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    ASTM F801 - 21

    Standard Test Method for Measuring Optical Angular Deviation of Transparent Parts

    Active Standard ASTM F801 | Developed by Subcommittee: F07.08

    Book of Standards Volume: 15.03

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    Significance and Use

    5.1 One of the measures of optical quality of a transparent part is its angular deviation. It is possible that excessive angular deviation, or variations in angular deviation throughout the part, will result in visible distortion of scenes viewed through the part. Angular deviation, its detection, and quantification are of extreme importance in the area of certain aircraft transparency applications, that is, aircraft equipped with Heads-up Displays (HUD). It is possible that HUDs will require stringent control over the optics of the portion of the transparency (windscreen or canopy) which lies between the HUD combining glass and the external environment. Military aircraft equipped with HUDs or similar devices require precise knowledge of the effects of the windscreen or canopy on image position in order to maintain weapons aiming accuracy.

    5.2 Two optical parameters have the effect of changing image position. The first, lateral displacement, is inherent in any transparency which is tilted with respect to the line of sight. The effect of lateral displacement is constant over distance, and seldom exceeds a fraction of an inch. The second parameter, angular deviation, is usually caused by a wedginess or nonparallelism of the transparency surfaces. The effect of angular deviation is related to the tangent of the angle of deviation, thus the magnitude of the image position displacement increases as does the distance between image and transparency. The quantification of angular deviation is then the more critical of the two parameters. Both parameters are illustrated in Fig. X1.1.

    1. Scope

    1.1 This test method covers measuring the angular deviation of a light ray imposed by transparent parts such as aircraft windscreens and canopies. The results are uncontaminated by the effects of lateral displacement, and it is possible to perform the procedure in a relatively short optical path length. This is not intended as a referee standard. It is one convenient method for measuring angular deviations through transparent windows.

    1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

    1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

    2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

    ASTM Standards

    E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

    F733 Practice for Optical Distortion and Deviation of Transparent Parts Using the Double-Exposure Method

    F2156 Test Method for Measuring Optical Distortion in Transparent Parts Using Grid Line Slope

    ICS Code

    ICS Number Code 49.035 (Components for aerospace construction)

    UNSPSC Code

    UNSPSC Code 25200000(Aerospace systems and components and equipment)

    Referencing This Standard
    Link Here
    Link to Active (This link will always route to the current Active version of the standard.)

    DOI: 10.1520/F0801-21

    Citation Format

    ASTM F801-21, Standard Test Method for Measuring Optical Angular Deviation of Transparent Parts, ASTM International, West Conshohocken, PA, 2021, www.astm.org

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