Focused ion beam secondary electron images of polycrystalline aluminum films are valuable in revealing the grain structure of the films with great clarity on a scale of < 50 nm. The contrast mechanism for focused ion beam images in crystalline materials was identified ten years ago as due to crystalline channeling of the ion beam. To our knowledge there has been no report which quantifies this mechanism either with respect to the magnitude of the intensity changes or their angular dependence. The "critical angles" of the angular dependence are expected to depend on the source of the secondary electrons, which may be different than the channeled angular dependence of backscattering or sputtering, for example, because of the difference in the distance of approach between the ion and a target atom at which the physical mechanism of interest takes place. The experiment described here quantifies the change in intensity and angular width in the case of channeled 30 keV Ga ions in aluminum thin films. This has been accomplished through a novel combination of focused ion beam images and electron backscatter diffraction measurements. Results of these experiments, and their possible implications for metallurgical imaging, will be discussed.