SATRA’s use of microscopy

Some principles of effective microscopy, and applications for the footwear and leather industries.

by David Smith

The first microscopes were likely just drops of water placed on top of a specimen, with the water droplet creating a magnified view of whatever was underneath it. Later, glass bottles filled with water were used, and then simple curved sections of polished glass, which had been discovered to have similar magnifying properties.

It was a Dutch optician and his son – Hans and Zacharias Jansen – who developed the first practical microscope in 1590. This useful tool was quickly seized upon by scientists and engineers, who made their own improvements to the basic design. Perhaps most famous of these was Galileo Galilei who, in 1609, developed a compound microscope with a convex and a concave lens.

At its most basic, a microscope is an arrangement of lenses encased in a tube that enables an operator to view a greatly magnified section of an item under observation. However, there are a myriad of designs available that are variations on this basic theme. Modern microscopes incorporate a variety of adjustable electronic lighting options, interchangeable objective lenses, computer connectivity and optional external viewing screens.

Effective usage

Using a microscope effectively for research or testing requires familiarity with a number of different techniques and lighting strategies. As with photography, the key to obtaining a good image is control of the lighting – its intensity, origin and direction.

The simplest light source, which will be familiar to most people who have used a basic microscope at school, is transmitted light – where a light source is shone up from underneath the specimen and through the glass slide on which it is mounted. On simple microscopes, this light is introduced via a mirror situated under the microscope stage, which is used to reflect either natural light or light from a small light source. This was originally an oil-burning lamp, but is now usually an electric light source. This type of illumination is suitable for assessing fibrous material, diaphanous items, or anything where it is desirable to backlight the subject to be sure of its silhouette or edge detail.

The other main source of illumination for microscopy is reflected light, where a light is shone directly down the optical path of the microscope and onto the specimen from above. This method has the advantage of only illuminating the area currently under assessment, thus avoiding problems associated with over-illumination (such as glare and haze).

With most good-quality microscopes, there are two options for supplying this reflected light: 'bright field' and 'dark field' (see figures 1a and 1b).

Bright field illumination is achieved by focusing the light from the light source (a lamp housing usually at the rear of the microscope), through the optical path and onto the specimen, providing illumination through the objective lenses themselves. Light is focused on only the viewed area of the item and at an intensity that can be precisely controlled by the operator. This kind of illumination is well suited for examining opaque materials or specimens that are reasonably flat, without significant surface texture.

Dark field illumination is a technique for viewing a specimen with greatly increased contrast, without having to treat or stain it prior to viewing. Although the light source is the same, and the light is focused down through the optical path (as with bright field illumination), the difference is that instead of lighting the item from directly overhead, the illumination is oblique – lighting the specimen from the sides. The effect can be likened to evening sun shining across a ploughed field – the low angle of lighting makes textures and contours stand out sharply. It is best suited for semi-transparent subjects, and can be used in combination with transmitted light to create striking images – typically, a subject clearly outlined against a dark, almost black background.

By using a carefully aligned light source, the quantity of directly transmitted light entering the image plane is reduced, and only the light scattered by the specimen is collected by the viewing lenses. Dark field images can suffer from low light intensity, as the lighting is not direct, but this can usually be adjusted for, especially if the end result is a digital photograph that can be brightened later.

It is, of course, possible to illuminate a specimen with both transmitted and reflected light at the same time, where it is desirable to do so, as this produces an image with both clearly defined texture and outline.

Another invaluable tool for the assessment of different items is polarised light. A 'polariser' is placed between the light source and the specimen, and an ‘analyser’ between the eyepiece and the item. When the analyser is rotated relative to the polariser, it creates interference patterns in the light coming from the illumination source, and the result is an image rich in colour and increased contrast. Typically, a rainbow of colours can be observed in transparent specimens. This type of lighting is very useful for fibre identification, as different materials reflect polarised light in different, and characteristic ways.

Zooming in

A range of different magnifications is possible with good quality microscopes – achieved through a combination of the power of the objective lens selected and the magnification factor of the viewing head. For example, a 20x objective lens on a microscope fitted with a 10x viewing head gives an overall magnification of 200x.

Such an ability can further be modified by the introduction of reducing rings, which can be fitted to reduce the overall magnification but increase the specimen viewing area. This enables the examination of whole footwear without it needing to be cut or damaged.

Microscopy at SATRA

SATRA uses high-end optical microscopes equipped with high-quality digital cameras to assist customers with specimen analysis. The software these microscopes use allows us to take measurements from a recorded digital image, thus enabling us to take particle counts, shape perimeter measurements, lengths and areas. It also allows for the superimposition of a scale or a grid over a digital microscope image.

The relatively low magnification of our bench-mounted stereo scope and its large specimen viewing area make it the perfect tool for examining features on whole footwear or other products that are not practical to cut and prepare for viewing on a metallurgical (or stage-based) microscope.

Optical microscopy offers an efficient and economical solution to many quality control issues. It has proved very useful for the inspection of electronic components (especially those incorporated into footwear), where it is invaluable for recording images of good- or poor-quality soldering as a record of variations in manufacture.

Optical microscopy compliments advanced tools such as scanning electron microscopy (SEM), with the advantage that it is practically instant, requires little or no preparation and items can be viewed 'live' by the customer at the point of inspection.

A valuable tool for shoemakers

As mentioned earlier, SATRA can carry out microscopic inspections of fabrics and threads for quality control purposes, or to examine in detail the effects of simulated weathering or abrasion on materials or products. Inspection of individual footwear components, along with accompanying detailed and annotated photographs is another valuable service that SATRA can provide. This can give support in cases of customer complaint, or quality control issues.

Microscopy is a tremendously useful tool for the examination of fabrics incorporating novel insulation strategies, such as phase change materials (PCMs). These are often microencapsulated into tiny spheres, which can then be applied to a fabric by means of a spray.

In theory, these should be evenly distributed amongst the warp and weft fibres of the fabric for maximum effectiveness, but their placement is often not that uniform. Microscopic analysis of a cross-section of a specimen of fabric at a magnification of around 100-200x reveals clumps of these capsules situated in irregular groupings.

When viewed with dark field illumination, these capsules become more visible, and it is possible to see them as three-dimensional shapes.

As part of our work on mould prevention, SATRA has used optical microscopy to examine leathers and fabrics contaminated with mould spores. Other examples of inspection work carried out on our microscopes are defects in plastic moulding, examining the quality of surface finishes – for instance, if paint or adhesive is evenly applied – and the measuring of hairline cracks or crazing in plastic coatings. Crack propagation in weathered or tested plastic items can also benefit from microscopic examination.
By examining cross sections of some coated leathers or materials such as metals or plastics which may be painted, it is possible to take accurate readings of the thickness of the layers of paint or coating (figure 2), which can be presented pictorially in a SATRA test report.

Adhesive application on footwear materials or flooring is another quality control issue that can be examined with microscopy. A combination of low-magnification overhead images to look at overall coverage can be combined with higher-magnification cross-sectional images to assess the profile of the adhesive, and check if it has been correctly applied. Where a bond has failed, the mode of adhesive failure can be assessed with microscopy.

Stitching, decorative trim or other embellishments are also good targets for microscopy. This is because the distance between stitches or decorative broguing needs to be consistent between shoes, and can be measured by microscopy.

The human eye has a remarkable range of vision, and remains one of the most effective tools for quality control and specimen inspection. However, viewing an item under even low microscopic magnification can make an amazing difference to the level of detail visible and, as a result, to the corresponding value of the assessment.

The inclusion of one or two well-chosen, concisely annotated microscope images within a technical report can greatly assist the reader to understand the technical content, and can also convey technical findings much more effectively than a block of text.

Publishing Data

This article was originally published on page 10 of the July/August 2016 issue of SATRA Bulletin.

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