ASTM F801-21

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Designation: F801 − 16 F801 − 21
Standard Test Method for
Measuring Optical Angular Deviation of Transparent Parts
This standard is issued under the fixed designation F801; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
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 safety, health, and healthenvironmental 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
2.1 ASTM Standards:
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
F2156 Test Method for Measuring Optical Distortion in Transparent Parts Using Grid Line Slope
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
3. Terminology
3.1 Definitions:
3.1.1 angular deviation—deviation, n—the departure of a light ray from its original path as it passes through a transparent material.
The change in angle of such a light ray. The displacement of an image due to the change in direction of the light ray.
3.1.2 lateral (or linear) displacement,—n—the shift or movement of a light ray from its original path as it passes through a
transparent material, while maintaining parallelism between the original and final paths. The change in location of an image due
to this change in path.
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 Transparent
Enclosures and Materials.
Current edition approved April 1, 2016May 1, 2021. Published April 2016June 2021. Originally approved in 1983. Last previous edition approved in 20082016 as
F801 – 96 (2008).F801 – 16. DOI: 10.1520/F0801-16.10.1520/F0801-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F801 − 21
3.1.3 modulation transfer function (MTF)—), n—the ratio of output modulation to the input modulation. The modulus of the
Fourier transform of the optical spread function.
4. Summary of Test Method
4.1 This test method provides a basic procedure for measuring the angular deviation induced by a transparent part. Angular
deviation measurements are made by an optoelectronic system employing collimated light from an appropriate target pattern, a
field lens, and a position-detecting device/system such as linear diode arrays or a two-dimensional diode array. Hold the transparent
part either in its installed angle or perpendicular to the collimated light source or any other orientation suitable for the purpose of
making the measurement. One specific optoelectronic system suitable for conducting this test method is provided in Appendix X2
and Appendix X3 and in Section 6 below.
5. 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.
6. Apparatus
6.1 Transmitter, capable of projecting collimated light rays from a suitable target. The target is allowed to be a small hole, a
transparent cross, or an “L” with one arm horizontal and one arm vertical, embedded in an opaque background. The stroke width
of the “L” or cross 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. The transmitter shall be firmly affixed to the floor or other stationary fixture. Note that the choice of the transmitter target
shall be compatible with the position detecting mechanism in the receiver.
6.2 Receiver, shall be firmly affixed to the floor or a stable platform, consisting of the following components:
6.2.1 Displacement Compensation and Imaging Lens—The sensitivity of the instrument is in part determined by the focal length
of the lens. An appropriate focal length is likely to be (but is not limited to) 10 in. (254 mm).
6.2.2 Optical Beam Splitter, to separate the incoming light into two orthogonal elements; one for elevation and the other for
azimuth. The type of beam splitter shall be chosen to keep both optical path lengths equal.
6.2.3 Two Linear Charge Coupled Devices (CCD or diode) Arrays, each located at the focal plane of the displacement
compensating lens. One array is oriented horizontally (for the measurement of azimuthal changes), and the other oriented vertically
(for the measurement of elevation changes). A convenient element spacing of the arrays is 0.001 in. (0.0254 mm). Using this
element spacing, and the 10-in. (254-mm)10 in. (254 mm) lens, each diode represents the equivalent of 0.1 milliradian (mrad)
angular deviation.
6.2.4 Electronic System capable of determining the center diode of the band of illuminated diodes on each CCD array.
6.2.5 Electronics System capable of converting the diode number to an angular deviation to be displayed on a digital readout.
F801 − 21
6.3 Transmitter and Receiver Lenses are permitted to be of achromatic construction to reduce the effect of aberrations on the
measurement.
6.4 Dioptometer, to verify attainment of collimated light.
6.5 For further information on the rationale and development of the design, see the appendixes.appendixes (Appendix X1 –
Appendix X4.)).
7. Test Specimen
7.1 The part to be tested shall be positioned in such a manner as to approximate its installed configuration or positioned as
specified by the agency requesting the test. No special conditioning other than cleaning is required.
8. Calibration and Standardization
8.1 Position the transmitter and receiver so that the optical axes of both are parallel and approximately colinear. The light from
the transmitter shall pass through the test specimen to fall on the receiver lens. Depending on the configuration of the test specimen,
locate the transmitter and receiver approximately 4 ft (1219 mm or less) apart. This distance depends on the specific equipment
used and is not critical (see Appendix X4 for more information).
8.2 If necessary, adjust the transmitter lens or target position to provide collimated light. A dioptometer is sufficient for this
adjustment.
8.3 If necessary, adjust the receiver field lens and positions of the CCD arrays so each array is at the focal plane of the receiver
lens. Perform rough adjustment by using the receiver lens to sharply focus the target from the previously adjusted transmitter.
Check by interposing a thick optical flat (plane parallel-sided transparent plate) in the optical path, and tilting the flat with respect
to the optical axis. When correctly adjusted, there will be no movement of the transmitter image at the plane of the CCD array.
If the image moves (the readout varies by more than 0.1 mrad), adjust the position of the appropriate CCD array to eliminate this
movement.
8.4 A method of conducting an accuracy test is made by interposing a standard or highly accurate optical wedge in the light path
between transmitter and receiver. The display shall accurately indicate the angular deviation imposed by the optical wedge in both
the vertical or horizontal meridians. An alternative method would be to tilt the transmitter or receiver on an accurate tilt table. The
tilt, converted to milliradians, shall equal that shown on the display. The latter method is usually preferable since it yields a
continuous accuracy check over the entire range of measurement.
8.5 A method of performing a check to ensure operation of all diodes is performed by illuminating the entire CCD array and noting
the default reading on the display. (This default reading is also dependent on the specific circuitry used, but is ideally a constant).
NOTE 1—The area of transparency being measured at any one time is related to the smallest diameter lens being used at the transmitter or receiver. The
system will average angular deviations throughout a subset of this area. Use of lenses of significantly larger or smaller diameters will affect repeatability
of measurement from one instrument to another. Use of lenses with small diameters will improve performance on transparencies with rapidly changing
angular deviations, but will reduce available light energy at the CCD array, possibly below its threshold. Lens size is further discussed in the Appendixes.
8.6 It is possible that certain variations will be as a result of the following sources of error:
8.6.1 Transmitter or receiver lens malfocus. Noncollimated light from the transmitter will cause the receiver to measure some
lateral displacement as well as angular deviation.
8.6.2 Poor transparency optics (MTF or modulation transfer function losses) will cause a blurred image on CCD arrays. If this blur
is asymmetric, it is possible that some error will be introduced. If the MTF loss is great enough, it is possible that the light energy
will fall below the threshold of the CCD array, and a no-reading condition will result.
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9. Procedure
9.1 Mount the transparent part on a fixture that allows accurate determination of the elevation and azimuth position of the part.
9.2 Locate and firmly mount the transmitter at an appropriate position corresponding to the observational point of interest (for
example: pilot’s pilot design eye designed position), or along a line connecting this point with the receiver lens.
9.3 Locate and firmly mount the receiver external to the transparent part and at a suitable distance from the transmitter (see
Appendix X4).
9.4 Establish a baseline or zero determination without a transparency in the optical path. Record the number as displayed on the
digital readout under this condition (depending on the electronic system used, this value will be zero or adjustable to zero).
9.5 Locate the transparency between the transmitter and receiver. Take readings at points specified by the using activity by rotating
the canopy about a critical point such as the pilot’s pilot design eye position or other position of interest specified by the using
activity. The difference between these readings and the baseline figures represent the angular deviation in milliradians through each
point.
10. Calculation
10.1 With appropriate selection of receiver lens focal length and CCD array diode separation, the display readout will be in
0.1-milliradian0.1 milliradian increments. It is possible to vary the sensitivity of the instrument by altering either of these
parameters. Assuming a 0.001 in. (0.025 mm) diode spacing as standard, increasing the focal length will improve the sensitivity
as follows:
a 5 arc tan 0.001/f
~ !
where:
a = sensitivity (minimum measurable angle), mrad and
f = focal length of receiver lens, in.
10.2 Although the separation distance between the projector and receiver is not critical and does not affect the measurement
accuracy, it does have an effect on both the light energy at the image plane and the maximum amounts of angular deviation that
can be measured. Calculate the largest distance from the optical axis at the image plane that does not produce vignetting as follows:
H 5 f 3 d 2 d /2S
~ !
2 2 1
where:
H = maximum unvignetted ray height at image plane,
d = diameter o
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