Observations of print sample measurements have shown that some inks are more prone to noisy measurements than other inks. After investigation it was found that these inks exhibited behavior that may be described as luminescent. More precisely, the measured density of these inks is higher when the samples are stored in the dark and lower when the samples have been exposed to light—either from recent testing or from ambient office light. For example, a sample that is tested in a Xenon light chamber then removed for measurement, will measure lower density if measured immediately after removal compared to a measurement several hours or a day later. This study identified which inks were sensitive to light exposure, the magnitude of their response to ambient light, and their recovery in the dark.