What is the shading rate? What is the optimal shading rate for a laser particle size analyzer?

The shading rate can be defined as the ratio of the shading cross-section of the particles in the light beam to the total area of the light beam. In use, it is often expressed by the percentage of light scattered and absorbed by the particles to the total output light (minus background scattering). Therefore, the shading rate is also called Optical Concentration.

The specific calculation method is to divide the difference between the light intensity I₀ at the center of the detector after the laser passes through the pure medium and the light intensity I_i at the center of the detector after the sample is added by the light intensity I₀.

The shading rate =(I₀ - I_i) / I₀*100%

The shading rate is a direct reflection of the sample concentration in the optical detection path. Usually the best shading rate of laser particle size analyzer is between 10% and 15%. Within this range, there are the following advantages.

1. Sufficient signal strength: Ensure that the detector obtains scattered light signals with high signal-to-noise ratio. Provide a solid foundation for accurate inversion of particle size distribution. Significantly improve the detection ability of submicron and nanoparticles.

2. Avoid multiple scattering: Moderate particle concentration and single scattering dominance can maximize the applicability of the theoretical model. The measurement results can best reflect the true state of the sample.

3. Optimize dynamic range: Balance the scattered light intensity of large and small particles to ensure that the instrument maintains good sensitivity and resolution throughout the entire measurement range (e.g., 0.01 microns to several thousand microns).

Optical concentration is different from the volume (weight) percentage concentration of particles. The former is related to the size of the particles. The smaller the particles, the larger the specific surface area and the stronger the light shielding ability. Therefore, in two samples with the same volume concentration, the smaller particles will show a larger optical concentration in the laser particle size analyzer.