1、Designation: F2156 11F2156 17Standard Test Method forMeasuring Optical Distortion in Transparent Parts UsingGrid Line Slope1This standard is issued under the fixed designation F2156; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 When an observer looks through an aerospace transparency, relative optical distortion results, specificall
3、y in thick, highlyangled, multilayered plastic parts. Distortion occurs in all transparencies but is especially critical to aerospace applications suchas combat and commercial aircraft windscreens, canopies, or cabin windows. This is especially true during operations such astakeoff, landing, and aer
4、ial refueling. It is critical to be able to quantify optical distortion for procurement activities.1.2 This test method covers the apparatus and procedures that are suitable for measuring the grid line slope (GLS) of transparentparts, including those that are small or large, thin or thick, flat or c
5、urved, or already installed. This test method is not recommendedfor raw material.1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.3.1 ExceptionThe values given in parentheses are for information only.1.4 This standard doe
6、s not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.1.5 This international standard was de
7、veloped in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Docu
8、ments2.1 ASTM Standards:2E177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test MethodF733 Practice for Optical Distortion and Deviation of Transparent Parts Using the Double-Exposure Method
9、F801 Test Method for Measuring Optical Angular Deviation of Transparent Parts3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 design eye, nthe reference point in aircraft design from which all anthropometrical design considerations are taken.3.1.2 distortion, nthe rate of chang
10、e of deviation resulting from an irregularity in a transparent part.3.1.2.1 DiscussionDistortion shall be expressed as the slope of the angle of localized grid line bending, for example, 1 in 5 (see Fig. 1).3.1.3 grid board, nan optical evaluation tool used to detect the presence of distortion in tr
11、ansparent parts.3.1.3.1 Discussion1 This test method is under the jurisdiction of ASTM Committee F07 on Aerospace and Aircraft, and is the direct responsibility of Subcommittee F07.08 on TransparentEnclosures and Materials.Current edition approved Dec. 1, 2011June 1, 2017. Published January 2012June
12、 2017. Originally approved in 2001. Last previous edition approved in 20062011 asF2156 - 06.F2156 - 11. DOI: 10.1520/F2156-11.10.1520/F2156-17.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvol
13、ume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adeq
14、uately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
15、ken, PA 19428-2959. United States1The grid board is usually, but not always, a vertical rectangular backboard with horizontal and vertical intersecting lines withmaximum contrast between the white lines and the black background.3.1.4 grid line slope, nan optical distortion evaluation parameter that
16、compares the slope of a deviated grid line to that of anondeviated grid line.3.1.4.1 DiscussionThe degree of deviation shall be indicated by a ratio, for example, 1 in 2, 1 in 8, or 1 in 20 (the visual optical quality improvesas the second number gets larger.)3.1.5 installed angle, nthe transparency
17、 orientation as installed in the aircraft, defined by the angle between a horizontal line(line of sight) and a plane tangent to the surface of the transparency (see Fig. 2).3.1.6 repeatability limit (rL), nfrom Practice E177, 27.3.2, “approximately 95 % of individual test results from laboratoriessi
18、milar to those in an Inter-laboratory Study (ILS) can be are expected to differ in absolute value from their average value by lessthan 1.96s (about 2s).”3.1.6.1 DiscussionFIG. 1 Optical Distortion Represented By TangentF2156 172in terms of this test method, approximately 95 % of all pairs of replica
19、tions from the same evaluator and the same photo differ inabsolute value by less than the rL.FIG. 2 Schematic Diagrams of GLS Photographic Recording DistancesF2156 1733.1.7 reproducibility limit (RL), nfrom Practice E177, 27.3.3, “approximately 95 % of all pairs of test results from laboratoriessimi
20、lar to those in the study can be are expected to differ in absolute value by less than 1.9602s (about 2.02s) = 2.77s (or about2.8s). This index is also known as the 95 % limit on the difference between two test results.”3.1.7.1 Discussionin terms of this test method, approximately 95 % of all pairs
21、of replications from different evaluators and the same photo differin absolute value by less than the RL.4. Summary of Test Method4.1 The transparent part shall be mounted, preferably at the installed angle, at a specified distance from a grid board test pattern.A photographic camera shall be placed
22、 so as to record the grid pattern as viewed through the part from the design eye (or otherspecified) viewing position. If the viewing position is not defined, the values in Table 1 mayshall be used as photographic testgeometry. The image is then analyzed to assess the level of optical distortion as
23、measured by grid line slope.4.2 Distortion shall be recorded using either a single-exposure photograph or a double-exposure photograph. The photographedgrid shall then be measured using either a drafting machine procedure or a manual procedure. Each procedure has its own levelof precision.5. Signifi
24、cance and Use5.1 Transparent parts, such as aircraft windshields, canopies, cabin windows, and visors, shall be measured for compliance withoptical distortion specifications using this test method. This test method is suitable for assessing optical distortion of transparentparts as it relates to the
25、 visual perception of distortion. It is not suitable for assessing distortion as it relates to pure angular deviationof light as it passes through the part. Either Test Method F801 or Practice F733 is appropriate and shall be used for this latterapplication. This test method is not recommended for r
26、aw material.6. Apparatus6.1 Test RoomThe test room shall be large enough to locate the required testing equipment properly.6.1.1 Setup A requires a room approximately 12 m (40 ft) long.6.1.2 Setup B requires a room approximately 7 m (23 ft) long.6.1.3 Setup C: other distances shall be used if desire
27、d. GLS results will vary with different distances, which means thatmeasurements of different parts taken at different distances cannot be compared.6.1.4 The walls, ceiling, and floor shall have low reflectance. A flat black paint or coating is preferred though not required.6.2 Grid BoardThe grid boa
28、rd shall provide a defined pattern against which the transparent part is examined. Grid boards shallbe one of the following types:6.2.1 Type 1The grid board shall be composed of white strings held taut, each spaced at a specific interval, with the stringsstretched vertically and horizontally. The gr
29、id board frame and background shall have a flat black finish to reduce light reflection.A bank of fluorescent lights at each side or evenly distributed natural sunlight conditions provide illumination of the strings.6.2.2 Type 2The grid board shall be a transparent sheet having an opaque, flat black
30、 outer surface except for the grid lines.The grid lines remain transparent, and when backlit with fluorescent or incandescent lights, provide a bright grid pattern againsta black background with excellent contrast characteristics.6.2.3 Type 3The grid board shall be a rigid sheet of material that has
31、 a grid pattern printed on the front surface. Details ofthe grid lines, pattern, and lighting shall be as specified by the procuring activity.TABLE 1 GLS Photographic Recording DistancesSetup ACamera-to-grid-board distance 1000 cm (32 ft 10 in.)ACamera-to-part distance 550 cm (18 ft 1 in.)Part-to-gr
32、id-board distance 450 cm (14 ft 9 in.)Setup BCamera-to-grid-board distance 450 cm (14 ft 9 in.)Camera-to-part distance 150 cm (4 ft 11 in.)Part-to-grid-board distance 300 cm (9 ft 10 in.)Setup CCamera-to-grid-board distance User definedCamera-to-part distance User definedBPart-to-grid-board distance
33、 User definedA All measurements shall be 3 cm or 3 %, whichever is smaller.B It is recommended that the camera-to-part distance be the design eye distance.F2156 1746.2.4 The grid board shall have a width and height large enough so that the area of the part to be imaged is superimposed withinthe peri
34、meter of the grid board. Details of the grid square size shall be as specified by the procuring activity. The recommendedgrid line spacing shall be not less than 1.27 cm (12 in.) or more than 2.54 cm (1 in.).6.3 CameraThe camera shall be used to photograph distortion for the evaluation of grid line
35、slope. For highest resolution, itis recommended that a large format camera be used, although a 35-mm camera is also acceptable. Black-and-white film shall be400 ASA (or slower). Use of a digital camera is permitted if it has sufficiently high resolution (that is, with no visible pixilationin the pri
36、nted image). When using a double-exposure recording technique (Fig. 3), the film-based camera shall have adouble-exposure capability. Separate digital images are superimposed using a computer-based photo editor. The camera lens shallhave very low distortion characteristics. The camera shall be firml
37、y mounted at design eye (or other specified viewing position)to prevent any movement during the photographic exposure.6.4 Drafting Machine ProcedureThe drafting machine shall consist of a vertical and horizontal scale attached to a rotatinghead that displays the angular position of the horizontal or
38、 vertical scale in degrees with a resolution of at least 1 arc minute or1100 of a degree. This common, commercially available apparatus, which is mounted to a drafting table, shall be used for theevaluation of grid line slope.6.5 Manual ProcedureMeasurements shall be made using high-quality drafting
39、 instruments (for example, metal scales, righttriangle).7. Test Specimen7.1 The transparency to be measured shall be cleaned, using the manufacturer or procuring agency approved procedure, toremove any foreign material that might cause localized optical distortion. Unless specified by the procuring
40、activity, no specialconditioning, other than cleaning, shall be required and the part shall be at ambient temperature. If required, a mask canshall beapplied to the transparency to eliminate non-optical zones.8. Calibration and Standardization8.1 Test Procedure GeometryDistance measurements shall be
41、 made using a high-quality tape measure.8.2 Drafting Machine ProcedureMeasurements shall be made using high-quality drafting instruments. The drafting machineshall be accurate to within its specified manufacturers tolerances.8.3 Manual ProcedureMeasurements shall be made using high-quality drafting
42、instruments (for example, metal scales, righttriangle).FIG. 3 GLS Double-Exposure Recording TechniqueF2156 1759. Procedure9.1 Photographic Recording Techniques:9.1.1 The procuring activity specifies whether Setup A, B, or C (other specified distances) shall be used to measure opticaldistortion. Tabl
43、e 1 contains the setup measurement distances. Fig. 2 illustrates the setup geometries. When the part is flat andmounted (nearly) vertically (for example, a passenger window), Setup A is a more stringent test than Setup B. Use Setup C whenthe part is curved (for example, an aircraft canopy). It is re
44、commended that measurements then be performed at the installedposition and the camera be placed at the design eye location. Choose the test conditions that will most accurately simulate theactual field viewing conditions under which the part is used.9.1.2 Firmly mount the transparent part to be exam
45、ined to prevent movement during photographing. The mounted angle of thepart shall be as specified by the procuring activity. It is recommended that the part be mounted at the installed angle. Record themounted angle and report it with the results.9.1.3 The camera shall be mounted at the design eye p
46、osition (or other position as specified by the procuring activity). Theoptical axis of the camera shall be perpendicular to the grid board surface and shall be aimed at the target panel. See 6.3 forphotographic recording requirements.9.1.4 Place the grid board at a given distance (see Table 1) from
47、the camera or as specified by the procurement agency, andensure that the grid board pattern is in good focus at the focal plane of the camera. It is highly desirable (but not required) for partof the grid board target to be directly visible from the camera position without passing through the transp
48、arency. If this is possible,this undistorted section of the grid board serves as an alignment reference when determining the GLS of the transparency.9.1.5 Photograph optical distortion through the part using one of two recording techniques. Both of these techniques recorddistortion of the grid. The
49、distortion from these two photographic techniques shall be analyzed using either the drafting machineor manual measurement procedures.9.1.5.1 Single-exposure photographPrepare a single-exposure photograph of the grid board viewed through the transparentpart. The camera shall be focused on the grid board.9.1.5.2 Double-exposure photographPhotograph the grid board through the transparent part. Then, without allowing anymovement of the camera or advancing the film, remove the part and make the second exposure of the grid board alone.9.1.6 Develop
