ASTM F801-96

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: F 801 – 96
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 installed angle. The positions of two images of a collimated
light source are recorded using two linear diode arrays. One
1.1 This test method covers measuring the angular deviation
array records azimuth or horizontal position while the other
of a light ray imposed by transparent parts such as aircraft
records elevation or vertical position. These arrays are at the
windscreens and canopies. The results are uncontaminated by
posterior focal plane of a field lens. The positions are again
the effects of lateral displacement, and the procedure may be
recorded after the interposition of a transparent part in the
performed in a relatively short optical path length. This is not
optical path. The difference in image position is directly related
intended as a referee standard. It is one convenient method for
to the angular deviation imposed by the transparent part. The
measuring angular deviations through transparent windows.
effects of lateral displacement are removed by the field lens.
1.2 This standard does not purport to address all of the
Sensitivity of measurement may be controlled by choosing
safety concerns, if any, associated with its use. It is the
appropriate focal length field lenses and spacing of elements on
responsibility of the user of this standard to establish appro-
the diode arrays.
priate safety and health practices and determine the applica-
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
part is its angular deviation. Excessive angular deviation, or
2.1 ASTM Standards:
variations in angular deviation throughout the part, result in
E 691 Practice for Conducting an Interlaboratory Study to
visible distortion of scenes viewed through the part. Angular
Determine the Precision of a Test Method
deviation, its detection, and quantification are of extreme
3. Terminology
importance in the area of certain aircraft transparency applica-
tions, that is, aircraft equipped with Heads-up Displays (HUD).
3.1 Definitions:
HUDs may require stringent control over the optics of the
3.1.1 angular deviation—the departure of a light ray from
portion of the transparency (windscreen or canopy) which lies
its original path as it passes through a transparent material. The
between the HUD combining glass and the external environ-
change in angle of such a light ray. The displacement of an
ment. Military aircraft equipped with HUDs or similar devices
image due to the change in direction of the light ray.
require precise knowledge of the effects of the windscreen or
3.1.2 lateral (or linear) displacement—the shift or move-
canopy on image position in order to maintain weapons aiming
ment of a light ray from its original path as it passes through a
accuracy.
transparent material, while maintaining parallelism between
5.2 Two optical parameters have the effect of changing
the original and final paths. The change in location of an image
image position. The first, lateral displacement, is inherent in
due to this change in path.
any transparency which is tilted with respect to the line of
3.1.3 modulation transfer function (MTF)—the ratio of
sight. The effect of lateral displacement is constant over
output modulation to the input modulation. The modulus of the
distance, and seldom exceeds a fraction of an inch. The second
Fourier transform of the optical spread function.
parameter, angular deviation, is usually caused by a wedginess
4. Summary of Test Method
or nonparallelism of the transparency surfaces. The effect of
angular deviation is related to the tangent of the angle of
4.1 This test method outlines how measurements can be
deviation, thus the magnitude of the image position displace-
made by an optoelectronic system employing collimated light,
ment increases as does the distance between image and
a field lens, and linear diode arrays as the part is held in its
transparency. The quantification of angular deviation is then
the more critical of the two parameters.
This test method is under the jurisdiction of ASTM Committee F-7 on
Aerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 on
6. Apparatus
Transparent Enclosures and Materials.
Current edition approved Nov. 10, 1996. Published January 1997. Originally
6.1 Transmitter, capable of projecting collimated light rays
e1
published as F 801 – 83. Last previous edition F 801 – 83 (1989) .
from a suitable target. The target may be a transparent cross or
Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
F 801
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. transmitter and receiver. The display should accurately indicate
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 be to tilt the transmitter or receiver on an accurate tilt table. 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
for azimuth. The type of beam splitter should be chosen to keep 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
NOTE 1—The area of transparency being measured at any one time is
measurement of azimuthal changes), and the other oriented
related to the smallest diameter lens being used at the transmitter or
vertically (for the measurement of elevation changes). An
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
affect repeatability of measurement from one instrument to another. Use of
mm). Using this element spacing, and the 10-in. (254-mm)
lenses with small diameters will improve performance on transparencies
lens, each diode will represent the equivalent of 0.1 milliradian
with rapidly changing angular deviations, but will reduce available light
(mrad) angular deviation.
energy at the CCD array, possibly below its threshold. Lens size is further
6.2.4 Electronic System that will determine the center diode
discussed in the annex.
of the band of illuminated diodes on each CCD array.
8.6 Certain variations may be as a result of the following
6.2.5 Electronics System that will convert the number to be
sources of error:
displayed on a digital readout.
8.6.1 Transmitter or receiver lens malfocus. Noncollimated
6.3 Transmitter and Receiver Lenses should be of achro-
light from the transmitter will cause the receiver to measure
matic construction to reduce the effect of aberrations on the
some lateral displacement as well as angular deviation.
measurement.
8.6.2 Poor transparency optics (MTF losses) will cause a
6.4 Dioptometer, to verify attainment of collimated light.
blurred image on CCD arrays. If this blur is asymmetric, some
6.5 For further information on the rationale and develop-
error will be introduced. If the MTF loss is great enough, the
ment of the design see the appendixes.Appendix X1-Appendix
light energy will fall below the threshold of the CCD array, and
X4
a no-reading condition will result.
7. Test Specimen
9. Procedure
7.1 The part to be tested should be positioned in such a
9.1 Mount the transparent part on a fixture that allows
manner as to approximate its installed configuration. No
accurate determination of the elevation and azimuth position of
special conditioning other than cleaning is required.
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 ate position corresponding to the observational point of interest
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
transmitter.
ft (305 mm or less) apart.
8.2 Adjust the transmitter lens or target position to provide 9.4 Establish a baseline or zero determination without a
transparency in the optical path. Record the number as dis-
collimated light. A dioptometer is sufficient for this adjustment.
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
interposing a thick optical flat (plane parallel-sided transparent eye position or other position of interest specified by the using
plate) in the optical path, and tilting the flat with respect to the activity. The difference between these readings and the baseline
optical axis. When correctly adjusted, there will be no move- figures solely represent the angular deviation in milliradians
ment of the transmitter image at the plane of the CCD array. If through each point.
F 801
10. Calculation the interlaboratory test may be repeated to obtain an updated
estimate of precision and bias.
10.1 With appropriate selection of receiver lens focal length
and CCD array diode separation, the display readout will be in 12.1.1 There are two primary sources of error with this
procedure: 1) those dealing with the measurement device itself,
0.1-mrad increments. The sensitivity of the instrument may be
varied by altering either of these parameters. Assuming a 0.001 and 2) those dealing with the positioning of the part to be
in. (0.025 mm) diode spacing as standard, increasing the focal measured. Since this procedure only addresses the measure-
length will improve the sensitivity as follows: ment device and not positioning equipment this section will be
confined to data relating to the precision of the measurement
a 5 arc tan ~0.001/f!
device itself.
where:
12.1.2 Measurements of azimuth and elevation angular
a 5 sensitivity (minimum measurable angle), mrad and
deviation of two windscreens by two organizations using
f 5 focal length of receiver lens, in.
precise positioning equipment resulted in a total of 880 data
10.2 Although the separation distance between the projector
points. These 880 points were measured twice to determine
and receiver is not critical and does not affect the measurement
repeatability. 832 of the 880 points, or 94 percent, were within
accuracy, it does have an effect on both the light energy at the
6 0.1 milliradian from the first measurement to the second
image plane and the maximum amounts of angular deviation
measurement. Thus the 95 % confidence interval for repeat-
that can be measured. The largest distance from the optical axis
ability for this Test Method is 6 0.1 (it should be noted that the
at the image plane that does not produce vignetting may be
least count of the device described in this test method is 0.1
calculated as follows:
milliradian so the confidence interval value has been rounded
H 5 f 3 ~d 2 d !/2S
2 2 1
off to the nearest 0.1 milliradian even though the statistically
calculated confidence interval would be slightly more than 0.1
where:
milliradians). The third organization that participated in these
H 5 maximum unvignetted ray height at image plane,
tests had a less precise manual positioning device that was not
d 5 diameter of receiver lens,
capable of repositioning the windscreens as accurately as the
d 5 diameter of transmitter lens,
S 5 separation between transmitter and receiver, and automated systems. Since the objective of the interlaboratory
f 5 focal length of receiver lens test was to assess the measurement device and not the
10.3 A typical linear CCD array containing 512 elements,
positioning equipment these data were not included in the
each with a 0.001 in. (0.025 mm) spacing, has an active surface determination of repeatability.
12.5 mm long. The maximum angular deviation that can be
12.1.3 A calibrated optical wedge with an angular deviation
detected by such an array may be calculated as follows.
of 5.07 milliradians was used to determine overall precision
M 5 2 3 arctan 12.5/f ! and bias of this test procedure. A total of 5 devices located in
~
3 laboratories were used in this evaluation. The wedge was
where:
positioned in each of 4 orientations (up, down,
...