Differences between Laser Spectrometers and Laser Particle Counters
Laser spectrometers are designed to provide aerosol particle detection with very high size resolution of particles detected. In order to provide the level of size resolution, many size bins are provided to collect the particle data. Size detection is often provided by a laser tube or solid state laser. Both devices have their advantages, yet many tools today use the laser diodes for the high levels of power provided and the small foot print. The downside of laser diodes is that the laser energy is often tri-modal, spread across a primary wavelength and several side lobes of lesser energy. Laser tubes may not offer the typical energy of a laser diode, but the energy is contained in a narrow wavelength of light. A differential mobility analyzer (DMA) can also be incorporated in place of a laser particle counter, but requires a sophisticated level of control. The DMA is based on mass analysis and has advantages of size accuracy over a laser particle counter. Airflow must be accurately controlled at a constant rate of flow. Air pressure changes would affect airflow measurements and air flow control; thus, pressure compensation would be important to consider. Particle counting can be accomplished using a laser particle counter or condensation particle counter. The laser particle count will have a more narrow size range, but be reasonably accurate in size response; while a CPC will be less expensive and offer no size data.
A laser particle counter was designed with laser light tubes in the 1990 time frame, but has since began using solid state lasers for the advantage of power. The same advantages and dis-advantages for laser diodes and laser tubes are applicable with a laser particle counter. The number os size bins and the level of particle size resolution is significantly less than with a aerosol particle spectrometer. As laser diode power has increased, particle size detection sensitivity began to drop from 1 micron to 100 nano-meters (NM), then down to 50 nm, more recently going to 10 nm and now down in the size range of 3-5 nm. In addition, the laser particle counter has shrunk in size from large desk top units down to portable, hand held laser particle counters. The larger versions of the LPC offer higher size resolution and more data bins; while the handheld portable laser particle counters offer a very small package, battery operation; but with few size bins and poor size resolution. Airflow control is important, but is not well controlled in portable laser particle counters. Yet, the industry makes use of the portable LPC being used more as a portable monitor.