New generations of infrared (IR) seekers adopt two or more detectors to define corresponding spectral lines of the target signature. An optical system composed of ordinary optical components (lenses and mirrors) are implemented to focus different incident spectral lines on the corresponding detectors. These ordinary optical components have diffraction losses of considerable amplitude which adds to their heavy weight, manufacturing complexity, and high production expenses. To overcome these drawbacks, a thin film Fresnel zone plate (FZP) lens is proposed to substitute the ordinary optical components in separating and focusing different spectral lines. The technology of FZP lenses is quite compatible with recent fabrication techniques. In the present paper, a theoretical analysis and numerical calculations are given to verify the achievements of the proposed FZP in decreasing the diffraction losses, focusing the incident radiation, and enhancing the performance of the detector. The focusing and spectroscopic characteristics of the FZP are studied with varying the wavelength.