Interpretation ID: nht68-1.39

DATE: 02/20/68

FROM: H. S. BEAGLE -- ELECTRICAL TESTING LABORATORIES, INC.

TO: EDWIN L. SLAGLE -- NATIONAL HIGHWAY SAFETY BUREAU FEDERAL HIGHWAY ADMINISTRATION DEPARTMENT OF TRANSPORTATION

COPYEE: J. R. SCHAEFFER -- ETL; A. R. CHICK -- ETL

TITLE: MOTOR VEHICLE SAFETY STANDARD NO. 205 -GLAZING MATERIALS

REF.1: ELECTRICALLY HEATED SAFETY GLASS FOR DEFOGGING

REF.2: TEST PROCEDURE PER USA STANDARD 226.1-1966

ATTACHMT: LETTER DATED 2/21/68 FROM EDWIN L. SLAGLE OF D.O.T. NATIONAL HIGHWAY SAFETY BUREAU TO H. S. BEAGLE OF ELECTRICAL TESTING LABORATORIES INC.

TEXT: The above subject reference No. 2 provides, in part, that safety glazing material be tested for regular (parallel) luminous transmittance (method unspecified), which value shall be not less than 70 per cent both before and after irradiation (ultraviolet are exposure). The word "parallel" means that the light beam used in making this measurement should be essentially collimated. This condition may be obtained by using a light source sufficiently distant from the test specimen that the rays are essentially parallel.

Our photometric laboratory has been performing this test satisfactorily for many years, using the apparatus shown in the attached photographs numbered 1 through 4, as follows:

Photo #1 - A general view, no glass in place, showing diaphragm plate and source of collimated beam above. Light source, positioned 48 inches above photo cell and shielded, is an incandescent monoplane tungsten-filament lamp calibrated for operation at the specified color temperature of 2854 degrees Kelvin.

#2 - Close up of diaphragm plate with 1-1/4-inch opening, positioned 3/4 inch above the color corrected photovoltaic cell. Cell is visible through diaphragm opening.

#3 - Set-up with glass specimen in place. Glass specimen shown has electrically conductive lines on one surface.

#4 - Close up of glass specimen on diaphragm plate.

In performing this measurement the test specimen is moved laterally across the diaphragm (aperture) to find and record the minimum transmittance value.

2

It is our belief that this test method is as suitable for evaluating the luminous transmittance of electrically heated safety glass as it is for the same glass not so treated. As a matter of fact, it has been so employed in the past with respect to imported glass of the electrically heated type.

However, to avoid possible future problems concerning the acceptability of our test method, we are submitting this matter for your consideration at this time. The urgency of it is related to the long loud time with respect to production of electrically heated safety glass to provide for defogging of rear windows in motor vehicles.

We therefore respectfully ask for your written approval of the test method, as herein described, as being acceptable for purposes of evaluating the luminous transmittance of electrically heated safety glass for compliance in this regard, with the requirements of MVSS No. 205.

The earliest possible response to this request would be most sincerely appreciated.

Sincerely yours,

Enclosure: photographs