A diffraction grating, diffraction structure or Fresnel zone device is formed on a first substrate for diffracting light components of different wavelengths. An array of detectors is formed on a second substrate for detecting different wavelength components diffracted where the second substrate is spaced apart from the grating, structure or device to form a spectrometer. Spectrometers sensitive to the particular spectral lines may be used for detecting the presence of substances. The spectral resolution at such spectral lines may be increased relative to other regions to enhance the sensitivity of detection. This is done by inverse Fourier transform of the desired discrete spectrum to obtain a desired transmission function and by half-toning the aperture function.