Laser Diffraction

Laser diffraction is a highly efficient and effective method of dispersing light to the size of particles ranging from submicron to millimetre scales.

What is Laser Diffraction?

Laser diffraction is a widely used method of measuring particles of matter ranging from hundreds of nanometres to a few millimetres in size.

In the early 1980’s, Sympatec introduced a new development that led to the separation of lasers in one direction: the scattering of dry powder and even the finest combined powder was fulfilled with RODOS. And with the R series of laser diffraction sensor HELOS, new precision benchmarks have been achieved. Recent improvements include improved use of Fraunhofer test-free parameters down to submicron control (less than 1 µm), greater signal frequency for better adjustment with distance technology of combination and more powerful test methods. Laser diffraction is now even closer to perfect levels. Increasingly, the needs and limitations of the industry are far exceeding.

The key reasons for its success are:

• Variable width – from submicron to width in millimetres.

• Fast estimates – results are produced in less than a minute.

• Multiplication – large numbers of particles are taken as a sample for each measure.

• Quick response – monitor and control the particle dispersion process.

• High sample output – hundreds of measurements per day.

• Evaluation is not required – it is easily verified using standard reference materials.

• Well-established approach – compiled by ISO 13320 (2009).

Laser diffraction has become the most common method of particle measurement in many industrial areas because it is faster, simpler and better than most conventional screening methods.

Principles of Laser Diffraction

Laser diffraction measures the particle size distribution by measuring the angular variation of the scattering light intensity as the laser beam exceeds the sample of the dispersed particle. Larger particles transmit light at smaller angles relative to the laser beam, and smaller particles transmit light at larger angles. To calculate the particle size responsible for creating the scatter pattern, the angular scattering intensity data is analysed using Mie theory for light scattering. Particle size is reported as a volume equal to the diameter of the sphere.

Optical Properties

To calculate the particle size distribution, taking a model-sized sphere model, Mie theory of light scattering is used in laser diffraction.

Mie theory requires knowledge of the visual cues (refractive index and assumed component) of both the measured sample as well as the refractive index of the dispersant. Usually, dispersant visual features are easily accessible from published data, and most modern tools will have a built-in website that includes standard distributors. In samples where visual characteristics are unknown, the user can measure using a repetitive method based on the beauty of the balance between the model data and the actual data collected in the sample.

The simplified method is to use Fraunhofer estimates, which do not require knowledge of sample sightings. This can provide precise results for huge particles. However, it should be used with care whenever working with samples that may have particles less than 50µm or where particles are visible on the other side.

Young’s double-slit experiment:

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