Particle number and size can also be determined through their light-scattering properties and through the use of nephelometer-based approaches, in which a light beam; typically from a near-IR diode laser is directed into an ambient airstream and any reflected light (from suspended particles) detected perpendicularly to the incident beam. The frequency of scattering events provides an indication of the particle concentrarion, while the intensity of the scattering light is related to the particle size - following assumptions about the shape and refractive index of the particles, an aerosol number and size distribution can be obtained (in practice, particle concentration in a certain number of discrete size bins is usually reported). Optical particle counters permit real time measurement particle size distribution and (unlike gravimetric methods) include any volatile component of the aerosol; however , the assumptions, required to derive the instrument calibration curves dictate that care is required when comparing optical particle counter measurements with, for example, filter or TEOM data, on in any situation where the particle population may vary significantly from the standard used to calibrate the instrument (typically, monodispherse, polystyrene latexspheres). Typical instrument performance is to measure particles over a diameter range of 0.1-10 ไมโครเมตร, divided into 10-20 size bins. Optical particle or dust monitors are frequently handheld units, used to monitor industrial and occupational health-related aerosol exposure, in addition to performing environmental measurements.