When particles are a few hundreds of nanometers, they are sufficiently large to be detected without enlarging them. An instrument suitable for that is an optical particle sizer (OPS). An OPS is a light and easily-transportable instrument, which provides fast and accurate measurements of particle number concentrations and particle size distributions by using single particle counting techniques. An OPS can count particles between 300 nm and 10 µm and therefore extends outside of the measurement limits for an SMPS system, which often has an upper limit of 1 µm. Often OPS systems and SMPS systems are combined, offering a measureable range from 2 nm to 10 µm.