LII: Laser Induced Incandescence
Instrument for soot characterization
Laser-induced incandescence (LII) is a technology that offers a reliable means for spatially and temporally measuring the soot (black carbon, elemental carbon) concentration and primary particle size in engine exhausts and in ambient air. It is an extremely sensitive measurement technique that offers unparalleled measured dynamic range and requires no sampling systems, diluting systems, or consumables such as filters. It is also provides real-time measurement (order of 10 Hz) and is suitable for dynamic measurements.
The Artium LII consists of a self-contained rugged optics enclosure, which includes the laser and all components needed for operating the instrument. The optical system consists of a computer-controlled automated laser beam energy detection and adjustment system that maintains the laser light fluence at the sampling volume at optimum conditions. The system automatically maintains constant laser fluence over a wide range of environmental conditions. Incandescence signal is made at 90 degrees to the transmitted beam which helps form a well-defined measurement volume. The incandescence signal is detected by a pair of detectors that use light filters centered at wavelengths of approximately 400 nm and 780 nm. The innovative two-color pyrometry technique permits accurate measurement of soot particle temperature and leads to an auto-compensating approach for soot concentration measurements. Measurement of the soot temperature allows the use of low fluence heating of the soot so that sublimation of soot is insignificant.
A custom 2-channel 12-bit, 500 MHz PCI A/D digital signal acquisition card is used for optimized high-speed data acquisition and transfer to the computer. A USB interface between the LII optics package and the system computer is used for the control functions. Besides measuring the soot volume fraction, the LII signal decay characteristics are also processed to infer the primary particle size.