Many natural and manmade objects contain fluorescent substance. To visualize the distribution of fluorescence emitting substance is of great importance for food freshness examination, molecular dynamics analysis and so on. Unfortunately, the presence of fluorescent substance is usually imperceptible under strong ambient illumination, since fluorescent emission is relatively weak compared with surface reflectance. Even assuming that surface reflectance could be somehow blocked out, shading effect on fluorescent emission that relates to surface geometry would still interfere with visibility of fluorescent substance in the scene. In this paper, we propose a visibility enhancement method to better visualize the distribution of fluorescent substance under unknown and uncontrolled ambient illumination. By using an image pair captured with UV and visible flash illumination, we obtain a shading-free luminance image that visualizes the distribution of fluorescent emission. We further replace the luminance of the RGB image under ambient illumination by using this fluorescent emission luminance, so as to obtain a full colored image. The effectiveness of our method has been verified when used to visualize weak fluorescence from bacteria on rotting cheese and meat.