The performance of an atomic clock or magnetometer based on CPT is determined by the resonance width and the signal-to-noise ratio. In particular, a large signal amplitude results in a high stability for a clock or a high sensitivity for a magnetometer. The signal amplitude is typically evaluated by the "contrast," either as an "absorption contrast" (which we define as the ratio of the change in light absorption on resonance to the absorption off resonance) or as a "transmission contrast," (which we define as the ratio of the change in light transmission on resonance to the transmission off resonance). In many initial experiments investigating CPT resonances, the resonance contrast was very small: typically the light absorption changed by only about 1 % when the resonance was excited. Since in principal an absorption contrast approaching 100 % is possible, it is of value to investigate ways to increase the absorption contrast and thereby improve the stability of CPT atomic clocks.