A variety of manufacturing environments require strict control over the presence of foreign debris in the air. Semiconductor manufacturing, for example, has long required “clean-rooms” that use extensive air filtering to reduce the number and size of particles in the air to some acceptable level. Other manufacturing environments have similar but distinct requirements. For example, in pharmaceutical and medical device manufacturing environments, hospitals, and food processing or preparation environments it is critical to control not only the number of particles in the air, but minimization of biological particles is also of particular importance. Microbial contamination, for example, can render an entire batch of pharmaceutical product unusable leading to significant monetary losses in the manufacturing process. Accordingly, it is advantageous to have instantaneous detection of contamination events, including instantaneous information about whether a contamination event is biological or non-biological, during the manufacturing process for pharmaceuticals or medical devices. Such a capability is also advantageous in post offices or other government facilities, which may be targets of biological or chemical terrorist attacks.
Various detectors have been designed to detect fluid borne particles by measuring the amount and directionality of light scattered by particles in a sampling area. Some of these detectors are described in U.S. Pat. Nos. 5,646,597, 5,969,622, 5,986,555, 6008,729, 6,087,947, and 7,053,783 all to Hamburger et al, and U.S. Pat. No. 7,430,046 to Jiang et al. These detectors all involve direction of a light beam through a sample of environmental air such that part of the beam will be scattered by any particles in the air, a beam blocking device for transmitting only light scattered in a predetermined angular range corresponding to the predetermined particle size range, and a detector for detecting the transmittal light. An alarm is actuated if the light detected at the detector is above a predetermined level. Some of these detectors, for example the detectors described in U.S. Pat. No. 7,430,046 to Jiang et al., also use the measurement of fluorescence exited in measured particles by illumination with source light to classify measured particles as biological or non-biological.
Conventional particle detectors that rely on scattering or fluorescence measurement confront a common set of challenges. For example, in detectors or particle counters that are designed for detection of scattered light, the scattered light signal must be extracted from the incident illumination light source signal. This involves detecting a weak signal (scattering from small particles) from a very noisy background (glare from the laser source). Additionally, conventional particle detectors that provide simultaneous scattering and fluorescence measurements must be able to (1) detect enough of the florescent light to result in a usable signal and (2) separate out the signal generated by detected fluorescent light from other electrical or optical noise in the system.
Some of these problems are exacerbated when the particles to be measured are suspended in some fluid other than air or a gas having a low index of refraction. If the particles to be measured are suspended in water, for example, which has an index of refraction higher than air, a variety of effects can conspire to reduce the amount of scattered or fluorescent light available for measurement. Wave guiding within the fluid line, for example, can trap light being fluorescently emitted or scattered at high angles. Additionally, Fresnel reflections at the fluid-air boundary or the boundary between the fluid line and the surrounding air can further reduce the amount of light available to measure. Additionally, the corners of a fluid conduit having a rectangular cross section or the curved surface of a fluid conduit having a circular cross section can cause lensing effects and other optical aberrations that degrade the signal to noise ratios at the detectors.