ASTM F801-96(2008)
(Test Method)Standard Test Method for Measuring Optical Angular Deviation of Transparent Parts
Standard Test Method for Measuring Optical Angular Deviation of Transparent Parts
SIGNIFICANCE AND USE
One of the measures of optical quality of a transparent part is its angular deviation. Excessive angular deviation, or variations in angular deviation throughout the part, 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). HUDs may 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.
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.
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 the procedure may be performed 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: F801 − 96(Reapproved 2008)
Standard Test Method for
Measuring Optical Angular Deviation of Transparent Parts
ThisstandardisissuedunderthefixeddesignationF801;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
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 4. Summary of Test Method
1.1 Thistestmethodcoversmeasuringtheangulardeviation 4.1 This test method outlines how measurements can be
of a light ray imposed by transparent parts such as aircraft made by an optoelectronic system employing collimated light,
windscreens and canopies. The results are uncontaminated by a field lens, and linear diode arrays as the part is held in its
the effects of lateral displacement, and the procedure may be installed angle. The positions of two images of a collimated
performed in a relatively short optical path length. This is not light source are recorded using two linear diode arrays. One
intended as a referee standard. It is one convenient method for array records azimuth or horizontal position while the other
measuring angular deviations through transparent windows. records elevation or vertical position. These arrays are at the
posterior focal plane of a field lens. The positions are again
1.2 This standard does not purport to address all of the
recorded after the interposition of a transparent part in the
safety concerns, if any, associated with its use. It is the
opticalpath.Thedifferenceinimagepositionisdirectlyrelated
responsibility of the user of this standard to establish appro-
to the angular deviation imposed by the transparent part. The
priate safety and health practices and determine the applica-
effects of lateral displacement are removed by the field lens.
bility of regulatory limitations prior to use.
Sensitivity of measurement may be controlled by choosing
2. Referenced Documents
appropriatefocallengthfieldlensesandspacingofelementson
the diode arrays.
2.1 ASTM Standards:
E691 Practice for Conducting an Interlaboratory Study to
5. Significance and Use
Determine the Precision of a Test Method
5.1 One of the measures of optical quality of a transparent
3. Terminology
part is its angular deviation. Excessive angular deviation, or
3.1 Definitions:
variations in angular deviation throughout the part, result in
3.1.1 angular deviation—the departure of a light ray from
visible distortion of scenes viewed through the part. Angular
its original path as it passes through a transparent material.The
deviation, its detection, and quantification are of extreme
change in angle of such a light ray. The displacement of an
importance in the area of certain aircraft transparency
image due to the change in direction of the light ray.
applications, that is, aircraft equipped with Heads-up Displays
3.1.2 lateral (or linear) displacement—the shift or move- (HUD). HUDs may require stringent control over the optics of
ment of a light ray from its original path as it passes through a the portion of the transparency (windscreen or canopy) which
lies between the HUD combining glass and the external
transparent material, while maintaining parallelism between
the original and final paths.The change in location of an image environment. Military aircraft equipped with HUDs or similar
due to this change in path. devices require precise knowledge of the effects of the wind-
screen or canopy on image position in order to maintain
3.1.3 modulation transfer function (MTF)—the ratio of
weapons aiming accuracy.
output modulation to the input modulation.The modulus of the
Fourier transform of the optical spread function.
5.2 Two optical parameters have the effect of changing
image position. The first, lateral displacement, is inherent in
This test method is under the jurisdiction of ASTM Committee F07 on
any transparency which is tilted with respect to the line of
Aerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 on
sight. The effect of lateral displacement is constant over
Transparent Enclosures and Materials.
distance, and seldom exceeds a fraction of an inch.The second
Current edition approved April 1, 2008. Published April 2008. Originally
parameter, angular deviation, is usually caused by a wedginess
approved in 1983. Last previous edition approved in 2002 as F801 – 96 (2002).
DOI: 10.1520/F0801-96R08.
or nonparallelism of the transparency surfaces. The effect of
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
angular deviation is related to the tangent of the angle of
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
deviation, thus the magnitude of the image position displace-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ment increases as does the distance between image and
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F801 − 96 (2008)
transparency. The quantification of angular deviation is then 8.3 Adjust the receiver field lens and positions of the CCD
the more critical of the two parameters. arrays so each array is at the focal plane of the lens. Perform
rough adjustment by using the receiver lens to sharply focus
6. Apparatus
the target from the previously adjusted transmitter. Check by
interposing a thick optical flat (plane parallel-sided transparent
6.1 Transmitter, capable of projecting collimated light rays
from a suitable target. The target may be a transparent cross or plate) in the optical path, and tilting the flat with respect to the
optical axis. When correctly adjusted, there will be no move-
an “L” with one arm horizontal and one arm vertical, embed-
ded in an opaque background. The stroke width of the “L” or ment of the transmitter image at the plane of the CCD array. If
the image moves (the readout varies by more than 0.1 mrad),
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 adjust the position of the appropriate CCD array to eliminate
this movement.
transmitter should be firmly affixed to the floor or other
stationary fixture.
8.4 An accuracy test may be made by interposing a standard
6.2 Receiver, firmly affixed to the floor or a stable platform, or highly accurate optical wedge in the light path between
consisting of the following components: transmitterandreceiver.Thedisplayshouldaccuratelyindicate
6.2.1 Displacement Compensation and Imaging Lens—The
the angular deviation imposed by the optical wedge in both the
sensitivity of the instrument is in part determined by the focal vertical or horizontal meridians. An alternative method would
length of the lens. An appropriate focal length may be 10 in.
betotiltthetransmitterorreceiveronanaccuratetilttable.The
(254 mm). tilt, converted to milliradians, should equal that shown on the
6.2.2 Optical Beam Splitter, to separate the incoming light
display. The latter method is usually preferable since it yields
into two orthogonal elements; one for elevation and the other a continuous accuracy check over the entire range of measure-
forazimuth.Thetypeofbeamsplittershouldbechosentokeep
ment.
both optical path lengths equal.
8.5 A check to ensure operation of all diodes may be
6.2.3 Two Linear Charge Coupled Devices (CCD or diode)
performed by illuminating the entire CCD array and noting the
Arrays, each located at the focal plane of the displacement
default reading on the display. (This default reading is also
compensating lens. One array is oriented horizontally (for the
dependent on the specific circuitry used, but should be a
measurement of azimuthal changes), and the other oriented
constant).
vertically (for the measurement of elevation changes). An
NOTE 1—The area of transparency being measured at any one time is
appropriate element spacing of the arrays is 0.001 in. (0.0254
related to the smallest diameter lens being used at the transmitter or
mm). Using this element spacing, and the 10-in. (254-mm)
receiver. The system will average angular deviations throughout a subset
lens, each diode will represent the equivalent of 0.1 milliradian
of this area. Use of lenses of significantly larger or smaller diameters will
(mrad) angular deviation.
affectrepeatabilityofmeasurementfromoneinstrumenttoanother.Useof
6.2.4 Electronic System that will determine the center diode
lenses with small diameters will improve performance on transparencies
with rapidly changing angular deviations, but will reduce available light
of the band of illuminated diodes on each CCD array.
energy at the CCD array, possibly below its threshold. Lens size is further
6.2.5 Electronics System that will convert the number to be
discussed in the annex.
displayed on a digital readout.
8.6 Certain variations may be as a result of the following
6.3 Transmitter and Receiver Lenses should be of achro-
sources of error:
matic construction to reduce the effect of aberrations on the
8.6.1 Transmitter or receiver lens malfocus. Noncollimated
measurement.
light from the transmitter will cause the receiver to measure
6.4 Dioptometer, to verify attainment of collimated light.
some lateral displacement as well as angular deviation.
6.5 For further information on the rationale and develop-
8.6.2 Poor transparency optics (MTF losses) will cause a
ment of the design see the appendixes. (Appendix X1 –
blurred image on CCD arrays. If this blur is asymmetric, some
Appendix X4.)
error will be introduced. If the MTF loss is great enough, the
lightenergywillfallbelowthethresholdoftheCCDarray,and
7. Test Specimen
a no-reading condition will result.
7.1 The part to be tested should be positioned in such a
manner as to approximate its installed configuration. No 9. Procedure
special conditioning other than cleaning is required.
9.1 Mount the transparent part on a fixture that allows
accuratedeterminationoftheelevationandazimuthpositionof
8. Calibration and Standardization
the part.
8.1 Position the transmitter and receiver so that the optical
9.2 Locate and firmly mount the transmitter at an appropri-
axes of both are parallel and approximately colinear. The light
atepositioncorrespondingtotheobservationalpointofinterest
from the transmitter shall pass through the test specimen to fall
(pilot’s eye designed position), or along a line connecting this
on the receiver lens. Depending on the configuration of the test
point with the receiver lens.
specimen, locate the transmitter and receiver approximately 4
ft (305 mm or less) apart.
9.3 Locate and firmly mount the receiver external to the
8.2 Adjust the transmitter lens or target position to provide transparent part and at a distance of 4.9 ft (1.5 m) from the
collimatedlight.Adioptometerissufficientforthisadjustment. transmitter.
F801 − 96 (2008)
9.4 Establish a baseline or zero determination without a Round Robin Test” and is available from ASTM International
transparency in the optical path. Record the number as dis- headquarters. It should also be noted that there are only a few
played on the digital readout under this condition. organizations capable of making these types of measurements
on aircraft transparencies. At the time of the inter-laboratory
9.5 Locate the transparency between the transmitter and
test program there were only 5 measurement devices available
receiver. Take readings at points specified by the using activity
at 3 facilities. Although this is a lower number than that
by rotating the canopy about a critical point such as the pilot’s
recommended by Practice E691, the results provide a reason-
eye position or other position of interest specified by the using
ableindicationoftheexpectedrepeatabilityandreproducibility
activity.Thedifferencebetweenthesereadingsandthebaseline
of the procedure. If more measurement systems become
figures solely represent the angular deviation in milliradians
available in the future, the interlaboratory test may be repeated
through each point.
to obtain an updated estimate of precision and bias.
10. Calculation 12.1.1 There are two primary sources of error with this
procedure: (1) those dealing with the measurement device
10.1 With appropriate selection of receiver lens focal length
itself, and (2) those dealing with the positioning of the part to
and CCD array diode separation, the display readout will be in
be measured. Since this procedure only addresses the measure-
0.1-mrad increments. The sensitivity of the instrument may be
ment device and not positioning equipment this section will be
variedbyalteringeitheroftheseparameters.Assuminga0.001
confined to data relating to the precision of the measurement
in. (0.025 mm) diode spacing as standard, increasing the focal
device itself.
length will improve the sensitivity as follows:
12.1.2 Measurements of azimuth and elevation angular
a 5 arc tan 0.001/f
~ !
deviation of two windscreens by two organizations using
precise positioning equipment resulted in a total of 880 data
where:
points. These 880 points were measured twice to determine
a = sensitivity (minimum measurable angle), mrad and
repeatability. 832 of the 880 points, or 94 %, were within 60.1
f = focal length of receiver lens, in.
milliradian from the first measurement to the second measure-
10.2 Although the separation distance between the projector
ment. Thus the 95 % confidence interval for repeatability for
and receiver is not critical and does not affect the measurement
this test method is 60.1 (it should be noted that the least count
accuracy, it does have an effect on both the light energy at the
of the device described in this test method is 0.1 milliradian so
image plane and the maximum amounts of angular deviation
the confidence interval value has been rounded off to the
thatcanbemeasured.Thelargestdistancefromtheopticalaxis
nearest 0.1 milliradian even though the statistically calculated
at the image plane that does not produce vignetting may be
confidence interval would be slightly more than 0.1 milliradi-
calculated as follows:
ans). The third organization that participated in these tests had
H 5 f 3 d 2 d /2S a less precise manual positioning device that was not capable
~ !
2 2 1
ofrepositioningthewindscreensasaccuratelyastheautomated
where:
systems. Since the objective of the interlaboratory test was to
H = maximum unvignetted ray height at image plane,
assess the measurement device and not the positioning equip-
d = diameter of receiver lens,
ment these data were not included in the determinati
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