An image acquisition instrument that is designed for non-invasive in vivo use, to detect bioluminescence, which is based on the expression of luciferase, the light-emitting enzyme of the firefly Photinus pyralis. After the administration of the substrate luciferin, an ATP- and O2-dependent photochemical reaction occurs, resulting in the release of photons by living cells containing luciferase. This photon emission can be detected by a cooled charge-coupled device (CCD) camera, minutes after the administration of the substrate.