Oven Lamp usually has two glass color, one is clear glass, the other one is smoked deep color glass.
Why glass oven light bulb is with smoked deep color? it is because that the tungsten filament is covered with a thick layer of red phosphorus, if the oven bulb is lighten on, tungsten filament will create a lot of heating, red phosphorus will be evaporating and adhere to the glass, then the glass looks smoked.
1>. All the lamp is with clear glass, filament is with red phosphorus, before life testing at factory, glass is clear.
2>. After life testing at factory, the glass will change to smoked deep color.
3>. However, because for different lamp, the filament has different amount of red phosphorus, so the glass color is a little different, some bulb maybe a little deep, some maybe a little lighter. This color difference is always there, because so far no one can control the red phosphorus same amount for each lamp.