Laser Diffraction (LD) for particle characterization down to the nano range

Microtrac has been a global leader in laser diffraction instrumentation for over 40 years - by continuously improving the instrument technology, we offer customers a robust portfolio of laser diffraction instruments which is ideal for particle sizing and characterization.

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Particle Size and Shape Analyzer SYNC

Measuring range: 0.01 µm - 4 mm
Measurement principle: Laser Diffraction & Dynamic Image Analysis
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Laser Diffraction (LD) Technology
$Laser Diffraction (LD) Technology$

Background information on the function principle, advantages and physics behind laser diffraction analyzers.
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Particle Size Analyzer S3500

Measuring range: 0.02 µm - 2.8 mm
Measuring principle: Laser diffraction
Lasers: 3x Red 780 nm
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Particle Size Analyzer BLUEWAVE

Measuring range: 0.01 µm - 2.8 mm
Measuring principle: Laser diffraction
Lasers: 1x Red 780 nm / 2x Blue 405 nm
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Spray particle & spray droplet size analyzer AEROTRAC II

Measurement principle: Laser diffraction
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Introduction to Laser Diffraction (LD)

Laser diffraction (LD) analysis, also known as static light scattering, is the most common method for the determination of particle size distributions other than traditional sieve analysis.

The method is based on the deflection of a laser beam by an ensemble of particles dispersed in either a liquid or an air stream. The angles of diffraction or scattering angles are characteristic of the particle size. ISO 13320 comprehensively describes the methodology of laser diffraction.

The following explains the advantages and limits as well as the working mechanisms and theory behind laser diffraction technology.

Microtrac was the very first company to develop, manufacture and market commercial laser diffraction analyzers starting in the 1970s. We have been a technological leader continuously pushing innovation ever since.

$The SYNC is Microtrac’s most advanced laser diffraction analyzer.$

The SYNC is Microtrac’s most advanced laser diffraction analyzer.

Advantages of laser diffraction

Using laser diffraction to analyze particle size distributions offers users many advantages.

1. Wide measurement range

Modern laser diffraction analyzers determine the particle size distribution over a very wide dynamic measurement range. Typically, a size range of 10 nm to 4 mm is covered, which corresponds to a factor of 400,000 between the smallest and the largest measurable particles. In practice, however, laser diffraction is usually applied over a size range of about 30 nm – 1,000 µm. It should be noted that this wide measurement range is always fully available with all modern analyzers. There is no need for prior adjustment of the size range by shifting lenses or selecting suitable optics, for example.

2. Versatility

Laser diffraction is used in many different industries for routine analysis and quality control as well as for demanding research and development tasks. This is also due to the fact that both wet samples, i.e. suspensions and emulsions, and dry powders can be easily characterized. In a wet measurement, powerful recirculators and pump systems, usually with integrated ultrasonic probes, ensure efficient homogenization so that in many cases sample preparation can be carried out completely in the instrument. In a dry measurement, the particles are separated by a Venturi nozzle in an air stream.

3. High sample throughput and easy operation

Short measurement times are a major advantage of laser diffraction. The analysis procedure, using a wet measurement as an example, includes: (1) Filling the instrument with dispersing liquid via an autofill pump, (2) Performing a setzero (blank measurement without sample particles), (3) Adding a sample, (4) Acquiring the diffraction data, (5) Cleaning of the instrument by means of automatic rinsing function. The whole run takes 1-2 minutes, depending on the use of ultrasonic energy and the number of cleaning cycles. In the case of dry measurements, the measurement time is 10 - 40 seconds.

4. Accuracy and reproducibility

The use of SOPs ensures that analysis by laser diffraction is always performed under the same conditions. This virtually eliminates software input errors and guarantees high reproducibility, even between analyzers at different locations. The accuracy of laser diffraction can be verified with standards. The requirements (for accuracy and reproducibility) are specified in ISO 13320 and are usually significantly exceeded. Incidentally, calibration of the devices by users is not necessary.

5. Robustness

Laser diffraction instruments are characterized by great robustness and low maintenance requirements. The method is hardly susceptible to external interference and many instruments are located in production facilities. However, to further decrease the required maintenance of the analyzer, it should ideally be equipped with long life diode lasers. Unlike Microtrac analyzers, many instruments still use HeNe lasers, which have a significantly diminished service life compared to laser diodes. These HeNe gas lasers must be replaced at regular intervals and also require additional warm-up time.

The Physics behind Laser Diffraction (LD)

When laser light (monochromatic, coherent, polarized) hits an object, diffraction phenomena occur. For example, diffraction can be observed from apertures, slits, gratings, and particles. From the edges of a particle, the light propagates in the form of spherical wave fronts, whose interference then leads to the observed phenomena.

The diffraction angle is determined by the wavelength of the light and the size of the particle, with angles becoming smaller with increasing particle size. For intermediately sized particles, Mie theory can be applied to the scattering patterns to determine the size. Particles in this range and larger have size dependent scattering patterns. Larger particles have higher scattering in the forward direction than smaller ones.

For very small particles, the interaction of light with these particles can be described by Rayleigh scattering. In the Rayleigh regime, scattered light is weaker and almost isotropic in all spatial directions.

diffraction of laser light at a spherical particle

Laser Diffraction (LD) in a particle size analyzer

In laser diffraction analysis, the scattered or diffracted light is recorded over the widest possible range of angles by means of a special laser and detector arrangement. The evaluation of this signal is based on the principle that large particles preferentially scatter light at small angles whereas small particles have their scattered light maximum at large angles.

When evaluating the signal, it must be taken into account that a particle size does not correspond to a specific angle, but that each particle scatters light in all directions at different intensities. This is therefore an indirect measurement method since the size is not measured directly on the particle but is calculated via a secondary property (diffraction pattern).

Furthermore, the recorded pattern is generated by particles of different sizes at the same time, so it is a superposition of the scattered light of many particles of different sizes. Therefore, laser diffraction is a so-called ensemble measurement method.

During evaluation, all signals are treated as if they were generated by ideal spherical particles. Particle shape is not detected. An irregular particle shape leads to broader size distributions, since both the width and the length of the particles contribute to the overall scattering signal and are included in the result. Appropriate considerations must be made to properly account for irregular particle shape.

Typical setup in a Microtrac Laser Diffraction (LD) analyzer with lasers hitting the sample cell and detectors determining the scattering pattern after passing through a collector lens.

Limits of Laser Diffraction (LD) particle size analyzers

The upper limit of the measuring range of laser diffraction is determined by the fact that with increasing particle size, the diffraction angles become smaller and smaller. As a result, small differences between particle sizes are more difficult to detect metrologically.

The lower limit of the measurement range is defined by the weak intensity of the scattered light from small particles. The use of shorter wavelength light, which generates increased scattering intensity, can extend the measurement range of laser diffraction to smaller particle sizes. This is the reason why many laser analyzers use blue light sources for improved performance in the submicron size range.

Fourier and reverse Fourier optics in laser diffraction analyzers

According to ISO 13320, measuring instruments for laser diffraction can be operated with either Fourier optics or reverse Fourier optics. With Fourier optics, the particles are illuminated by a parallel beam, whereas with an inverse Fourier arrangement a convergent laser beam is used.

Fourier optics offer the advantage that the diffraction signal is always correctly detected regardless of the position of a particle in the laser beam, and equal diffraction conditions prevail at any point in the interrogated sample volume.

With the inverse Fourier setup, the particle stream must be relatively narrow, and in addition, particles of the same size in the convergent beam have different diffraction angles relative to the optical axis. All this generally leads to blurry diffraction patterns compared to the Fourier setup. The advantage of the inverse Fourier method is that one can collect a wider angular range on a smaller detector array.

However, with a suitable design, an angular range of 0-163 ° can also be covered with the Fourier arrangement. Therefore, Microtrac laser diffraction analyzers use the Fourier arrangement.

$Laser Diffraction (LD) - Figure 3a$

Laser diffraction with Fourier setup (left, MICROTRAC) and reverse Fourier setup (right)

Fraunhofer approximation and Mie evaluation in laser diffraction analyzers

Laser Diffraction and Static Light Scattering Analysis are often used interchangeably, although the term laser diffraction has become established in many industries and laboratories.

Diffraction produces maxima and minima in the intensity distribution at characteristic angles. This distribution is described by the so-called Fraunhofer theory. The advantage of the Fraunhofer approximation is that no other material properties of the sample need to be known. However, this approach is not applicable for smaller and transparent particles, since here the optical properties of the particles also have an influence on the intensity distribution at the detectors.

These optical properties, primarily the refractive index, must be known for the evaluation of the particle size distribution. This kind of evaluation is done according to Mie Theory, named after the physicist Gustav Mie. Strictly speaking, Fraunhofer diffraction is only a special case of Mie theory, which describes all diffraction and scattering phenomena comprehensively.

The scattered light pattern changes depending on the particle size. For particles with a diameter d significantly larger than the wavelength of the light, the Fraunhofer approximation is applicable. For smaller particles, the Mie evaluation must be used. Scattering from very small particles is called Rayleigh scattering.

Microtrac Products & Contact

$Laser diffraction analyzer SYNC$

Different Microtrac analyzers such as the SYNC use Laser Diffraction to characterize particles.

Laser diffraction analyzers

Our team of experts will be happy to advise you on your application and on our product range.

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Laser Diffraction (LD) - FAQ

What is Laser Diffraction (LD)?

Laser diffraction is a measuring technology for the determination of particle size distributions. In this method, a laser beam is targeted towards an ensemble of particles dispersed in either a liquid or an air stream. The resulting deflection pattern of scattering angles of the laser is characteristic of the particle size of the material and detected by an according sensor.

Which standards and norms relate to Laser Diffraction (LD)?

The measuring technology is described in standard ISO 13320 "Particle size analysis - Laser diffraction methods". The way in which results are calculated and displayed is described in standards ISO 9276-1 and ISO 9276-2 "Representation of results of particle size analysis" part 1 and part 2.

How long does a Laser Diffraction (LD) measurement take?

A typical Laser Diffraction measurement usually takes 1-2 minutes for particles dispersed in a liquid. Dry measurements of particles dispersed in an air stream that use laser diffraction are even faster with measurement times of only 10-40 seconds.

What are the advantages of Laser Diffraction (LD)?

The advantages of Laser Diffraction analysis include a wide measurement range (10 nm to 4 mm), great versatility (suitable for many different materials), high sample throughput, easy operation, accuracy, and reproducibility as well as the general robustness of Laser Diffraction analyzers.

What is the measuring range of Laser Diffraction (LD)?

Typically, Laser Diffraction analyzers cover a particle size range of 10 nm to 4 mm. This corresponds to a factor of 400,000 between the smallest and the largest measurable particles. In most applications, Laser Diffraction is typically used for particle size distributions between 30 nm – 1 mm.

Who uses Laser Diffraction (LD) analyzers?

Laser Diffraction equipment is typically used in research or quality control applications. In research, Laser Diffraction analyzers are used to explore and develop new materials; in quality control they are used to ensure the according properties of the manufactured goods are continuously met.