Pain receptors are free nerve endings- nociceptors- that are stimulated by the chemical released from damaged tissue. These receptors alert us for possible danger but they don't display sensory adaptation. Sensory adaptation it's when the response to stimulus decreases even though the stimulus is the same, we just become unaware of it. In pain sensations, that doesn't happen.
From the brightness of the light of course. If the brightness is increasing it means the the light source is approaching and if the brightness is decreasing it means the light source is leaving.
