  Laser induced line narrowing effect, discovered more than thirty years ago,
can also be applied to recent studies in high resolution spectroscopy based on
electromagnetically induced transparency. In this paper we first present a
general form of the transmission width of electromagnetically induced
transparency in a homogeneously broadened medium. We then analyze a Doppler
broadened medium by using a Lorentzian function as the atomic velocity
distribution. The dependence of the transmission linewidth on the driving field
intensity is discussed and compared to the laser induced line narrowing effect.
This dependence can be characterized by a parameter which can be regarded as
``the degree of optical pumping''.
