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 entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising theref
2、rom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions.Copyright 2013 SAE International All rights reserved. No part of this pub
3、lication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970
4、(outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP924BAEROSPACERECOMMENDED PRACTICEARP924 REV. BIssued 1967-02 Reaffirmed 2005-09 R
5、evised 2013-10 Superseding ARP924A Specification and Inspection of Glass for Integrally Lighted Aerospace InstrumentsRATIONALEARP924B was revised to update the current revision levels in the references, to provide more technical clarification in the definitions section, and to further define glass s
6、ubstrates and light wedges. 1. SCOPE This SAE Aerospace Recommended Practice (ARP) covers the requirements for the types of glass to be utilized in the fabrication of cover glasses and lighting wedges used in aerospace instruments. It defines the maximum extent of physical defects and recommends sta
7、ndard methods of inspection and evaluation. Definitions of terminology used in this document are covered in 2.2. 2. REFERENCES 2.1 Applicable Documents The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applic
8、able issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regul
9、ations unless a specific exemption has been obtained. 2.1.1 ASTM Publications Available from ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, Tel: 610-832-9585, www.astm.org.ASTM C1036 Standard Specification for Flat Glass SAE INTERNATIONAL ARP924B Page 2 o
10、f 6 2.1.2 U.S. Government Publications Available from DLA Document Services, Building 4/D, 700 Robbins Avenue, Philadelphia, PA 19111-5094, Tel: 215-697-6396, http:/quicksearch.dla.mil/.MIL-G-174 Glass, Optical (INACTIVE FOR NEW DESIGN) MIL-C-675 Coating of Glass Optical Elements (Anti-Reflection) M
11、IL-PRF-13830 Optical Components for Fire Control Instruments; General Specification Governing the Manufacture, Assembly, and Inspection of 2.2 Definitions BACK COATING: Coating material which is deposited on the side opposite that which is being coated in the evaporated coating process, generally re
12、sulting in an objectionable appearing film or haze. BEAUTY DEFECTS: Beauty defects are those imperfections of components and elements of an optical system which do not affect the optical characteristics. They are undesirable but may be accepted if they do not cause a significant degradation of image
13、 quality or environmental stability. BUBBLES: Larger air pockets or gaseous inclusions trapped within the glass (typically 0.010 inch). CHIPS: Chips are areas from which glass has broken away from the surface, edge, or bevel of an optical element. COATING HOLES (VOIDS): Areas void of coating arising
14、 from dust, dirt, lint, or improperly cleaned surfaces beneath the film.COLOR SHIFT: A change in the dominant wavelength or the color purity of light which has been reflected from or has passed through the glass. DIGS: Digs are breaks or the polished surface of a round, oval, square, etc. shape incl
15、uding pits, holes, and surface-broken bubbles. DISTORTION: Inhomogeneities in the glass or irregularities in the surface of the element causing displacement of images.FIRE POLISH: A specular reflective finish that is produced by flame or molding. FRACTURES: Unintentional separation of the glass such
16、 as cracks. INCLUSION: Foreign matter trapped within the glass substrate or trapped on the surface by a coating. INTEGRALLY LIGHTED INSTRUMENT: An instrument where the lighting system is designed such that the light sources are contained within the instrument enclosure, thus not relying on any outsi
17、de source of illumination; the instrument may employ reflected or transilluminated light. REFRACTIVE INDEX: When a light ray is refracted at the surface of separation between air and glass, the ratio of the sine of the angle of incidence in the air to the sine of the angle of refraction in the glass
18、 is equal to the refractive index ofthe glass. SCRATCHES: Furrows or grooves in the surface of the glass caused by the removal of glass, usually made by coarse grit, fragments of glass, sharp tools, etc. rubbed over the surface. SEEDS: Small bubbles of air or gaseous inclusions entrapped within the
19、glass (typically 0.010 inch). SHORT FINISH: Gray appearance of a polished glass surface resulting from this operation not being carried to the point where all traces of the previous grinding or smoothing operation are removed. SAE INTERNATIONAL ARP924B Page 3 of 6 SLEEK: A scratch having boundaries
20、which appear polished and tend to disappear as the glass is rotated. SPATTER: A term used to denote the condition resulting when large particles of coating material condense on the glass surface, and adhere there, in evaporated coating processes. STAIN: A stain is a discoloration of the glass surfac
21、e, caused by the deposit of foreign matter on the surface or changes produced on the surface of the glass by chemical action of some substance with the glass. STRIAE (CORDS): Striae are apparent streaks or veins in the glass which are the result of minor variations in the index of refraction within
22、the body of the glass. WAVE: Surface irregularity causing objects viewed through the glass to appear wavy or rippled. 3. RECOMMENDATIONS 3.1 Cover Glass 3.1.1 Glass Substrate The types of glass used for a cover glass are soda lime or borosilicate such as optical crown glass, polished plate glass, fl
23、oat glass, and clear sheet glass. A further definition of the above is covered in ASTM C1036 (flat glass). When optical crown glass is selected, a six digit number should be used to specify the glass: the first three numbers pertain to the index of refraction and the last three digits pertain to the
24、 Abbe Constant (dispersion value) - one example is type 523-586. The control of the characteristics should be 0.005 for the index of refraction and 1.0 for the dispersion value. The chemical and physical properties of the glass need not be supplied unless required. The range of index of refraction o
25、f a cover glass when used with an anti-reflective coating should be between 1.47 and 1.55 and should be specified on the glass drawing. For cover glasses, uncoated, of a maximum thickness of 1/4 inch, the minimum transmission of any wavelength between 400 and 700 nm should be 87%. The selection of t
26、he cover glass, with respect to cost, should be governed by the following consideration: optical crown glass will be more expensive because plate and sheet (flat) glass are available in standard thicknesses. ASTM C1036 outlines the standard thicknesses available. Normal, flat glasses are preferred w
27、hen platinizing is a requirement. The glass shall be essentially free of color such that a standard white target shall appear unchanged in color when viewed through the thickness of the glass. Section 4 contains the inspection procedure. 3.1.2 Physical Defects Physical defects are as follows: a. Ext
28、ernal: chips - edge scratches - long in nature digs - generally round in nature distortion - impairs normal viewing short finish - incomplete polishingwave - surface appears wavy or rippled b. Internal: bubbles, seeds - considered as digs fracture - unacceptable striae (cords) - unacceptable if in t
29、he plane of viewing inclusion - considered as a dig SAE INTERNATIONAL ARP924B Page 4 of 6 The acceptance of chips in a cover glass should be specified relating to design requirements, however the depth should not be more than half the thickness of the glass. Edge chips that do not encroach on the ex
30、posed aperture should be allowable, providing the chip does not interfere with the sealing of the cover glass in the bezel. Edge chips should be stoned, sandblasted, or beveled out. In order to specify an instrument cover glass, the magnitude of acceptable scratches and digs must be defined. It is r
31、ecommended that the following limits apply when using the surface quality standards for optical elements: medium scratch; number 20 dig or inclusion or coating spatter; number 40 pinhole or coating voids as specified in MIL-PRF-13830. The following glass surface quality conditions shall govern the a
32、cceptance of a cover glass: 1. A medium scratch is one which cannot be detected through the glass at perpendicular incidence, using adequate background lighting, at a distance of greater than 11 feet. The combined length of medium scratches shall not exceed one-quarter the diameter of the glass. 2.
33、When a medium scratch is present, the maximum combined length of acceptable scratches shall not exceed three-quarters the diameter of the glass. 3. When a medium scratch is not present, the maximum combined length of acceptable scratches shall not exceed twice the diameter of the glass. 4. The dig n
34、umber is the actual diameter of the defect specified in units of 1/100 mm: a number 20 dig represents a 0.0079 inch diameter defect. In the case of irregular shaped digs the mean diameter will be utilized. The permissible number of maximum size digs or voids shall be one per each 20 mm of diameter:
35、the sum of all dig diameters shall not exceed twice the diameter of the maximum size specified per 20 mm diameter. The physical defect limits apply to each surface of the glass and successively to each coated surface. In computing maximum length scratches in other than circular glass, the mean measu
36、rement of the part shall be used in lieu of the diameter. The free aperture of the instrument should be specified on the print and a more liberal tolerance allowed for any area beyond the free aperture. The procedure to inspect the glass for defects shall be accomplished as outlined in Section 3. In
37、 order to apply an anti-reflective coating to a glass substrate, a non-coated border must be permitted to accommodate a jig or fixture. It is recommended that a minimum of 0.040 inch be permitted and more if possible (as measured from the edge of the glass). Imperfections attributable to the anti-re
38、flection coatings are as follows: spatter, pinholes or voids, andback coating. MIL-C-675 may be referenced for the requirements of anti-reflection coatings. An example of the notes appearing on a cover glass drawing is as follows: 1. Optical crown glass type 523-586 grade C per MIL-G-174 (INACTIVE F
39、OR NEW DESIGN) 2. Glass quality q3-Glazing select in accordance with ASTM C1036 on front and back surfaces 3. The glass shall be coated with an anti-reflection coating in accordance with MIL-C-675; the coating shall cover the front and back surfaces excluding a border of 0.060 +0.0/-0.060 4. The edg
40、es of the glass shall have a fine ground finish 3.2 Lighting Wedge 3.2.1 Glass Substrate The types of flat glass used for a lighting wedge are optical crown, clear sheet, and polished float. The polished flat glass is available in different grades as specified in ASTM C1036. The designer should be a
41、ware that the specification of other than optical crown or q2-Mirror grade glass for wedge lighting purposes could cause an undesirable color shift. The glass shall be essentially free of color such that a standard white target shall appear unchanged in color when viewed through the thickness of the
42、 glass (reference Section 4). A lighting wedge fabricated with normal (green) plate glass would cause a definite color shift to the reflected light illuminating display. The range of index of refraction of a lighting wedge when used with an anti-reflection coating should be between 1.47 and 1.55 and
43、 should be specified on the wedge drawing. For lighting wedges, uncoated, of a maximum thickness of 1/4 in, the minimum transmission of any wavelength between 400 and 700 nm should be 90%. SAE INTERNATIONAL ARP924B Page 5 of 6 3.2.2 Physical Defects The physical defects of a lighting wedge are the s
44、ame as those listed for a cover glass. The recommendation for surface quality is: a light scratch; a number 20 dig or inclusion or coating spatter; number 40 pinhole or coating void. The following glass surface quality conditions shall govern the acceptance of a lighting wedge: a. A light scratch is
45、 one which cannot be detected through the glass at perpendicular incidence, using adequate background lighting, at a distance of greater than 3 feet. The combined length of light scratches shall not exceed one-quarter the diameter of the glass. b. When a light scratch is present, the maximum combine
46、d length of acceptable scratches shall not exceed three-quarters the diameter of the glass. c. When a light scratch is not present, the maximum combined length of acceptable scratches shall not exceed twice the diameter of the glass. d. The dig conditions are the same as those listed for the cover g
47、lass: 3.1.2 (4). When evaluating a lighting wedge, a fracture between holes or between a hole and the edge of the glass may be acceptable providing it is outside the viewing area, and does not affect the lighting or mechanical design and providing there is no change of propagation. 4. TEST METHODS 4
48、.1 The cover glass or lighting wedge will be inspected for physical defects as per Figure 1, utilizing two 15 W fluorescent daylight lamps in a diffusing reflector or similar fixture in a darkened area. The surface quality should be inspected at an angle corresponding to the maximum brightness of a
49、visible scratch and at the front surface of the fixture. A scratch shall always be inspected in a horizontal plane and at the distance appropriate to the magnitude of the scratch criteria. The glass should then be rotated and reflected light utilized to inspect for other physical defects including pinholes or voids and spatter if the glass has an anti-reflection coating. Instrument glasses being rejected for physical defects may be re-inspec
