1、SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirelyvoluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefro
2、m, is the sole responsibility of the user.”SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions.Copyright 2003 SAE InternationalAll rights reserved. No part of this publication may be
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4、776-0790Email: custsvcsae.orgSAE WEB ADDRESS: http:/www.sae.orgSURFACEVEHICLERECOMMENDEDPRACTICEJ1246REAF.FEB2003Issued 1982-05Reaffirmed 2003-02Superseding J1246 JUN1997Measuring the Radius of Curvature of Convex Mirrors1. ScopeThe SAE Recommended Practice is intended for use in measuring the radiu
5、s of curvature (ROC) ofspherical convex mirrors.2. ReferencesThere are no referenced publications specified herein.3. ApparatusThe apparatus shall consist of a linear spherometer (see Figure 1) with two fixed posts of equalheight and ends so constructed that only point contact is made with the mirro
6、r. The distance between the twoposts, referred to as the chord length, should be known to 0.02 mm (0.001 in). A center probe is locatedmidway between the fixed posts on the line running through the contact points of the posts. It should alsomake only point contact with the mirror. The center probe s
7、hould be capable of vertical movement and beattached to a gauge which can display the linear displacement of the probe in units not larger than 0.002 mm(0.0001 in).Optionally, a third post may be placed rearward of the center probe (see Figure 1) to provide stability andfacilitate easier measurement
8、. It should be the same distance from the center probe as the two outside postsand the same height as the outside posts.3.1 Chord Length for Automotive ApplicationsFor automotive applications, the gauge shall have a chordlength long enough to permit ROC to be calculated to within 2.5% of the average
9、 ROC of the mirror. Inaddition, to allow surface variation to be detected, the chord length shall not be any longer than that requiredto calculate ROC to within 1.0% of the average ROC. Tables 1 and 2 list the range of ROC that can bemeasured within these requirements for gauges with various chord l
10、engths. Table 1 gives values for a metricgauge which reads in units of 0.002 mm and Table 2 gives values for an English gauge which reads in units of0.0001 in. A gauge with the chord length listed in the first column should be used to measure any mirror witha ROC within the range listed in the secon
11、d column. For gauges with other chord lengths or mirrors with otherROC, see Appendix A.SAE J1246 Reaffirmed FEB2003-2-FIGURE 1SCHEMATIC OF MEASURING APPARATUSTABLE 1CHORD LENGTHS REQUIRED TO DETERMINE ROC 1.0 TO 2.5%(CENTER PROBE UNITS = 0.002 mm)Gauge with Chord Length Measures Mirrors with ROC25.0
12、0 mm 391 to 977 mm37.50 mm 879 to 2197 mm50.00 mm 1563 to 3906 mm62.5 mm 2441 to 6104 mm75.00 mm 3516 to 8789 mmTABLE 2CHORD LENGTHS REQUIRED TO DETERMINE ROC 1.0 TO 2.5%(CENTER PROBE UNITS = 0.0001 in)Gauge with Chord Length Measures Mirrors with ROC1.000 in 12.5 to 31.3 in1.500 in 28.1 to 70.3 in2
13、.000 in 50.0 to 125.0 in2.500 in 78.1 to 195.3 in3.000 in 112.5 to 281.3 inSAE J1246 Reaffirmed FEB2003-3-4. Procedure4.1 Radius of Curvature at One LocationThe spherometer shall be placed on a flat surface such as an opticalflat accurate to at least 76 x 105 mm (3 x 105 in) over 13 mm (0.5 in) long
14、er than the chord length and thegauge shall be set to zero. The spherometer shall then be placed on the mirror such that the device is normalto the surface (all fixed posts are in contact with the mirror) and the displacement of the center probe read.Radius is defined by Equation 1:(Eq. 1)where:R =
15、Radius of Curvature (ROC)C = Chord LengthH = Displacement of Center Probe4.2 Radius of Curvature of a MirrorRadius of curvature of a mirror shall be defined as the average of at leastten readings taken over the surface of the mirror. The readings shall consist of: two readings at the center ofthe mi
16、rror, one parallel and one perpendicular to the longitudinal axis of the mirror; four readings along theperimeter of the mirror with the spherometer placed parallel to and 3.0 mm (0.12 in) from the edge; and fourreadings at the same locations along the perimeter of the mirror with the gauge placed p
17、erpendicular to theedge and with the outer post 3.0 mm (0.12 in) from the edge. (See Figure 2.)For mirrors with an area greater than 200 cm2 (31 in2), readings shall be taken in addition to those prescribedsuch that there is at least one reading for each 20 cm2 (3.0 in 2) of surface area. These addi
18、tional readingsshall be taken at enough locations to obtain an accurate average of the radius of curvature over the entiresurface.The chord length of the gauge used shall be reported with the results of any ROC reading.FIGURE 2MEASUREMENT LOCATIONSPREPARED BY THE SAE DRIVER VISION STANDARDS COMMITTE
19、ER C28 H-H2-+=SAE J1246 Reaffirmed FEB2003-4-APPENDIX AFACTORS IN SELECTING THE APPROPRIATE CHORD LENGTHA.1 Radius of curvature (ROC) is calculated using the displacement of the center post relative to the outer posts.The ROC is actually the radius of a circle on which the tips of the three posts wo
20、uld fall. Any variation in thesurface between the posts will not be detected. Therefore, gauges with shorter chord lengths give a truerindication of surface variation. Longer chord lengths can mask variation and lead to inaccurate measurementof ROC. A chord length should be selected which is short e
21、nough to include any expected variations in thesurface.Chord length will also determine the precision of the calculated radius of curvature. Longer chord lengths willgive a more precise ROC. In Table A1, the ROC listed in the first column cannot be calculated more preciselythan plus or minus the val
22、ues in the adjacent columns. For example, a 1000 mm radius of curvature mirror canonly be determined to within 12 mm using 38.1 chord length gauge or to within 5 mm using 62.25 chordlength gauge.To determine the minimum chord length required to attain a given level of precision, use Equation A1:(Eq.
23、 A1)where:R = Radius of Curvature (ROC)C = Chord LengthH = Precision in Measuring Center Probe DisplacementR = Precision in Calculating ROCTABLE A1EFFECT OF CHORD LENGTH ON DETERMINING RADIUS OF CURVATURE(CENTER PROBE CAN BE READ 0.002 mm)ROCChord Length38.10 mmChord Length62.25 mm1000 mm 12 51500 m
24、m 24 102000 mm 44 16C 2 R 2 H R-SAE J1246 Reaffirmed FEB2003RationaleNot applicable.Relationship of SAE Standard to ISO StandardNot applicable.ApplicationThis SAE Recommended Practice is intended for use in measuring the radius of curvature(ROC) of spherical convex mirrors.Reference SectionThere are no referenced publications specified herein.Developed by the SAE Driver Vision Standards Committee
