Fiber Fineness Measurement by Projection Microscope

The projection microscope is the standard method for measuring wool fibre diameter, and all other methods have to be checked for accuracy against it. The method is also applicable to any other fibres with a circular cross-section. The method involves preparing a microscope slide of short lengths of fibre which is then viewed using a microscope that projects an image of the fibres onto a horizontal screen for ease of measurement. The apparatus is shown diagrammatically in Fig. Techniques are followed that avoid bias and ensure a truly random sample. 

Method of Test
A suitable random and representative sample is conditioned for 24 h in a standard testing atmosphere. Using a modified Hardy microtome the fibres are cut to a suitable length (0.4mm for fibres below 27 (im) and a slide is prepared by carefully mixing the fibres into the mountant. The use of short fibres gives a length-biased sample so that proportionally more of the longer fibres will have their diameter measured. The mounting agent should be non-swelling and have a suitable refractive index (for example liquid paraffin). The mixture of fibres and mountant is spread thinly on the slide and covered with a cover glass, carefully avoiding air bubbles and finger prints.
The projection microscope
The slide is placed on the stage, coverglass down (microscope inverted) and fibres are selected for measurement in the following way. The slide is traversed in a zigzag fashion, measuring every fibre that complies with the following requirements: 1 has more than half its length visible in the 7.5cm circle which is drawn in the centre of the field of view; 2 is not in contact with any other fibre at the point of measurement. The traverse of the slide is continued until the required number of fibres has been measured. The magnification of the microscope is adjusted to be 50Ox so that on the scale used to measure the fibres each millimetre represents 2 um.

For accurate tests three slides should be measured from randomly selected areas of the material and not less than 150 fibres per slide should be measured. The coefficient of variation of diameter for unblended wool lies between 20% and 28%. From this value the number of tests to give certain confidence limits has been calculated . 
  
 

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