My father has just given me a Weston Master III meter in very good condition which seems accurate. Well, it was calibrated recently (1998!).

It comes with the dome for ambient measurement and also with a disc which appears to be a neutral density filter. When this is fitted, the numbers revert back to the same set as when the door is closed and it gives a similar reading to one taken with the door closed.

Can anyone tell me what it is called and what it was used for? i.e. in what circumstances would you use the ND filter instead of the closed door. Different angle of light acceptance perhaps?