Interpretation ID: nht75-1.1
DATE: 11/26/75
FROM: AUTHOR UNAVAILABLE; James B. Gregory; NHTSA
TO: Libby-Owens-Ford Company
TITLE: FMVSS INTERPRETATION
TEXT:
N41-42
H. M. Alexander, Vice President Technical Development & Services Libbey-Owens-Ford Company 1701 East Broadway Toledo, Ohio 43605
Dear Mr. Alexander:
This is a further reply to your letter of September 11, 1975, requesting an interpretation of the test procedures for measuring light transmission of glazing materials in accordance with Federal Motor Vehicle Safety Standard No. 205, Glazing Materials.
As you know, the purpose of the luminous transmittance requirement is to assure that safety glazing used in motor vehicles at levels requisite for driving visibility, does not restrict the vehicle operator's vision below that necessary for safe operation. Further, the apparent effective luminous transmittance of safety glazing as viewed by the human eye is the average transmittance of the entire area surveyed by the eye. In view of this, the National Highway Traffic Safety Administration would not consider glazing with an average luminous transmittance over its entire area of 70 percent or more to be in non-compliance with the standard.
As you pointed out, the luminous transmittance test procedure specified by Standard No. 205 does not specify the diameter of the measuring light beam. Thus, you are free to use any diameter light beam that is appropriate.
In regard to the distinct bend lines in your back window, we would consider for purposes of determining luminous transmittance, your back window to be divided into three sections - two wings and the central section. Thus, in this case, the distinct bend lines would not be considered as glazing material.
Sincerely,
James B. Gregory Administrator
September 11, 1975
Dr. James B. Gregory, Administrator National Highway Traffic Safety Administration Nassif Building Washington, D.C. 20590
Dear Dr. Gregory:
On August 25, we met at the Department of Transportation with Messrs. Guy Hunter, Doug Pritchard and Chuck Kaehn and discussed a new type automotive backlight.
This new type backlight offers advantages toward greater safety by better rear visibility with minimal optical distortion. However, we questioned whether this type backlight meets the 70% light transmission requirement of the ANSI Code Z26 which is a requirement of FMVSS 205.
This new backlight achieves its advantages as the result of bending sharply around a "hot line" electrical conductor which results in a .190" wide opaque line at the boundary between the intersecting bent glass surfaces. In the heated version of such a backlight, the conducting lines in the wing areas are only 17/32" apart whereas in the central area of the backlight, the lines are 1-1/8" apart. See attached sketch of "hot line bent" back window dated September 5, 1975.
The ANSI Code which requires that such an automotive backlight have a light transmission above 70%, does not specify the diameter of the measuring light beam or its placement on the backlight. In the backlight discussed here, if a 1-1/4" diameter measuring beam is used and positioned with one heating line centered in the beam in the central or major rear vision area, it meets the 70% requirement, see Figure 1 in the attached memorandum by Paul Mattimoe dated September 5, 1975. However, if two heating lines are located symmetrically within the same beam as shown in Figure 3 in attached memo, the transmission does not meet the code requirement.
Therefore, we respectfully request that the NHTSA provide an interpretation which will recognize that this improved design of backlight meets the intent of FMVSS 205. Specifically we would suggest that the interpretation specify the positioning of one frit conductor line centered in the light beam as shown in Figure 1 of the attachment. This orientation will provide additional objectivity in the test conditions while the 70% transmissibility requirement will continue to limit the amount of abscuration which could be caused by the electrical conductors. In addition, we suggest that transmission measurements should not be required at the boundaries formed by the intersection of any two glazing surfaces.
We will appreciate your consideration of this request. We will be glad to meet at DOT offices again if advisable. It is urgent that a decision be received since production tooling for the car model involved in this request is proceeding at the present time.
Very truly yours,
H. M. Alexander Vice President Technical Development & Services
HMA:pjp
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