While performing an aerial application flight, the pilot was tasked with identifying the target vegetation and then spraying the applicant. The pilot had performed a reconnaissance of the general area and was aware of the powerlines nearby. He stated that it was getting late in the day and the sun was now setting to the west. After making a spray run, the pilot pulled up but had misjudged his distance from the power lines and did not see them clearly, possibly due to sun blindness. The helicopter collided with the power lines, impacted terrain, and sustained substantial damage. The pilot sustained head and chest injuries. He stated that his head injuries would not have occurred had he worn a helmet. His helmet had been stolen from his helicopter about 10 days prior to the accident and with only 20 days left to spray opted to fly without one until he could purchase a replacement.