boroscopes Shade 3 passes an average of 55%, so 55% of 0.513 = 0.282

Phillips Boroscopes Shade 5 passes an average of 40%, so 40% of 0.513 = 0.205

Aura Lens AGW 203 passes an average of 3.4%, so 3.4% of .513 = 0.017

Aura Lens AGW 286 Shade 5 passes an average of 1.5%, so 1.5% of .513 = 0.008

Copyright © 2005, Aura Lens Products, Inc.