1、Designation: F 801 96 (Reapproved 2002)Standard Test Method forMeasuring Optical Angular Deviation of Transparent Parts1This standard is issued under the fixed designation F 801; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method covers meas
3、uring the angular deviationof a light ray imposed by transparent parts such as aircraftwindscreens and canopies. The results are uncontaminated bythe effects of lateral displacement, and the procedure may beperformed in a relatively short optical path length. This is notintended as a referee standar
4、d. It is one convenient method formeasuring angular deviations through transparent windows.1.2 This standard does not purport 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 practi
5、ces and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method23. Terminology3.1 Definitions:3.1.1 angular deviationthe departure of a light ray fr
6、omits original path as it passes through a transparent material. Thechange in angle of such a light ray. The displacement of animage due to the change in direction of the light ray.3.1.2 lateral (or linear) displacementthe shift or move-ment of a light ray from its original path as it passes through
7、 atransparent material, while maintaining parallelism betweenthe original and final paths. The change in location of an imagedue to this change in path.3.1.3 modulation transfer function (MTF)the ratio ofoutput modulation to the input modulation. The modulus of theFourier transform of the optical sp
8、read function.4. Summary of Test Method4.1 This test method outlines how measurements can bemade by an optoelectronic system employing collimated light,a field lens, and linear diode arrays as the part is held in itsinstalled angle. The positions of two images of a collimatedlight source are recorde
9、d using two linear diode arrays. Onearray records azimuth or horizontal position while the otherrecords elevation or vertical position. These arrays are at theposterior focal plane of a field lens. The positions are againrecorded after the interposition of a transparent part in theoptical path. The
10、difference in image position is directly relatedto the angular deviation imposed by the transparent part. Theeffects of lateral displacement are removed by the field lens.Sensitivity of measurement may be controlled by choosingappropriate focal length field lenses and spacing of elements onthe diode
11、 arrays.5. Significance and Use5.1 One of the measures of optical quality of a transparentpart is its angular deviation. Excessive angular deviation, orvariations in angular deviation throughout the part, result invisible distortion of scenes viewed through the part. Angulardeviation, its detection,
12、 and quantification are of extremeimportance in the area of certain aircraft transparency applica-tions, that is, aircraft equipped with Heads-up Displays (HUD).HUDs may require stringent control over the optics of theportion of the transparency (windscreen or canopy) which liesbetween the HUD combi
13、ning glass and the external environ-ment. Military aircraft equipped with HUDs or similar devicesrequire precise knowledge of the effects of the windscreen orcanopy on image position in order to maintain weapons aimingaccuracy.5.2 Two optical parameters have the effect of changingimage position. The
14、 first, lateral displacement, is inherent inany transparency which is tilted with respect to the line ofsight. The effect of lateral displacement is constant overdistance, and seldom exceeds a fraction of an inch. The secondparameter, angular deviation, is usually caused by a wedginessor nonparallel
15、ism of the transparency surfaces. The effect ofangular deviation is related to the tangent of the angle ofdeviation, thus the magnitude of the image position displace-ment increases as does the distance between image andtransparency. The quantification of angular deviation is thenthe more critical o
16、f the two parameters.1This test method 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 Nov. 10, 1996. Published January 1997. Originallypublished as F 801
17、83. Last previous edition F 801 83 (1989)e1.2Annual Book of ASTM Standards, Vol 14.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.6. Apparatus6.1 Transmitter, capable of projecting collimated light raysfrom a suitable target. The
18、 target may be a transparent cross oran “L” with one arm horizontal and one arm vertical, embed-ded in an opaque background. The stroke width of the “L” orcross shall be uniform. Choice of an “L” or a cross is optional,since only one half of the cross target is used at any time. Thetransmitter shoul
19、d be firmly affixed to the floor or otherstationary fixture.6.2 Receiver, firmly affixed to the floor or a stable platform,consisting of the following components:6.2.1 Displacement Compensation and Imaging LensThesensitivity of the instrument is in part determined by the focallength of the lens. An
20、appropriate focal length may be 10 in.(254 mm).6.2.2 Optical Beam Splitter, to separate the incoming lightinto two orthogonal elements; one for elevation and the otherfor azimuth. The type of beam splitter should be chosen to keepboth optical path lengths equal.6.2.3 Two Linear Charge Coupled Device
21、s (CCD or diode)Arrays, each located at the focal plane of the displacementcompensating lens. One array is oriented horizontally (for themeasurement of azimuthal changes), and the other orientedvertically (for the measurement of elevation changes). Anappropriate element spacing of the arrays is 0.00
22、1 in. (0.0254mm). Using this element spacing, and the 10-in. (254-mm)lens, each diode will represent the equivalent of 0.1 milliradian(mrad) angular deviation.6.2.4 Electronic System that will determine the center diodeof the band of illuminated diodes on each CCD array.6.2.5 Electronics System that
23、 will convert the number to bedisplayed on a digital readout.6.3 Transmitter and Receiver Lenses should be of achro-matic construction to reduce the effect of aberrations on themeasurement.6.4 Dioptometer, to verify attainment of collimated light.6.5 For further information on the rationale and deve
24、lop-ment of the design see the appendixes. (Appendix X1-Appendix X4.)7. Test Specimen7.1 The part to be tested should be positioned in such amanner as to approximate its installed configuration. Nospecial conditioning other than cleaning is required.8. Calibration and Standardization8.1 Position the
25、 transmitter and receiver so that the opticalaxes of both are parallel and approximately colinear. The lightfrom the transmitter shall pass through the test specimen to fallon the receiver lens. Depending on the configuration of the testspecimen, locate the transmitter and receiver approximately 4ft
26、 (305 mm or less) apart.8.2 Adjust the transmitter lens or target position to providecollimated light. A dioptometer is sufficient for this adjustment.8.3 Adjust the receiver field lens and positions of the CCDarrays so each array is at the focal plane of the lens. Performrough adjustment by using t
27、he receiver lens to sharply focusthe target from the previously adjusted transmitter. Check byinterposing a thick optical flat (plane parallel-sided transparentplate) in the optical path, and tilting the flat with respect to theoptical axis. When correctly adjusted, there will be no move-ment of the
28、 transmitter image at the plane of the CCD array. Ifthe image moves (the readout varies by more than 0.1 mrad),adjust the position of the appropriate CCD array to eliminatethis movement.8.4 An accuracy test may be made by interposing a standardor highly accurate optical wedge in the light path betwe
29、entransmitter and receiver. The display should accurately indicatethe angular deviation imposed by the optical wedge in both thevertical or horizontal meridians. An alternative method wouldbe to tilt the transmitter or receiver on an accurate tilt table. Thetilt, converted to milliradians, should eq
30、ual that shown on thedisplay. The latter method is usually preferable since it yieldsa continuous accuracy check over the entire range of measure-ment.8.5 A check to ensure operation of all diodes may beperformed by illuminating the entire CCD array and noting thedefault reading on the display. (Thi
31、s default reading is alsodependent on the specific circuitry used, but should be aconstant).NOTE 1The area of transparency being measured at any one time isrelated to the smallest diameter lens being used at the transmitter orreceiver. The system will average angular deviations throughout a subsetof
32、 this area. Use of lenses of significantly larger or smaller diameters willaffect repeatability of measurement from one instrument to another. Use oflenses with small diameters will improve performance on transparencieswith rapidly changing angular deviations, but will reduce available lightenergy a
33、t the CCD array, possibly below its threshold. Lens size is furtherdiscussed in the annex.8.6 Certain variations may be as a result of the followingsources of error:8.6.1 Transmitter or receiver lens malfocus. Noncollimatedlight from the transmitter will cause the receiver to measuresome lateral dis
34、placement as well as angular deviation.8.6.2 Poor transparency optics (MTF losses) will cause ablurred image on CCD arrays. If this blur is asymmetric, someerror will be introduced. If the MTF loss is great enough, thelight energy will fall below the threshold of the CCD array, anda no-reading condi
35、tion will result.9. Procedure9.1 Mount the transparent part on a fixture that allowsaccurate determination of the elevation and azimuth position ofthe part.9.2 Locate and firmly mount the transmitter at an appropri-ate position corresponding to the observational point of interest(pilots eye designed
36、 position), or along a line connecting thispoint with the receiver lens.9.3 Locate and firmly mount the receiver external to thetransparent part and at a distance of 4.9 ft (1.5 m) from thetransmitter.9.4 Establish a baseline or zero determination without atransparency in the optical path. Record th
37、e number as dis-played on the digital readout under this condition.9.5 Locate the transparency between the transmitter andreceiver. Take readings at points specified by the using activityby rotating the canopy about a critical point such as the pilotseye position or other position of interest specif
38、ied by the usingF 8012activity. The difference between these readings and the baselinefigures solely represent the angular deviation in milliradiansthrough each point.10. Calculation10.1 With appropriate selection of receiver lens focal lengthand CCD array diode separation, the display readout will
39、be in0.1-mrad increments. The sensitivity of the instrument may bevaried by altering either of these parameters. Assuming a 0.001in. (0.025 mm) diode spacing as standard, increasing the focallength will improve the sensitivity as follows:a 5 arc tan 0.001/f!where:a = sensitivity (minimum measurable
40、angle), mrad andf = focal length of receiver lens, in.10.2 Although the separation distance between the projectorand receiver is not critical and does not affect the measurementaccuracy, it does have an effect on both the light energy at theimage plane and the maximum amounts of angular deviationtha
41、t can be measured. The largest distance from the optical axisat the image plane that does not produce vignetting may becalculated as follows:H 5 f23 d22 d1!/2Swhere:H = maximum unvignetted ray height at image plane,d2= diameter of receiver lens,d1= diameter of transmitter lens,S = separation between
42、 transmitter and receiver, andf2= focal length of receiver lens10.3 A typical linear CCD array containing 512 elements,each with a 0.001 in. (0.025 mm) spacing, has an active surface12.5 mm long. The maximum angular deviation that can bedetected by such an array may be calculated as follows.M 5 2 3
43、arctan 12.5/f2!where:M = maximum angular deviation from one end of array tothe other andf2= focal length of receiver lens11. Report11.1 Draft a graph or chart, derived from the digital data,showing the angular deviation found at each point of interest.12. Precision and Bias12.1 PrecisionThe data use
44、d to develop this section wasobtained as the result of a round-robin test reported at theSeptember 1990 F7.08 subcommittee meeting. The writtenreport was entitled “Angular Deviation Revisited: Results of aRound Robin Test” and is available from ASTM Internationalheadquarters.3It should also be noted
45、 that there are only a feworganizations capable of making these types of measurementson aircraft transparencies. At the time of the inter-laboratorytest program there were only 5 measurement devices availableat 3 facilities. Although this is a lower number than thatrecommended by Practice E 691, the
46、 results provide a reason-able indication of the expected repeatability and reproducibilityof the procedure. If more measurement systems becomeavailable in the future, the interlaboratory test may be repeatedto obtain an updated estimate of precision and bias.12.1.1 There are two primary sources of
47、error with thisprocedure: (1) those dealing with the measurement deviceitself, and (2) those dealing with the positioning of the part tobe measured. Since this procedure only addresses the measure-ment device and not positioning equipment this section will beconfined to data relating to the precisio
48、n of the measurementdevice itself.12.1.2 Measurements of azimuth and elevation angulardeviation of two windscreens by two organizations usingprecise positioning equipment resulted in a total of 880 datapoints. These 880 points were measured twice to determinerepeatability. 832 of the 880 points, or
49、94 %, were within 60.1milliradian from the first measurement to the second measure-ment. Thus the 95 % confidence interval for repeatability forthis test method is 60.1 (it should be noted that the least countof the device described in this test method is 0.1 milliradian sothe confidence interval value has been rounded off to thenearest 0.1 milliradian even though the statistically calculatedconfidence interval would be slightly more than 0.1 milliradi-ans). The third organization that participated in these tests hada less precise manual positioning device that was not capab
