An Optical Particle Sizer (OPS) is a light, portable, and highly accurate instrument used for the rapid measurement of particle concentration and size distribution, primarily for particles in the micrometer size range. It employs single particle counting technology to provide real-time aerosol data. OPS instruments operate on the principle of light scattering, typically using a 120° light scatter detection method, to detect and size individual particles as they pass through a laser beam. They are designed to measure particles in the size range of 0.3 micrometers (µm) to 10 µm, often offering up to 16 user-adjustable size channels for detailed size fractionation. These devices can measure particle concentrations up to 3,000 particles/cm³ (equivalent to 3,000,000 particles/liter). Data acquisition can be very fast, with measurements as frequent as once per second, providing dynamic insights into aerosol populations. OPS instruments are typically compliant with international standards such as ISO 21501-1, ensuring measurement accuracy and reliability. Optical Particle Sizers find extensive applications across various fields. They are commonly used in filter testing, industrial measurements, and both indoor and outdoor air quality monitoring to assess particulate matter levels. In health effects studies, they help characterize exposure to airborne particles. Their versatility makes them suitable for a wide range of environmental and occupational hygiene applications where rapid and accurate particle sizing is required. Many OPS models are portable and battery-powered, offering extended operation times (e.g., 12 to 20 hours), making them ideal for field studies. They feature built-in data logging capabilities for storing large numbers of samples and often include color touch screens with intuitive user interfaces for easy operation. A significant feature is the ability to collect particles on an integrated filter for subsequent gravimetric or chemical analysis, allowing for complementary characterization. Comprehensive software packages, such such as Aerosol Instrument Manager®, are typically supplied for instrument control, data management, and advanced analysis.