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Date: 5/21/73 Number: 9
Related Program Areas:
Sterile Packaging - Foods, Drugs, Devices, Hardened Lenses

The polariscope is an optical inspection device used to detect internal stresses in glass and other transparent materials such as plastics, synthetic resins, etc. A polariscope is composed chiefly of a light source and two crossed polarized lenses such as Polaroid {{Registered Trademark}}. The polariscope light source is mounted beneath one lens, and is powered by either self-contained batteries or an external power source. Material to be examined is placed between the two polariscope lenses and viewed through the lens opposite the light source lens. Polariscopes are manufactured in configurations that very from standard portable units, ordinarily carried in stock, to instruments custom made for specific applications. The Bethlehem model P5 shown in Figure 1 is manufactured by Bethlehem Apparatus Company, Inc., Hellertown, Pennsylvania, and is a good representative of a commercially available portable polariscope. The model P5 consists chiefly of two crossed Polaroid {{Registered Trademark}} lenses, is lightweight (1 3/4 pounds), fairly inexpensive ($118 each), and requires no operating supplies. The light source is a 15 watt, 115 volt lamp, and is mounted beneath the lower Polaroid {{Registered Trademark}} lens in a reflective enclosure. The adjustable frame provides a five inch field between the Polaroid {{Registered Trademark}} lenses for material inspection. The model P5 is constructed of aluminum and steel with a baked hammertone gray finish.

Figure 1

image of the Bethlehem model P5

(image size 27KB)

Some materials, such as glass and plastics, are not normally doubly refractive, but become so when they are internally stressed by external forces or by manufacturing processes. These materials exhibiting internal stresses are subject to failure by bursting or deformation. Improperly annealed glass may be internally stressed to an extent that small external forces, such as applied when glass lenses are mounted in eyeglass frames, would cause the lens to shatter. Heat and sonic sealing of transparent packaging, such as pharmaceutical and sterile medical product plastic packaging, produces stressed areas in the seals. When held between the lenses of the polariscope, these stress areas can be seen as patches or bands of color. A continuous color band across the seal indicates a probable good seal. Discontinuities in the color band indicates a probable incomplete seal through which contaminants can enter the package. When a material is observed through the polariscope, the presence or lack of stress areas should not be construed to be conclusively indicative of improper or proper heat treating or sealing. The polariscope has not been established as a conclusive analytical test instrument in these areas.

Ideas for utilizing test instruments may occur more easily if the basic elements of the theory of operation is understood. Therefore, a brief explanation of polariscope theory follows. Figure 2 has been drawn to help clarify the narrative.

Figure 2

draw of brief explanation of polariscope theory to help clarify the narrative.

(image size 25KB)

Any light source emits a very large number of photons of light oriented at random. These photons are a light wave mixture linearly polarized in all possible transverse directions. To utilize the polariscope light source, this mixture of light waves must be "sorted out" or linearly polarized in one particular direction. When light is incident on a polarizer such as Polaroid {{Registered Trademark}}, linear light only is transmitted, i.e., only the component parallel to the transmission direction of the polarizer will be transmitted. When two polarizers are mounted in the crossed position, i.e., with their transmission directions at right angles to each other, no light is transmitted through the combination. If a material that refracts light is inserted between the crossed polarizers, the linearized light passing through the first polarizer will have its direction altered by the refracting material and some light will pass through the second polarizer to the viewer. The areas of the doubly refracting material that refracting light from the first polarizer will be seen as patches of color when viewed through the second polarizer.

The Bethlehem model P5 polariscope is being utilized by some districts as an experimental inspection tool. There are presently six of the Bethlehem model P5 polariscopes (with photos illustrating typical stresses in plastics) in District use, on loan from DEIO/Investigations Branch. If any District discovers a new use for the polariscope, information should be sent to DEIO/Investigations Branch for transmittal to all Districts.

Figure 1 - Bethelehem Model P5 Polariscopre

image of the Bethlehem model P5

Figure 2 (Below)

draw of brief explanation of polariscope theory to help clarify the narrative.

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