Optical systems are normally usable over a restricted spectral waveband. A refractive optical system will only transmit radiation over a limited range of wavelengths, and it may produce well color-corrected images over only part of this transmission band. This paper discusses the design of lens systems for use over the extended waveband from about 0.4 gm to 12µm, which encompasses the visible, the 3 to 5µm mid-infrared, and the 8 to 12 um thermal wavebands. Discussion is given to the available optical materials, including glasses formed by chemical vapor deposition and crystalline materials. The relationships between the refractive and dispersive properties required for wideband color cor-rection are formulated, and several designs are described that use two or three optical materials. Some discussion is given to the coatings required for such optics, viz., ultrawideband antireflection, mirror, and beam splitting coatings. The potential use of this type of optical system is in multisensor applications such as dual visual/thermal observation systems, perhaps employing staring array technology and/or CO2 laser incorporation.