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Interpretation ID: nht93-5.5

TYPE: INTERPRETATION-NHTSA

DATE: July 2, 1993

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Ray Kesler -- Kesler Research Enterprises

TITLE: None

ATTACHMT: Attached to letter dated 5/17/93 from Ray Kesler to John Womack (OCC 8660)

TEXT:

This responds to your follow-up letter to the agency in which you request further interpretation of the requirements in Standard No. 111 relating to convex mirrors. Specifically, you asked about how section S5.4.1, which limits the radii of curvature's permissible variance, relates to S12, which specifies the procedures for determining a convex mirror's average radius of curvature. I am pleased to have this opportunity to respond to your request.

As Marvin Shaw of my staff informed your associate Lawrence Hufstedler in a telephone conversation, section S12 sets forth a detailed multi-step procedure for calculating a convex mirror's average radius of curvature. The first step is to take ten readings on the mirror surface with a 3-point linear spherometer as specified in Figure 1 of the Standard. (See S12.1.) The second step is to convert each of the ten readings to a "radius of curvature calculation" using Table 1. (See S12.5.) The third step is to calculate the "average radius of curvature" by adding all 10 radius of curvature calculations and dividing by 10. (See S12.6.)

Mr. Hufstedler asked how S5.4.1 affects the calculations. That section states "none of the radii of curvature readings shall deviate from the average radius of curvature by more than 12.5 percent." This means that some of the radii of curvature readings may be up to 12.5 percent different than the average radius of curvature. In numerical terms, this means that if a mirror had an average radius of curvature of 36 inches a given radii of curvature reading could be as low as 31.5 inches and as high as 40.5.

I hope this information is helpful. Please feel free to contact Marvin Shaw of my staff at this address or by telephone at (202) 366-2992 if you have any further questions or need additional information.