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.
Oven Light bulbs that is used for oven has too be high temperature resistant, usually the temperature of the oven is not more than 300 degree centigrade, that is why the oven lamp is 300 degree centigrade high temperature resistant. High temperature resistant oven light bulb looks same as normal oven lamps, but the filament, lamp base and glue is different from normal oven lamps, all these materials are 300 degree centigrade high temperature resistant.