We have recently developed a narrow band imager (NBI) using an air gap based Fabry-P\'erot (FP) interferometer at the Indian Institute of Astrophysics, Bangalore. Narrow band imaging is achieved by using an FP interferometer working in combination with an order sorting pre-filter. The NBI can be tuned to a different wavelength position on the line profile by changing the plate separation of the FP. The interferometer has a 50 mm clear aperture with a bandpass of \(\sim\)247.8 m\AA and a free spectral range of \(\sim\)5.3\AA at \(\lambda\) = 656.3 nm. The developed NBI is used to observe the solar filament in the H\(\alpha\) wavelength. The instrument is being used to image the Sun at chromospheric height and it is also able to scan the H\(\alpha\) spectral line profile at different wavelength positions. We have also made Doppler velocity maps at chromospheric height by taking the blue and red wing images at \(\pm\)176 m\AA wavelength positions separately away from the line center of the spectral line. In this paper, we present a description of the NBI including lab test results of individual components and some initial observations carried out with this instrument.