Fridge light dims (doesn't go out) when door shut?

We have an oldish (10yo) Bosh Upright Fridge Freezer and we've just noticed that the light in the fridge doesn't go out completely when you shut the door. Instead the filament light bulb just dims down to an orange glow.

I've checked the door switch and that works fine any ideas what the problem could be? Is it possible there is a circuit that keeps a small current going to the bulb just to warm the filament slightly to make it last longer (I'm probably over thinking this).

Edit> So google provided an answer in that it does that when the fridge is in a cold location to stop the fridge freezing up, some bosh fridges have a switch near the light which can be switched off when the fridge is installed in a warmer area like a kitchen. Ours doesn't have a switch but I'm hoping changing the temperature setting to a lower one will have the same effect.