Identification of any kind of defect inside the material is very crucial for the engineer, whether in manufacturing, quality assurance, or material science. Some materials may seem to be perfectly good at first sight, but they might have serious problems in terms of internal stresses.

The stress generated due to improper cooling or deformation may serve as ticking bombs in the material, which makes them susceptible to failure even under slight loading.

This is where the polariscope comes into play, a beautifully designed optical tool that relies on the principles of polarized light to reveal stresses and structures within materials that cannot be seen with the naked eye.

What is a Polariscope?

The polariscope is an optical device designed for identifying stress, strain differences, and birefringence within opaque or translucent specimens. Whereas some techniques rely on damaging the test material by breaking it down to measure its strengths, the polariscope works without disturbing the specimen at all. Using the physical principles of light polarization, it converts stresses in the material into beautiful colors, making inspections both qualitative and quantitative.

Definition of Birefringence: Double refraction can be referred to as birefringence, which is the optical phenomenon whereby there is a refractive index for a substance that is dependent on the orientation and polarization of the light.

Glass and unstressed plastics have identical optical characteristics since they are isotropic; however, stress results in anisotropy, which causes the splitting of light.

How Does a Polariscope Work?

The essential principle of operation in a polariscope is based on the effect of polarized light on a stressed medium. Generally, there are four important parts in a polariscope testing equipment placed along the optical axis, which include a light source, a polarizer, a sample holder, and an analyzer.

Step 1: Light Polarization

The natural light from the built-in light source inside the instrument vibrates in all possible directions perpendicular to the path of travel of the light beam. Upon passing the light through the first filter, called the polarizer, the vibrating light will be limited to vibrating in one plane.

Step 2: Interaction with Stressed Material (The Photoelastic Effect)

Upon the entry of the plane-polarized light into the isotropic body subject to internal stress, the phenomenon called photoelasticity occurs. The stress inside makes the body behave like a birefringent substance. The incoming light wave splits into two mutually perpendicular waves: the “fast” wave and the “slow” wave; these are oriented along the principal axes of the stress field.

Since these two beams propagate with different speeds within the stressed medium, there is a phase difference between them. This phase difference, called the optical path difference, is directly dependent on the stress inside the object.

Step 3: Reconstruction and Analysis

The two light waves that are out of phase leave the specimen and enter a second polarized lens referred to as the analyzer. The analyzer is always placed in relation to the initial polarizer at a 90-degree angle (a system referred to as "crossed polars"). When there is no specimen or when the specimen has no internal stress, the analyzer will block all light and create a dark field.

But when there is a stress field that alters the light waves to either fast or slow rays, then upon their passing through an analyzer, they will combine and lead to constructive or destructive optical interference, showing as bright colored fringes (isochromatic fringes) and dark fringes (isoclinic fringes). The analysis of the patterns formed will help in understanding the nature of stress in terms of its location and intensity.

Primary Configuration Types

There are two major designs for polariscopes in optical terms based on the depth of investigation needed:

• Plane Polariscopes: The equipment uses just a polarizer and an analyzer. It shows both isochromatic fringes and isoclinic fringes. Even though it is quite effective in identifying stress directions, the overlap of the lines makes it difficult to measure stresses precisely.
• Circular Polariscope: In addition to the linearly polarizing plates used earlier, it incorporates two more plates called quarter-wave plates. One is inserted between the polarizing plates, and another is inserted between the analyzers. The two quarter-wave plates change the linearly polarized light into circularly polarized light, resulting in no dark isoclinic fringe pattern.

Key Industrial Applications

Glass Vessels and Glassware Production

When manufacturing glass bottles, vials, and glassware, the melted glass should cool down slowly inside a special furnace known as an annealing lehr. Otherwise, the thermal differences created will create tremendous stresses within the glass that can make it crack due to minor changes in temperature. Industrial polariscope testers are installed at the end of production lines for examining the quality of finished glassware.

Plastic and Injection Molding

Transparent parts made by the injection molding process, such as automobile parts, syringes for medical applications, and casings for electronics equipment, usually face the problem of molded-in stress because of uneven cooling or excessive pressures around the injection gate. Polariscopes are used for analyzing these injection-molded parts.

Optical Lenses Fabrication

Camera lenses, microscope lenses, aviation sensors, and eye wear lenses need to have no optical distortions whatsoever. Internal stresses that occur in a lens will affect the local refractive indices of the lenses, resulting in optical distortions. Such laboratory stress testing equipment is used in the optical fabrication of lenses and glasses.

Perform Inspection for PET Bottles

Bottles in the beverages industry are manufactured by blowing small parts made through injection molding called PET preforms. Inspecting the preforms through a polariscope helps detect problems with molecular structure and crystallization. It is much easier to detect issues before the expensive blow-molding phase.

Technical Standards and Standardization

Polariscope has many industrial standards like-

• ASTM C148: This method gives a standard for the examination of glass containers through a polariscope. It gives an idea about finding relative residual stress.
• ASTM D4093: This method is the standard for testing photoelastic measurements and residual strengths in transparent plastic materials.

Which industries use Polariscopes?

Industries that use polariscopes are-

• Packaging film and container manufacturers
• Gem and jewelry manufacturers
• Pharma companies
• Optics and eyewear manufacturers
• Aerospace and automotive structural manufacturers