Using Photometry to Measure Fibre Diameter Distributions

This paper is Part 13 in a series titled Fundamental Principles of Fibre Diameter Measurement.

Abstract

Photometry is the analytical use of light (luminous) intensity to measure the physical and chemical properties of solids, liquids and gases, and mixtures or solutions thereof.  Wavelengths in the infrared, visible and ultra-violet portions of the electromagnetic radiation spectrum are commonly used in photometric measurements.  Photometry is probably the most extensively used of all analytical technologies.

In principle the application of photometry to the measurement of wool fineness is very simple.  Photometers consist of a source of light of constant radiance, a sample cell and a photo-detector.  In the specific case of wool the measurement of fineness is based on the principle of light scattering and presumes that the wool fibre is opaque.  The impact of the light beam on the photo-detector in the absence of any interference will generate a detectable electrical signal.  If a fibre at right angles to the beam intersects the beam the fibre will project a shadow onto the photo-detector, due to the light incident on the fibre being scattered.  The shadow of the fibre will reduce the signal from the photo-detector by an amount that is proportional to the projected area of the fibre.  If the length of the fibre intersecting the beam is constant then the output from the photo-detector will be proportional to the transverse dimension of the fibre.

However, this is a very simplistic description of the physics of photometry as applied to wool fibre diameter measurement.  The specific details of the physics in particular instruments are dependent upon the instrument design.
When measuring wool fibre diameter the critical step required for photometric instruments is a technique for aligning the fibres so that they are always at right angles or near right angles to the beam of light, and of ensuring that the orientations of the individual fibres across the beam are similar.

There is an extensive literature describing aspects of photometric techniques for the measurement of the diameter distribution characteristics of wool i.e.

• Mean Fibre Diameter (MFD);
• Standard Deviation of Diameter (SD); and
• Coefficient of Variation of Diameter (CVD).

This is a very large topic.  Because of this reviewconsiders the use of Photometry in several parts, each part focusing on key developments within specific time periods i.e.

• The First Efforts (1950-1970);
• Leaping Forward with Laser Optics (1970-1983);
• Resolving Problems (1983-1989); and
• The Sirolan Laserscan (1989-today).

Citation

"Fundamental Principles of Fibre Diameter Measurement. Part 13: Photometry", P.J. Sommerville, AWTA Ltd Newsletter, April 2005