The threat of bioterrorism has inspired much research into optical techniques that can detect pathogenic aerosols in real time. Now a team of scientists from Yale University in New Haven, Conn., and the Naval Research Laboratory in Washington has developed an elastic-scattering technique capable of differentiating between life-threatening aerosols and normal materials in the background.Figure 1. An elastic-scattering technique distinguishes between pathogenic and benign aerosols. An ellipsoidal mirror collects the green laser light backscattered from the sample and focuses it to the CCD camera. In the experimental arrangement, a Q-switched, frequency-doubled Nd:YAG laser illuminates an aerosol emitted from a nozzle at the top of the apparatus (Figure 1). An ellipsoidal mirror collects the light that is backscattered from the aerosol and focuses it into a CCD camera.Virtually all the backscattered light can be collected, and the intensity distribution as a function of both polar and azimuthal angles can be inferred from the camera image. A diode laser facilitates synchronization of the laser pulse and the arrival of the aerosol in the beam path. A beam block prevents the laser beam from entering the camera, and spatial filtering eliminates forward-scattered light.Figure 2. The setup yields markedly different images of the scattered light from a polystyrene sphere (A) and a Bacillus subtilis spore (B). Figure 2 shows camera images of the light scattered from a polystyrene latex sphere and a Bacillus subtilis spore, which was used to simulate anthrax in the experiment. (Note the scanning electron microscope picture of the aerosol beneath each image.) The two are clearly distinguishable.Fast computer image processing, combined with this experimental technique, could provide reliable monitoring for life-threatening aerosols, the scientists contend.