Depending upon your door, frame or wall thickness you may need an extension piece. This extension will increase the overall length by between 10-20mm.
To determine what extension you need measure the door, wall or frame thickness you require and see what the overall requirement is. If you need to add between 10-20mm to the overall viewer length then this is the extension for you.
If you need to add up to 10mm - see AL1210BR
If you need to add up to 55mm see - 160DVEXT
If using a viewer with a door knocker add 10mm to the overall length required.
Product Ref: AL1220BR