MSHA - Code of Federal Regulations - 30 CFR 18.66
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MSHA - Title 30 CFR
30 CFR § 18.66
Tests of windows and lenses.


(a) Impact tests. A 4-pound cylindrical weight with a 1-inch-diameter hemispherical striking surface shall be dropped (free fall) to strike the window or lens in its mounting, or the equivalent thereof, at or near the center. Three of four samples shall withstand without breakage the impact according to the following table: 
                +----------------------------+--------------+
                |                            |  Height of   |
                | Lens diameter, (D), inches | fall, inches |
                +----------------------------+--------------+
                | D<4........................|            6 |
                | 4<= D<5.....................|            9 |
                | 5<= D<6.....................|           15 |
                | 6<= D.......................|           24 |
                +----------------------------+--------------+
Windows or lenses of smaller diameter than 1 inch may be tested by alternate methods at the discretion of MSHA.

(b) Thermal-shock tests. Four samples of the window or lens will be heated in an oven for 15 minutes to a temperature of 150° C. (302° F.) and immediately upon withdrawal of the samples from the oven they will be immersed in water having a temperature between 15° C. (59° F) and 20° C. (68° F.). Three of the four samples shall show no defect or breakage from this thermal-shock test.