Sheldon Schultz, a scientist at the University of California at San Diego, has used "plasmon resonant particles" to detect HIV antibodies in blood samples. When particles resonate in this way, they absorb the wavelengths that set off the resonance. This is why stained glass, with its tiny metal particles, absorbs some colors and lets others pass through. His idea springs from an interesting optical phenomenon that occurs with stained-glass windows: Photons send electrons rushing back and forth in the metal, generating an electron wave, or plasmon. Schultz's idea is to coat metal particles with antigens recognized by the target antibodies. If antibodies are present, the particles latch on and emit certain wavelengths. San Diego-based Seashell Technology will make the antigen-coated particles.