Particle Counter is an instrument that detects and counts particles. By its very nature a particle counter is a single particle counter, meaning it detects and counts particles one at a time. The nature of particle counting is based upon either light scattering, light obscuration, or direct imaging. A high energy light source is used to illuminate the particle as it passes through the detection chamber. The particle passes through the light source (typically a laser or halogen light) and if light scattering is used, then the redirected light is detected by a photo detector. If direct imaging is used, a halogen light illuminates particles from the back within a cell while a high definition, high magnification camera records passing particles. Recorded video is then analyzed by computer software to measure particle attributes. If light blocking (obscuration) is used the loss of light is detected. The amplitude of the light scattered or light blocked is measured and the particle is counted and tabulated into standardized counting bins. The image to the right shows a light scattering particle counter diagram. More information about types of particle counters and types of particle detection follow in this article. Direct imaging particle counting employs the use of a high resolution camera and a light to detect particles. Vision based particle sizing units obtain two dimensional images that are analyzed by computer software to obtain particle size measurement in both the laboratory and online. Along with particle size, color and shape analysis can also be determined.thumb|right|380px|Diagram of a Vision Based Particle Counter Applications of particle counters are separated into three primary categories: *Aerosol particle counters *Liquid particle counters *Solid particle counters. (An OSH glossary used in safety and health at work which is, adopted by ILO