ASTM F2156-01
(Test Method)Standard Test Method for Measuring Optical Distortion in Transparent Parts Using Grid Line Slope
Standard Test Method for Measuring Optical Distortion in Transparent Parts Using Grid Line Slope
SCOPE
1.1 When an observer looks through an aerospace transparency, relative optical distortion results, specifically in thick, highly angled, multilayered plastic parts. Distortion occurs in all transparencies but is especially critical to aerospace applications such as combat and commercial aircraft windscreens, canopies, or cabin windows. This is especially true during operations such as takeoff, landing, and aerial refueling. It is critical to be able to quantify optical distortion for procurement activities.
1.2 This test method covers apparatus and procedures that are suitable for measuring the grid line slope (GLS) of transparent parts, including those that are small or large, thin or thick, flat or curved, or already installed. This test method is not recommended for raw material.
1.3 The values stated in SI units shall be regarded as the standard. The values given in parentheses are for information only.
1.4 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:F2156–01
Standard Test Method for
Measuring Optical Distortion in Transparent Parts Using
Grid Line Slope
This standard is issued under the fixed designation F 2156; 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.
1. Scope F 801 Test Method for Measuring Optical Angular Devia-
tion of Transparent Parts
1.1 When an observer looks through an aerospace transpar-
ency, relative optical distortion results, specifically in thick,
3. Terminology
highly angled, multilayered plastic parts. Distortion occurs in
3.1 Definitions of Terms Specific to This Standard:
all transparencies but is especially critical to aerospace appli-
3.1.1 design eye, n—the reference point in aircraft design
cations such as combat and commercial aircraft windscreens,
from which all anthropometrical design considerations are
canopies, or cabin windows. This is especially true during
taken.
operations such as takeoff, landing, and aerial refueling. It is
3.1.2 distortion, n—the rate of change of deviation resulting
critical to be able to quantify optical distortion for procurement
from an irregularity in a transparent part.
activities.
3.1.2.1 Discussion—Distortion shall be expressed as the
1.2 This test method covers apparatus and procedures that
slope of the angle of localized grid line bending, for example,
are suitable for measuring the grid line slope (GLS) of
1 in 5 (see Fig. 1).
transparent parts, including those that are small or large, thin or
3.1.3 grid board, n—an optical evaluation tool used to
thick, flat or curved, or already installed. This test method is
detect the presence of distortion in transparent parts.
not recommended for raw material.
3.1.3.1 Discussion—The grid board is usually, but not
1.3 The values stated in SI units shall be regarded as the
always, a vertical rectangular backboard with horizontal and
standard. The values given in parentheses are for information
vertical intersecting lines with maximum contrast between the
only.
white lines and the black background.
1.4 This standard does not purport to address all of the
3.1.4 grid line slope, n—an optical distortion evaluation
safety concerns, if any, associated with its use. It is the
parameterthatcomparestheslopeofadeviatedgridlinetothat
responsibility of the user of this standard to establish appro-
of a nondeviated grid line.
priate safety and health practices and determine the applica-
3.1.4.1 Discussion—The degree of deviation shall be indi-
bility of regulatory limitations prior to use.
cated by a ratio, for example, 1 in 2, 1 in 8, or 1 in 20 (the
2. Referenced Documents visual optical quality improves as the second number gets
larger.)
2.1 ASTM Standards:
3.1.5 installed angle, n—the transparency orientation as
E 177 Practice for Use of the Terms Precision and Bias in
2 installed in the aircraft, defined by the angle between a
ASTM Test Methods
horizontal line (line of sight) and a plane tangent to the surface
E 691 Practice for Conducting an Interlaboratory Study to
2 of the transparency (see Fig. 2).
Determine the Precision of a Test Method
3.1.6 repeatability limit (rL), n—from Practice E 177,
F 733 Practice for Optical Distortion and Deviation of
27.3.2, “approximately 95 % of individual test results from
Transparent Parts Using the Double-Exposure Method
laboratories similar to those in an ILS can be expected to differ
in absolute value from their average value by less than 1.96s
(about 2s).”
This test method is under the jurisdiction of ASTM Committee F07 on
Aerospace andAircraft , and is the direct responsibility of Subcommittee F07.08 on
3.1.6.1 Discussion—in terms of this test method, approxi-
Transparent Enclosures and Materials.
mately 95 % of all pairs of replications from the same
Current edition approved Nov. 10, 2001. Published March 2002.
2 evaluator and the same photo differ in absolute value by less
Annual Book of ASTM Standards, Vol 14.02.
Annual Book of ASTM Standards, Vol 15.03. than the rL.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F2156–01
FIG. 1 Optical Distortion Represented By Tangent
FIG. 2 Schematic Diagrams of GLS Photographic Recording Distances
3.1.7 reproducibility limit (RL), n—from Practice E 177, differ in absolute value by less than 1.960=2s (about 2.0=2s)
27.3.3, “approximately 95 % of all pairs of test results from = 2.77s (or about 2.8s). This index is also known as the 95 %
laboratories similar to those in the study can be expected to limit on the difference between two test results.”
F2156–01
3.1.7.1 Discussion—in terms of this test method, approxi- 6.1.4 The walls, ceiling, and floor shall have low reflec-
mately 95 % of all pairs of replications from different evalua- tance. A flat black paint or coating is preferred though not
tors and the same photo differ in absolute value by less than the required.
RL. 6.2 Grid Board—The grid board shall provide a defined
pattern against which the transparent part is examined. Grid
4. Summary of Test Method
boards shall be one of the following types:
6.2.1 Type 1—The grid board shall be composed of white
4.1 The transparent part shall be mounted, preferably at the
strings held taut, each spaced at a specific interval, with the
installed angle, at a specified distance from a grid board test
strings stretched vertically and horizontally. The grid board
pattern. A photographic camera shall be placed so as to record
frame and background shall have a flat black finish to reduce
the grid pattern as viewed through the part from the design eye
light reflection. A bank of fluorescent lights at each side or
(or other specified) viewing position. If the viewing position is
evenly distributed natural sunlight conditions provide illumi-
not defined, the values in Table 1 may be used as photographic
nation of the strings.
test geometry.The image is then analyzed to assess the level of
optical distortion as measured by grid line slope. 6.2.2 Type 2—The grid board shall be a transparent sheet
having an opaque, flat black outer surface except for the grid
4.2 Distortion shall be recorded using either a single-
lines. The grid lines remain transparent, and when backlit with
exposure photograph or a double-exposure photograph. The
fluorescent or incandescent lights, provide a bright grid pattern
photographed grid shall then be measured using either a
against a black background with excellent contrast character-
drafting machine procedure or a manual procedure. Each
istics.
procedure has its own level of precision.
6.2.3 Type 3—The grid board shall be a rigid sheet of
5. Significance and Use
material that has a grid pattern printed on the front surface.
Details of the grid lines, pattern, and lighting shall be as
5.1 Transparent parts, such as aircraft windshields, cano-
specified by the procuring activity.
pies, cabin windows, and visors, shall be measured for com-
6.2.4 The grid board shall have a width and height large
pliance with optical distortion specifications using this test
enough so that the area of the part to be imaged is superim-
method. This test method is suitable for assessing optical
posed within the perimeter of the grid board. Details of the grid
distortion of transparent parts as it relates to the visual
square size shall be as specified by the procuring activity. The
perception of distortion. It is not suitable for assessing distor-
recommended grid line spacing shall be not less than 1.27 cm
tion as it relates to pure angular deviation of light as it passes
( ⁄2 in.) or more than 2.54 cm (1 in.).
through the part. Either Test Method F 801 or Practice F 733 is
6.3 Camera—The camera shall be used to photograph
appropriate and shall be used for this latter application. This
distortion for the evaluation of grid line slope. For highest
test method is not recommended for raw material.
resolution, it is recommended that a large format camera be
6. Apparatus used, although a 35-mm camera is also acceptable. Black-and-
white film shall be 400 ASA (or slower). Use of a digital
6.1 Test Room—The test room shall be large enough to
camera is permitted if it has sufficiently high resolution (that is,
locate the required testing equipment properly.
with no visible pixilation in the printed image). When using a
6.1.1 Setup A requires a room approximately 12 m (40 ft)
double-exposure recording technique (Fig. 3), the film-based
long.
camera shall have a double-exposure capability. Separate
6.1.2 Setup B requires a room approximately 7 m (23 ft)
digital images are superimposed using a computer-based photo
long.
editor. The camera lens shall have very low barrel distortion
6.1.3 Setup C: other distances shall be used if desired. GLS
characteristics. The camera shall be firmly mounted at design
results will vary with different distances, which means that
eye (or other specified viewing position) to prevent any
measurements of different parts taken at different distances
movement during the photographic exposure.
cannot be compared.
6.4 Drafting Machine Procedure—The drafting machine
shall consist of a vertical and horizontal scale attached to a
TABLE 1 GLS Photographic Recording Distances
rotating head that displays the angular position of the horizon-
Setup A
tal or vertical scale in degrees with a resolution of at least 1 arc
A 1
Camera-to-grid-board distance 1000 cm (32 ft 10 in.)
minute or ⁄100 of a degree. This common, commercially
Camera-to-part distance 550 cm (18 ft 1 in.)
available apparatus, which is mounted to a drafting table, shall
Part-to-grid-board distance 450 cm (14 ft 9 in.)
be used for the evaluation of grid line slope.
Setup B
6.5 Manual Procedure—Measurements shall be made using
Camera-to-grid-board distance 450 cm (14 ft 9 in.)
high-quality drafting instruments (for example, metal scales,
Camera-to-part distance 150 cm (4 ft 11 in.)
right triangle).
Part-to-grid-board distance 300 cm (9 ft 10 in.)
Setup C
7. Test Specimen
Camera-to-grid-board distance User defined
7.1 The transparency to be measured shall be cleaned, using
B
Camera-to-part distance User defined
the manufacturer or procuring agency approved procedure, to
Part-to-grid-board distance User defined
A remove any foreign material that might cause localized optical
All measurements shall be 63cmor 63 %, whichever is smaller.
B
Itisrecommendedthatthecamera-to-partdistancebethedesigneyedistance. distortion.Unlessspecifiedbytheprocuringactivity,nospecial
F2156–01
recommended that the part be mounted at the installed angle.
Record the mounted angle and report it with the results.
9.1.3 The camera shall be mounted at the design eye
position (or other position as specified by the procuring
activity). The optical axis of the camera shall be perpendicular
to the grid board surface and shall be aimed at the target panel.
See 6.3 for photographic recording requirements.
9.1.4 Place the grid board at a given distance (see Table 1)
from the camera or as specified by the procurement agency,
and ensure that the grid board pattern is in good focus at the
focal plane of the camera. It is highly desirable (but not
required) for part of the grid board target to be directly visible
from the camera position without passing through the trans-
parency. If this is possible, this undistorted section of the grid
board serves as an alignment reference when determining the
GLS of the transparency.
9.1.5 Photograph optical distortion through the part using
one of two recording techniques. Both of these techniques
record distortion of the grid. The distortion from these two
photographic techniques shall be analyzed using either the
drafting machine or manual measurement procedures.
9.1.5.1 Single-exposure photograph—Prepare a single-
exposure photograph of the grid board viewed through the
transparentpart.Thecamerashallbefocusedonthegridboard.
FIG. 3 GLS Double-Exposure Recording Technique
9.1.5.2 Double-exposure photograph—Photograph the grid
board through the transparent part. Then, without allowing any
movementofthecameraoradvancingthefilm,removethepart
conditioning, other than cleaning, shall be required and the part
and make the second exposure of the grid board alone.
shall be at ambient temperature.
9.1.6 Develop the film and produce 8- by 10-in. matte finish
prints (minimum size). The matte finish will reduce reflection
8. Calibration and Standardization
problems during measurement.
8.1 Test Procedure Geometry—Distance measurements
9.2 Grid Line Slope Measurement Procedures:
shall be made using a high-quality tape measure.
9.2.1 Drafting Machine Procedure:
8.2 Drafting Machine Procedure—Measurements shall be
9.2.1.1 Tape the photograph to the drafting board.
made using high-quality drafting instruments. The drafting
9.2.1.2 Align the horizontal scale of the drafting machine
machineshallbeaccuratetowithinitsspecifiedmanufacturer’s
with a horizontal line of the grid board in the photo that is in
tolerances.
the directly viewed section of the photo outside of the
8.3 Manual Procedure—Measurements shall be made using
windscreen. If there is no such area, align the scale with the
high-quality drafting instruments (for example, metal scales,
most undistorted horizontal grid board line within the picture,
right triangle).
to serve as a reference.
9.2.1.3 Zero the drafting machine.
9. Procedure
9.2.1.4 Systematically scan the photograph horizontally and
9.1 Photographic Recording Techniques:
vertically to find the most distorted area (the line with the
9.1.1 The procuring activity specifies whether Setup A, B,
greatest slope).
orC(otherspecifieddistances)shallbeusedtomeasureoptical
9.2.1.5 Place the horizontal straight edge tangent to the
distortion. Table 1 contains the setup measurement distances.
horizontal line displaying the maximum slope (bending from
Fig. 2 illustrates the setup geometries.When the part is flat and
horizontal) within the area of the transparency to be measured.
mounted (nearly) vertically (for example, a passenger win-
9.2.1.6 Record the angle indicated on the drafting machine
dow), SetupAis a more stringent test than Setup B. Use Setup
angle readout and convert the value to degrees if it is not in
C when the part is curved (for example, an aircraft canopy). It
degrees already.
is recommended that measurements then be performed at the
9.2.1.7 For vertical GLS, align the vertical scale of the
installed position and the camera be placed at the design eye
drafting machine with the
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