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Why has laser diffraction endured?

27 December 2010 One Comment

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Laser diffraction – ultimately sophisticated

Leonardo da Vinci is reported to have said that, “Simplicity is the ultimate sophistication.” Well, here I want to look at why the simplicity and ease of use offered by laser diffraction has led to such a sophisticated list of applications to which it is routinely applied. With so many industries and scientific fields demanding particle size data, it doesn’t take a great deal of delving to discover why laser diffraction is so often the method of choice.

If da Vinci were alive today it may not be such a wild stretch of the imagination to suppose that he might have taken great joy in a technique that measures the response of a material to light. If nothing else, he most certainly would have been fascinated by the fact that the pigment industry now relies on particle size data to control the optical properties of its products!

Applications of particle size data

Different industries measure particle size for essentially the same reasons. Some of the most important properties influenced, or in some cases controlled, by particle size are: reaction rate, dissolution rate, packing density, stability and ease of inhalation (in the case of inhaled pharmaceuticals), optical properties and consumer perception.

The basic benefits of laser diffraction

Some key attributes required of a particle sizing technique, all of which laser diffraction offers are:

o        Flexibility (wet and dry measurement)

o        Broad measurement range

o        Speed of data acquisition

o        Automated measurement

o        Non-destructive

Laser diffraction offers the benefits of being a non-destructive technique that, because it relies on the laws of light behaviour, requires no instrument calibration. Functioning within a range that is applicable in so many manufacturing applications, the technique delivers rapid measurement times for both wet and dry samples.

Over the years, however, it is the reliability and ease-of-use offered by laser diffraction systems that has made them so attractive. It is the drive towards simplicity through automation that has inspired instrument manufacturers in their design development over the last ten to fifteen years.

Laser diffraction: how it works

Laser diffraction calculates the particle size of a sample by measuring the light scattered by particles as they pass through the path of a laser. Large particles generate a high scattering intensity at relatively narrow angles to the incident beam, while smaller particles produce a lower intensity signal but at much wider angles. By measuring the intensity of scattered light across multiple angles, using an array of detectors, laser diffraction systems can generate data for particle size distributions extending over a very wide range (typically 0.1 – 3500 microns). Thousands of particles are measured at any instant during a measurement, and as such that the technique is referred to as an ‘ensemble technique’.

The particle size distribution of the sample is calculated from the detected scattering data using an appropriate theory of light behaviour. The latest version of ISO13320 [1] (the ISO standard for laser diffraction) recommends the use of Mie theory for all particles in the size range over which laser diffraction is applied.

The story continues…

In my next blog in the ‘Why has laser diffraction endured?’ series I will outline the developments in optical configuration, data analysis and automation that have led to the dominance of laser diffraction as a particle sizing technique.