Due to the life span of sodium, light bulbs using high pressure sodium light bulbs should be started at a low voltage level, but as they start to warm up, the internal gas pressure located inside the arc tube starts to go up and at that time more voltage is needed if you want to maintain the arc discharge. Like everything else in life the HPS bulb is going to get older and loose its effectiveness. When a high pressure sodium light bulbs starts getting older you will notice that maintaining the v Finish Reading this Post