1、Designation: F2156 11Standard 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, the year
2、 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 transpar-ency, relative optical distortion results, specifically in th
3、ick,highly angled, multilayered plastic parts. Distortion occurs inall transparencies but is especially critical to aerospace appli-cations such as combat and commercial aircraft windscreens,canopies, or cabin windows. This is especially true duringoperations such as takeoff, landing, and aerial ref
4、ueling. It iscritical to be able to quantify optical distortion for procurementactivities.1.2 This test method covers the apparatus and proceduresthat are suitable for measuring the grid line slope (GLS) oftransparent parts, including those that are small or large, thin orthick, flat or curved, or a
5、lready installed. This test method isnot recommended for raw material.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3.1 ExceptionThe values given in parentheses are forinformation only.1.4 This standard does not purport
6、to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E177 Pr
7、actice for Use of the Terms Precision and Bias inASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodF733 Practice for Optical Distortion and Deviation ofTransparent Parts Using the Double-Exposure MethodF801 Test Method for Measuring Opti
8、calAngular Deviationof Transparent Parts3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 design eye, nthe reference point in aircraft designfrom which all anthropometrical design considerations aretaken.3.1.2 distortion, nthe rate of change of deviation resultingfrom an irregul
9、arity in a transparent part.3.1.2.1 DiscussionDistortion shall be expressed as theslope 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 todetect the presence of distortion in transparent parts.3.1.3.1 DiscussionThe grid
10、board is usually, but notalways, a vertical rectangular backboard with horizontal andvertical intersecting lines with maximum contrast between thewhite lines and the black background.3.1.4 grid line slope, nan optical distortion evaluationparameter that compares the slope of a deviated grid line to
11、thatof a nondeviated grid line.3.1.4.1 DiscussionThe degree of deviation shall be indi-cated by a ratio, for example, 1 in 2, 1 in 8, or 1 in 20 (thevisual optical quality improves as the second number getslarger.)3.1.5 installed angle, nthe transparency orientation asinstalled in the aircraft, defi
12、ned by the angle between ahorizontal line (line of sight) and a plane tangent to the surfaceof the transparency (see Fig. 2).3.1.6 repeatability limit (rL), nfrom Practice E177,27.3.2, “approximately 95 % of individual test results fromlaboratories similar to those in an Inter-laboratory Study (ILS)
13、can be expected to differ in absolute value from their averagevalue by less than 1.96s (about 2s).”3.1.6.1 Discussionin terms of this test method, approxi-mately 95 % of all pairs of replications from the sameevaluator and the same photo differ in absolute value by lessthan the rL.1This test method
14、is under the jurisdiction of ASTM Committee F07 onAerospace and Aircraft , and is the direct responsibility of Subcommittee F07.08 onTransparent Enclosures and Materials.Current edition approved Dec. 1, 2011. Published January 2012. Originallyapproved in 2001. Last previous edition approved in 2006
15、as F2156 - 06. DOI:10.1520/F2156-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM Internati
16、onal, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.7 reproducibility limit (RL), nfrom Practice E177,27.3.3, “approximately 95 % of all pairs of test results fromlaboratories similar to those in the study can be expected todiffer in absolute value by less t
17、han 1.960=2s (about 2.0=2s)= 2.77s (or about 2.8s). This index is also known as the 95 %limit on the difference between two test results.”FIG. 1 Optical Distortion Represented By TangentFIG. 2 Schematic Diagrams of GLS Photographic Recording DistancesF2156 1123.1.7.1 Discussionin terms of this test
18、method, approxi-mately 95 % of all pairs of replications from different evalua-tors and the same photo differ in absolute value by less than theRL.4. Summary of Test Method4.1 The transparent part shall be mounted, preferably at theinstalled angle, at a specified distance from a grid board testpatte
19、rn. A photographic camera shall be placed so as to recordthe grid pattern as viewed through the part from the design eye(or other specified) viewing position. If the viewing position isnot defined, the values in Table 1 may be used as photographictest geometry. The image is then analyzed to assess t
20、he level ofoptical distortion as measured by grid line slope.4.2 Distortion shall be recorded using either a single-exposure photograph or a double-exposure photograph. Thephotographed grid shall then be measured using either adrafting machine procedure or a manual procedure. Eachprocedure has its o
21、wn level of precision.5. Significance and Use5.1 Transparent parts, such as aircraft windshields, cano-pies, cabin windows, and visors, shall be measured for com-pliance with optical distortion specifications using this testmethod. This test method is suitable for assessing opticaldistortion of tran
22、sparent parts as it relates to the visualperception of distortion. It is not suitable for assessing distor-tion as it relates to pure angular deviation of light as it passesthrough the part. Either Test Method F801 or Practice F733 isappropriate and shall be used for this latter application. Thistes
23、t method is not recommended for raw material.6. Apparatus6.1 Test RoomThe test room shall be large enough tolocate 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 dis
24、tances shall be used if desired. GLSresults will vary with different distances, which means thatmeasurements of different parts taken at different distancescannot be compared.6.1.4 The walls, ceiling, and floor shall have low reflec-tance. A flat black paint or coating is preferred though notrequire
25、d.6.2 Grid BoardThe grid board shall provide a definedpattern against which the transparent part is examined. Gridboards shall be one of the following types:6.2.1 Type 1The grid board shall be composed of whitestrings held taut, each spaced at a specific interval, with thestrings stretched verticall
26、y and horizontally. The grid boardframe and background shall have a flat black finish to reducelight reflection. A bank of fluorescent lights at each side orevenly distributed natural sunlight conditions provide illumi-nation of the strings.6.2.2 Type 2The grid board shall be a transparent sheethavi
27、ng an opaque, flat black outer surface except for the gridlines. The grid lines remain transparent, and when backlit withfluorescent or incandescent lights, provide a bright grid patternagainst a black background with excellent contrast character-istics.6.2.3 Type 3The grid board shall be a rigid sh
28、eet ofmaterial that has a grid pattern printed on the front surface.Details of the grid lines, pattern, and lighting shall be asspecified by the procuring activity.6.2.4 The grid board shall have a width and height largeenough so that the area of the part to be imaged is superim-posed within the per
29、imeter of the grid board. Details of the gridsquare size shall be as specified by the procuring activity. Therecommended grid 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 photographdistortion for the evaluation of grid line sl
30、ope. For highestresolution, it is recommended that a large format camera beused, although a 35-mm camera is also acceptable. Black-and-white film shall be 400 ASA (or slower). Use of a digitalcamera is permitted if it has sufficiently high resolution (that is,with no visible pixilation in the printe
31、d image). When using adouble-exposure recording technique (Fig. 3), the film-basedcamera shall have a double-exposure capability. Separatedigital images are superimposed using a computer-based photoeditor. The camera lens shall have very low distortion charac-teristics. The camera shall be firmly mo
32、unted at design eye (orother specified viewing position) to prevent any movementduring the photographic exposure.6.4 Drafting Machine ProcedureThe drafting machineshall consist of a vertical and horizontal scale attached to arotating head that displays the angular position of the horizon-tal or vert
33、ical scale in degrees with a resolution of at least 1 arcminute or1100 of a degree. This common, commerciallyavailable apparatus, which is mounted to a drafting table, shallbe used for the evaluation of grid line slope.6.5 Manual ProcedureMeasurements shall be made usinghigh-quality drafting instrum
34、ents (for example, metal scales,right triangle).7. Test Specimen7.1 The transparency to be measured shall be cleaned, usingthe manufacturer or procuring agency approved procedure, toremove any foreign material that might cause localized opticaldistortion. Unless specified by the procuring activity,
35、no specialTABLE 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-grid-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
36、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 User definedAAll measurements shall be 63cmor63 %, whichever is smaller.BIt is recommended that the camera-to-part distance be the
37、design eyedistance.F2156 113conditioning, other than cleaning, shall be required and the partshall be at ambient temperature. If required, a mask can beapplied to the transparency to eliminate non-optical zones.8. Calibration and Standardization8.1 Test Procedure GeometryDistance measurementsshall b
38、e made using a high-quality tape measure.8.2 Drafting Machine ProcedureMeasurements shall bemade using high-quality drafting instruments. The draftingmachine shall be accurate to within its specified manufacturerstolerances.8.3 Manual ProcedureMeasurements shall be made usinghigh-quality drafting in
39、struments (for example, metal scales,right triangle).9. 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. Table 1 contains the setup measurement distances.Fig. 2 illust
40、rates the setup geometries. When the part is flat andmounted (nearly) vertically (for example, a passenger win-dow), Setup A is a more stringent test than Setup B. Use SetupC when the part is curved (for example, an aircraft canopy). Itis recommended that measurements then be performed at theinstall
41、ed position and the camera be placed at the design eyelocation. Choose the test conditions that will most accuratelysimulate the actual field viewing conditions under which thepart is used.9.1.2 Firmly mount the transparent part to be examined toprevent movement during photographing. The mounted ang
42、leof the part shall be as specified by the procuring activity. It isrecommended that the part be mounted at the installed angle.Record the mounted angle and report it with the results.9.1.3 The camera shall be mounted at the design eyeposition (or other position as specified by the procuringactivity
43、). The optical axis of the camera shall be perpendicularto the grid board surface and shall be aimed at the target panel.See 6.3 for photographic recording requirements.9.1.4 Place the grid board at a given distance (see Table 1)from the camera or as specified by the procurement agency,and ensure th
44、at the grid board pattern is in good focus at thefocal plane of the camera. It is highly desirable (but notrequired) for part of the grid board target to be directly visiblefrom the camera position without passing through the trans-parency. If this is possible, this undistorted section of the gridbo
45、ard serves as an alignment reference when determining theGLS of the transparency.9.1.5 Photograph optical distortion through the part usingone of two recording techniques. Both of these techniquesrecord distortion of the grid. The distortion from these twophotographic techniques shall be analyzed us
46、ing either thedrafting machine or manual measurement procedures.9.1.5.1 Single-exposure photographPrepare a single-exposure photograph of the grid board viewed through thetransparent part. The camera shall be focused on the grid board.9.1.5.2 Double-exposure photographPhotograph the gridboard throug
47、h the transparent part. Then, without allowing anymovement of the camera or advancing the film, remove the partand make the second exposure of the grid board alone.9.1.6 Develop the film and produce 8- by 10-in. matte finishprints (minimum size). The matte finish will reduce reflectionproblems durin
48、g measurement. Alternatively, high qualityprinting paper can be used as well.9.2 Grid Line Slope Measurement Procedures:9.2.1 Drafting Machine Procedure:9.2.1.1 Tape the photograph to the drafting board.9.2.1.2 Align the horizontal scale of the drafting machinewith a horizontal line of the grid boar
49、d in the photo that is inthe directly viewed section of the photo outside of thewindscreen. If there is no such area, align the scale with themost undistorted horizontal grid board line within the picture,to serve as a reference.9.2.1.3 Zero the drafting machine.9.2.1.4 Systematically scan the photograph horizontally andvertically to find the most distorted area (the line with thegreatest slope).9.2.1.5 Place the horizontal straight edge tangent to thehorizontal line displaying the maximum slope (bending fromhorizontal) within the area of the transparenc
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