The present invention relates to a thermal donor comprising a dye mixture and a urea light-stabilizing compound, wherein the light-stabilizing compound is represented by structure I: ##STR00001## wherein: R.sub.1, R.sub.2 and R.sub.3 represent a hydrogen atom, alkyl group, cycloalkyl group, alkenyl group, aralkyl group, aryl group or heterocyclic group, provided that at least one of R.sub.1, R.sub.2, and/or R.sub.3 is a hydrogen atom; R.sub.4-R.sub.8 represent a hydrogen atom, hydroxyl group, alkyl group, cycloalkyl group, alkenyl group, aralkyl group, aryl group, heterocyclic group, aryloxy group, or alkoxy group; and R.sub.3 forms a heterocyclic ring with either R.sub.4 or R.sub.8, and an imaging system comprising a donor sheet having a layer containing a dye and an image receiving sheet having a dye receiving layer, wherein either the donor sheet or the image receiving sheet comprises a dye mixture and a urea light-stabilizing compound of structure I.