Generally, a spectrometer uses a diffraction grating to spread the spectral components of a light incident on the diffraction grating on a one-dimensional detector. The detector is composed of pixels linearly distributed along a line. However, the diffracted light is not spread linearly on the detector which means that the wavelength on pixel 1 is not linearly correlated with the wavelength on pixel 2. The resolution is therefore not constant for all the spectral range of the spectrometer. To know which pixel corresponds to which wavelength, a nonlinear calibration process must be applied to get a linear relation between the diffracted light position and the wavelength of this light. The new spectrometer that we present uses a special lens configuration name F-sin(theta) lens to provide a linear relation in the image plane between the pixel position and the wavelength of the light. The f- sin(theta) characteristic is defined as the difference in percent between the design position of the wavelength in the image plane and its ideal position, if the lens is perfect. The characteristic of the lens used in the spectrometer is better than 0.1%. The concept can also be used for WDM and DWDM applications.