A porton counting reticle is used to count and measure abrasives and dust particles - most commonly asbestos, paint particles or varnish particles.
In order to determine if dust particles in the air are at a safe level, a sample is collected on a filter that will reflect the concentrations of dust at the time and place of sampling. Once the sample is collected, it is placed under a phase contrast microscope and examined using a porton counting reticle. This must be done within two days of sample collection.
You can learn more about Porton Counting Reticles here.