Give your microscope the peronal touch

Clive Cowen, owner of microscope equipment supplier, Micro Instruments Limited, describes how his company makes customized solutions for practical applications which are often beyond the scope of conventional commercial products

There is a multitude of microscope manufacturers, each offering an extensive range of models for a myriad of applications. Despite this, there are many requirements that cannot be satisfied with off-the-shelf products. In such cases, solutions are provided where microscope optics and accessories are built into customized systems. Sourcing such parts can be a minefield. After many years building custom solutions Micro Instruments like to use components from Japanese microscope manufacturer, Meiji Techno.

One obvious example is the use of microscope objectives. Designed primarily for use on a microscope, objectives can be used as ready-made high resolution systems for a broad variety of imaging applications. It has been said that all microscopes are basically a tube with an objective at one end and an eye piece at the other. Of course there are multiple “extras” available to expand a microsocpe’s capabilities so that techniques such as fluorescence microscopy, interference and polarising work may be performed.
Nevertheless, the basic premise holds true. When such imaging components are incorporated into a special system, it is important that the optics are used in the way they were designed. Failure to do so will result in a drastic loss in performance. As has been said, “The laws of physics are quite impersonal and always answer a fool according to his folly.”

Microscope objectives were designed to produce a real image which is further magnified by an eyepiece, the focus of which being positioned at the real image produced by the objective. These are known as finite tube length objectives. (fig 1). The length of the tube separating the objective and the eyepiece is known as the mechanical tube length (MTL). This has been widely standardised at 160mm and it is the required distance between the seating face of the objective and the seating face of the eyepiece. The objectives produce their finite image, conventionally 10mm down the eyepiece end of the tube – at the primary image plane (PIP). So that objectives remain parfocal when they are interchanged, different magnification microscope objectives are produced with the same parfocalising distance (PFD).  This is the distance from the seating face of the objective to the focus of the objective – typically at 45mm on finite tube length objectives. ‘WD’ represents the free working distance of the objective.

Figure 1. Finite tube length microscope

Figure 2. Inifinite tube length microscope


Several manufactures are now producing infinity objectives (fig 2).  Here, the objectives are designed so that the object being viewed is at the focus of the objective. This means that the imaging rays coming from the objective are parallel, i.e. they do not produce a real image, as in the case of finite tube length objectives. A real image is created by incorporating a tube lens. Since the rays from the objective are parallel, a real image will be focused at the focal length of the tube lens. This can be viewed by an eyepiece in exactly the same way as the primary image is dealt with, in the case of finite tube length objectives. An advantage of this system is that, since the ray path is parallel from the objective to the tube lens, the length of this part optical system can be extended. On a microscope, the extra space can be used for additional items such as reflectors, used in fluorescence and epi-illumination. In designing special systems, this gives flexibility in positioning the viewing eyepiece in relation to the object being viewed.

Counting and identifying plankton

The Sir Alister Hardy Foundation for Ocean Science (SAHFOS) is an international charity registered in the UK, operating the Continuous Plankton Recorder (CPR) survey.  Based in Plymouth, the Foundation has been collecting data from the North Atlantic and the North Sea on biogeography and ecology of plankton since 1931.  Since 1948, the sampling equipment and procedures used by the survey have hardly varied. This consistency has enabled temporal and spatial comparisons of plankton data collected.  In recent years when the old CPR analysis microscopes required replacement suitable off the shelf instruments were not available. Specialist instrument makers were required to design a microscope that would have the correct specifications, to maintain consistency, have the advantages of modern technology and also accommodate health and safety requirements.

Micro Instruments working with  Tanya Jonas a marine para-taxonomist responsible for the management of the analysis programme, designed a system for counting and identifying plankton. The instrument incorporates a large stage to support the net on which the plankton is collected (fig 3). Identification of the plankton is a visual process and requires users to sit at the microscope for long periods of time. This makes ergonomic design critical. In manufacturing the system, Micro Instruments uses the Meiji infinity corrected objectives which enable the viewing head to be positioned at a comfortable height matching the requirements for each user. The infinity tube length enables the height of the viewing tube to be varied without affecting the magnification and the field of view.

Such special systems still require to be focused. The depth of field of high resolution, high magnification objectives is very small, typically less than 1µm. This means that a precise, fine focusing mechanism is required. One solution is to use Meiji’s FU1000 focus block. This may be fitted with a variety of viewing heads and objectives and may be mounted to a pillar or screwed to a support plate. It makes for a simple and economic solution.


                                   Figure 3. Rotifer eggs on silk

Studying oil particulates                               
                                                                
Micro Instruments has developed a system to study the cleanliness of oil used in hydraulic applications.  Hydraulic systems are used extensively in industry. If the oil becomes contaminated, costly breakdowns may result. While expensive scanning systems are available to automatically count particles in hydraulic fluid as they pass a detector, such systems do not give information on the nature of the particles. The type of contaminant may be identified by looking through a microscope. It is also necessary to have the ability to count particle numbers, specifying size ranges to report the level of cleanliness of the fluid.

The simple solution is to compare the test sample (a filter membrane) with reference standards of known particle content. In the specially made system, a Meiji Monozoom objective is used to provide images of matching magnification to the reference samples. Magnification is typically 100x. Images are collected using a CCTV camera and printed for easy comparison. (fig.5).

These two examples clearly illustrate the ease at which custom solutions may be built from readily available components to provide economic solutions for laboratories.

By Clive Cowen, Micro Instruments Limited, Long Hanborough, Witney

Clive Cowen would like to thank Tanya Jonas of SAHFOS for provision of materials used in this article.