Because the direct use of tungsten wire heating and heating out of infrared light which most people can not see, so the luminous efficiency is very low in 15 ~ 25 lumens \ tile with high heat. Increased tungsten filament temperature issued visible increase, but the tungsten melting point is only 3000 degrees, so tungsten filament life is relatively short. The advantage is that direct heating light has a low price and easy to use.