THE POLARISCOPE

Components of a Polariscope:

• Polarizer and Analyzer If the polarizer and analyzer are crossed, so that no light exits from the analyzer, the setup is called dark field. Light will only be transmitted due to effects resulting from birefringence in the model. If the polarizer and analyzer are aligned to pass all the light going through the system, the setup is called light field. The effect of the model is to then darken the image. Almost all work is done in the dark field setup. Two types of fringes occur in the image: isochromatics, which identify the magnitude of the maximum shearing stress vector by their color, and isoclinics, which give the direction of the maximum shearing stress vector.

• Two Quarter-Wave Plates The two quarter wave plates are aligned in crossed mode with the fast axis of one aligned with the slow axis of the other. The purpose of the quarter wave plates is to convert the incoming linearized polarized light to circularly polarized light before the light goes through the model, and then convert the light emerging from the model back into linearly polarized light before it passes through the analyzer. Actually, these plates are not necessary, but they remove the isoclinic lines from the pattern so as to produce a clear isochromatic pattern. They can be removed physically, or optically by aligning their axes with the polarizer axes.

• Loading Mechanism Some type of mechanism is required to load the model specimen. It must be placed between the polarizer and quarter-wave pairs.

• Light Source with Filters The light source can either be standard white light or a monochromatic source (usually with a wavelength matching the tint-of- passage, which is also the design wavelength of the quarter-wave plates). White light gives colored fringes, isochromatics, which aid in estimating stresses, but has the disadvantage that only a very few colored fringes can be distinguished easily. Monochromatic light, on the other hand, gives much narrower fringes which can be observed up to much higher order. In either case, the light must be expanded to the size of the model area it is desired to study. The beam can be either from an extended source (fluorescent or incandescent bulbs) diffused by a ground glass, or from a point source (arc lamp or discharge tube) with the light beam expanded and collimated by a collimating lens. Large collimating lenses are expensive, so much work is done with diffuse light sources. In this case however, a telecentric imaging system should be used to select out the light which traveled parallel through the specimen.

• Image Detector This can be a camera film (preferably one with high contrast), a CCD digital camera, or the human eye. Frequently, a ground glass plate is fixed to a camera film plane for visual inspection and alignment, then replaced with film for obtaining a permanent image. In recent times, there has of course been great interest in digital images from a CCD.

• Collimating and Field Lenses The collimating lens is used as described above. The field lens is used to focus the large image exiting from the polariscope down onto an image detector.

Example:

The following image is taken from the Measurements Group Wall Chart and shows the isochromatic fringe pattern obtained when a curved plastic (PSM-1) model of a curved bar is subjected to tensile stress.