A long-range surface plasmon resonance-based tunable optical filter is described. An attenuated total reflection device can be configured so that the reflected intensity of light is a maximum when the resonance condition is achieved. When the device is illuminated with polychromatic light at a fixed angle, the wavelength at which resonance is achieved shows a transmission maximum, with a full width at half maximum of less than 3 nm. By using an appropriate electro-optic material in the device, the wavelength of maximum transmission can be varied. Numerical results are presented. © 1997 Society of Photo-Optical Instrumentation Engineers.