There are two ways to count pollen, one that requires a skilled operator and the other that is fully automated. But essentially the two approaches achieve the same end.
Let’s first look at how manual pollen counting is done. While laborious, it's a well-established method that's still in widespread use. A short video describing the manual pollen counting method is here.
The manual method uses an air-sampling device called a Burkard spore trap to capture airborne pollen on a glass slide. A dye is used to stain any pollen on the slide so it can be counted by a skilled operator using a microscope. When a pollen monitoring site is active, the operator removes the slide from the trap at the same time each day, stains and counts it and identifies the different pollen types based on their distinctive shapes. For each identified pollen type, a report is generated of the average concentration of that pollen over the previous 24 hours. This information is used when generating the pollen forecasts for the next few days.
In principle, the automated method is the same as the manual method except that there isn’t a skilled operator. In the automated pollen counter, air drawn into the machine passes by a series of imaging devices that measure any particles that are it. Sophisticated computer programs then use these measurements to classify the different particles and identify any pollen grains. Automated pollen counters are sometimes described as being ‘real time’ because pollen is identified as it passes through the machine, rather than a day after the slide has been removed.