If I were doing this, I’d probably take two small pieces of metal (e.g. sheet metal cut to identical size, preferably aluminum because it has a fairly good thermal conductivity and fairly low heat capacity), attach temperature sensors to the back with epoxy, and paint the front - one black, one white. And put them both on an insulating surface (e.g. Styrofoam). And take them outside to be illuminated by direct sunlight.
Or better yet, paint one black, and leave the other one bare metal. Bare metal has low absorptivity and low emissivity, which means it absorbs very little visible light and IR.
If sunlight weren’t available, I’d try LED lamps. Not flashlights or desk lamps, something more serious. Like a floodlight.