ASTM F801-96(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:F801–96 (Reapproved 2002)
Standard Test Method for
Measuring Optical Angular Deviation of Transparent Parts
This standard is issued under the fixed designation F 801; 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 (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. Scope a field lens, and linear diode arrays as the part is held in its
installed angle. The positions of two images of a collimated
1.1 Thistestmethodcoversmeasuringtheangulardeviation
light source are recorded using two linear diode arrays. One
of a light ray imposed by transparent parts such as aircraft
array records azimuth or horizontal position while the other
windscreens and canopies. The results are uncontaminated by
records elevation or vertical position. These arrays are at the
the effects of lateral displacement, and the procedure may be
posterior focal plane of a field lens. The positions are again
performed in a relatively short optical path length. This is not
recorded after the interposition of a transparent part in the
intended as a referee standard. It is one convenient method for
opticalpath.Thedifferenceinimagepositionisdirectlyrelated
measuring angular deviations through transparent windows.
to the angular deviation imposed by the transparent part. The
1.2 This standard does not purport to address all of the
effects of lateral displacement are removed by the field lens.
safety concerns, if any, associated with its use. It is the
Sensitivity of measurement may be controlled by choosing
responsibility of the user of this standard to establish appro-
appropriatefocallengthfieldlensesandspacingofelementson
priate safety and health practices and determine the applica-
the diode arrays.
bility of regulatory limitations prior to use.
5. Significance and Use
2. Referenced Documents
5.1 One of the measures of optical quality of a transparent
2.1 ASTM Standards:
part is its angular deviation. Excessive angular deviation, or
E 691 Practice for Conducting an Interlaboratory Study to
variations in angular deviation throughout the part, result in
Determine the Precision of a Test Method
visible distortion of scenes viewed through the part. Angular
3. Terminology
deviation, its detection, and quantification are of extreme
importance in the area of certain aircraft transparency applica-
3.1 Definitions:
tions,thatis,aircraftequippedwithHeads-upDisplays(HUD).
3.1.1 angular deviation—the departure of a light ray from
HUDs may require stringent control over the optics of the
its original path as it passes through a transparent material.The
portion of the transparency (windscreen or canopy) which lies
change in angle of such a light ray. The displacement of an
between the HUD combining glass and the external environ-
image due to the change in direction of the light ray.
ment. Military aircraft equipped with HUDs or similar devices
3.1.2 lateral (or linear) displacement—the shift or move-
require precise knowledge of the effects of the windscreen or
ment of a light ray from its original path as it passes through a
canopy on image position in order to maintain weapons aiming
transparent material, while maintaining parallelism between
accuracy.
the original and final paths.The change in location of an image
5.2 Two optical parameters have the effect of changing
due to this change in path.
image position. The first, lateral displacement, is inherent in
3.1.3 modulation transfer function (MTF)—the ratio of
any transparency which is tilted with respect to the line of
output modulation to the input modulation.The modulus of the
sight. The effect of lateral displacement is constant over
Fourier transform of the optical spread function.
distance, and seldom exceeds a fraction of an inch. The second
4. Summary of Test Method
parameter, angular deviation, is usually caused by a wedginess
or nonparallelism of the transparency surfaces. The effect of
4.1 This test method outlines how measurements can be
angular deviation is related to the tangent of the angle of
made by an optoelectronic system employing collimated light,
deviation, thus the magnitude of the image position displace-
ment increases as does the distance between image and
This test method is under the jurisdiction of ASTM Committee F07 on
transparency. The quantification of angular deviation is then
Aerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 on
the more critical of the two parameters.
Transparent Enclosures and Materials.
Current edition approved Nov. 10, 1996. Published January 1997. Originally
e1
published as F 801 – 83. Last previous edition F 801 – 83 (1989) .
Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F801
6. Apparatus interposing a thick optical flat (plane parallel-sided transparent
plate) in the optical path, and tilting the flat with respect to the
6.1 Transmitter, capable of projecting collimated light rays
optical axis. When correctly adjusted, there will be no move-
from a suitable target. The target may be a transparent cross or
ment of the transmitter image at the plane of the CCD array. If
an “L” with one arm horizontal and one arm vertical, embed-
the image moves (the readout varies by more than 0.1 mrad),
ded in an opaque background. The stroke width of the “L” or
adjust the position of the appropriate CCD array to eliminate
cross shall be uniform. Choice of an “L” or a cross is optional,
this movement.
since only one half of the cross target is used at any time. The
8.4 An accuracy test may be made by interposing a standard
transmitter should be firmly affixed to the floor or other
or highly accurate optical wedge in the light path between
stationary fixture.
transmitterandreceiver.Thedisplayshouldaccuratelyindicate
6.2 Receiver, firmly affixed to the floor or a stable platform,
the angular deviation imposed by the optical wedge in both the
consisting of the following components:
vertical or horizontal meridians. An alternative method would
6.2.1 Displacement Compensation and Imaging Lens—The
betotiltthetransmitterorreceiveronanaccuratetilttable.The
sensitivity of the instrument is in part determined by the focal
tilt, converted to milliradians, should equal that shown on the
length of the lens. An appropriate focal length may be 10 in.
display. The latter method is usually preferable since it yields
(254 mm).
a continuous accuracy check over the entire range of measure-
6.2.2 Optical Beam Splitter, to separate the incoming light
ment.
into two orthogonal elements; one for elevation and the other
8.5 A check to ensure operation of all diodes may be
forazimuth.Thetypeofbeamsplittershouldbechosentokeep
performed by illuminating the entire CCD array and noting the
both optical path lengths equal.
default reading on the display. (This default reading is also
6.2.3 Two Linear Charge Coupled Devices (CCD or diode)
dependent on the specific circuitry used, but should be a
Arrays, each located at the focal plane of the displacement
constant).
compensating lens. One array is oriented horizontally (for the
measurement of azimuthal changes), and the other oriented
NOTE 1—The area of transparency being measured at any one time is
vertically (for the measurement of elevation changes). An
related to the smallest diameter lens being used at the transmitter or
receiver. The system will average angular deviations throughout a subset
appropriate element spacing of the arrays is 0.001 in. (0.0254
of this area. Use of lenses of significantly larger or smaller diameters will
mm). Using this element spacing, and the 10-in. (254-mm)
affectrepeatabilityofmeasurementfromoneinstrumenttoanother.Useof
lens, each diode will represent the equivalent of 0.1 milliradian
lenses with small diameters will improve performance on transparencies
(mrad) angular deviation.
with rapidly changing angular deviations, but will reduce available light
6.2.4 Electronic System thatwilldeterminethecenterdiode
energy at the CCD array, possibly below its threshold. Lens size is further
of the band of illuminated diodes on each CCD array.
discussed in the annex.
6.2.5 Electronics System that will convert the number to be
8.6 Certain variations may be as a result of the following
displayed on a digital readout.
sources of error:
6.3 Transmitter and Receiver Lenses should be of achro-
8.6.1 Transmitter or receiver lens malfocus. Noncollimated
matic construction to reduce the effect of aberrations on the
light from the transmitter will cause the receiver to measure
measurement.
some lateral displacement as well as angular deviation.
6.4 Dioptometer, to verify attainment of collimated light.
8.6.2 Poor transparency optics (MTF losses) will cause a
6.5 For further information on the rationale and develop-
blurred image on CCD arrays. If this blur is asymmetric, some
ment of the design see the appendixes. (Appendix X1-
error will be introduced. If the MTF loss is great enough, the
Appendix X4.)
lightenergywillfallbelowthethresholdoftheCCDarray,and
a no-reading condition will result.
7. Test Specimen
7.1 The part to be tested should be positioned in such a 9. Procedure
manner as to approximate its installed configuration. No
9.1 Mount the transparent part on a fixture that allows
special conditioning other than cleaning is required.
accuratedeterminationoftheelevationandazimuthpositionof
the part.
8. Calibration and Standardization
9.2 Locate and firmly mount the transmitter at an appropri-
8.1 Position the transmitter and receiver so that the optical atepositioncorrespondingtotheobservationalpointofinterest
axes of both are parallel and approximately colinear. The light (pilot’s eye designed position), or along a line connecting this
from the transmitter shall pass through the test specimen to fall point with the receiver lens.
on the receiver lens. Depending on the configuration of the test 9.3 Locate and firmly mount the receiver external to the
specimen, locate the transmitter and receiver approximately 4 transparent part and at a distance of 4.9 ft (1.5 m) from the
ft (305 mm or less) apart. transmitter.
8.2 Adjust the transmitter lens or target position to provide 9.4 Establish a baseline or zero determination without a
collimatedlight.Adioptometerissufficientforthisadjustment. transparency in the optical path. Record the number as dis-
8.3 Adjust the receiver field lens and positions of the CCD played on the digital readout under this condition.
arrays so each array is at the focal plane of the lens. Perform 9.5 Locate the transparency between the transmitter and
rough adjustment by using the receiver lens to sharply focus receiver. Take readings at points specified by the using activity
the target from the previously adjusted transmitter. Check by by rotating the canopy about a critical point such as the pilot’s
F801
eye position or other position of interest specified by the using organizations capable of making these types of measurements
activity.Thedifferencebetweenthesereadingsandthebaseline on aircraft transparencies. At the time of the inter-laboratory
figures solely represent the angular deviation in milliradians
test program there were only 5 measurement devices available
through each point.
at 3 facilities. Although this is a lower number than that
recommended by Practice E 691, the results provide a reason-
10. Calculation
ableindicationoftheexpectedrepeatabilityandreproducibility
10.1 With appropriate selection of receiver lens focal length
of the procedure. If more measurement systems become
and CCD array diode separation, the display readout will be in
available in the future, the interlaboratory test may be repeated
0.1-mrad increments. The sensitivity of the instrument may be
to obtain an updated estimate of precision and bias.
varied by altering either of these parameters.Assuming a 0.001
12.1.1 There are two primary sources of error with this
in. (0.025 mm) diode spacing as standard, increasing the focal
procedure: (1) those dealing with the measurement device
length will improve the sensitivity as follows:
itself, and (2) those dealing with the positioning of the part to
a 5 arc tan ~0.001/f!
be measured. Since this procedure only addresses the measure-
ment device and not positioning equipment this section will be
confined to data relating to the precision of the measurement
where:
a = sensitivity (minimum measurable angle), mrad and
device itself.
f = focal length of receiver lens, in.
12.1.2 Measurements of azimuth and elevation angular
10.2 Although the separation distance between the projector
deviation of two windscreens by two organizations using
and receiver is not critical and does not affect the measurement
precise positioning equipment resulted in a total of 880 data
accuracy, it does have an effect on both the light energy at the
points. These 880 points were measured twice to determine
image plane and the maximum amounts of angular deviation
repeatability. 832 of the 880 points, or 94 %, were within 60.1
thatcanbemeasured.Thelargestdistancefromtheopticalaxis
milliradian from the first measurement to the second measure-
at the image plane that does not produce vignetting may be
ment. Thus the 95 % confidence interval for repeatability for
calculated as follows:
this test method is 60.1 (it should be noted that the least count
H 5 f 3 ~d 2 d !/2S
2 2 1
of the device described in this test method is 0.1 milliradian so
the confidence interval value has been rounded off to the
where: nearest 0.1 milliradian even though the statistically calculated
H = maximum unvignetted ray height at image plane,
confidence interval would be slightly more than 0.1 milliradi-
d = diameter of receiver lens,
ans). The third organization that participated in these tests had
d = diameter of transmitter lens,
1 a less precise manual positioning device that was not capable
S = separation between transmitter and receiver, and
ofrepositioningthewindscreensasaccuratelyastheautomated
f = focal length of receiver lens
systems. Since the objective of the interlaboratory test was to
10.3 A typical linear CCD array containing 512 elements,
assess the measurement device and not the positioning equip-
eachwitha0.001in.(0.025mm)spacing,hasanactivesurface
ment these data were not included in the determination of
12.5 mm long. The maximum angular deviation that can be
repeatability.
detected by such an array may be calculated as follows.
...