Back-lighting polymeric film with polarized light from a polarizing plate and viewing it through a second plate cross-polarized to the back-lighting provides visual contrast between the flaw and the transparent film

Von Volkli, A. D.

Wiedekamp, K. E.

English

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NASA TECH BRIEF 
.,
April 1970	 Brief 70-10027 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
ii 
A Method for the Visual Detection of Holes in Thin Polymeric Films 
Hole 
___-	 I I 
Light 
Source V 
Polarizing Plate	 Test Film	 Cross- Polarizing Plate 
Schematic of Test Apparatus 
The use of thin polymeric films for the containment 
of liquids and gases has necessitated the development 
of inspection methods for detecting, locating, arid 
evaluating small holes and tears in the films. Such 
flaws can be detected by the leakage of a pressurizing 
gas applied to an edge-sealed film structure mounted 
in an assembly or a test fixture. This method, how-
ever, cannot be used on disassembled structures or un-
supported films. Consequently, visual inspection must 
be used for determining the size and nature of the 
flaws. The latter factor is particularly important as a 
basis for ascertaining probable causes of failure when 
testing films of different composition or configuration. 
Since thin films of the commonly used polymers 
(i.e., polyesters, polyimides, polyethylenes, fluoro-
carbons) are highly transparent to visible light, the 
detection of a hole with the unaided eye is very difficult 
and time-consuming under standard lighting condi-
tions. There is no contrast between the light which has
passed through or been reflected from the film and the 
light which has passed through the hole. A method 
was therefore developed to provide the required visual 
contrast between the hole and the transparent film. 
The section of film being inspected is back-lighted 
with polarized light from a polarizing plate and viewed 
through a second plate cross-polarized to the back-
lighting. The dispersion, refraction, and stress-optic 
rotation of the polarized light as it passes through 
the film renders intact film areas visible when viewed 
through the ' cross-polarized plate. The polarized light 
which passes through a hole in the film appears as a 
black area when viewed through the cross-polarized 
plate. (Polarized sunglasses are an effective substi-
tute for the second polarizing plate when inspecting 
large areas of film.) 
The major optical effects which render different 
films visible vary with the base polymers and mole-
cular orientations of the films. Stretch-oriented films 
(continued overicaf) 
Now
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such as polyester and polyimide films show a high 
degree of birefringence effects and appear in rainbow 
hues, particularly when the films are wrinkled. 
Thermoplastic films such as polyethylene, polypropy-
lene, and FEP fluorocarbon are visible as a uniform 
gray color. Holes always show as black areas in the 
colored background of the film. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000Brookpark Road 
Cleveland, Ohio 44135 
Reference: B70-10027
Patent status: 
No patent action is contemplated by NASA. 
Source: K. E. Wiedekamp and A. D. Von Volkli of 
The Boeing Company 
under contract to 
Lewis Research Center 
(LEW- 10876)
. 
Brief 70-10027	 Category 04


A method for the visual detection of holes in thin polymeric films

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