A general arrangement drawing below, shows the spotlights used to judge the height of the aircraft. When the two beams met on the surface the aircraft was at 60 feet.

DC72/28
This modification was designed by Benjamin Lockspeiser, the Deputy Director of Scientific Research at the Ministry of Aircraft Production, who recalled a proposed system to find inshore submarines using aircraft fitted with searchlights. The fitting of the lights was made necessary after the special low-level altimeter developed by EMI was found to be unreliable.
To commemorate the 65th anniversary of the "Dambusters" Raid the Royal Air Force Museum in conjunction with the Newsdesk Media Group has produced a new book "Breaching The German Dams - Flying into History" which uncovers elements of hidden history crucial to the success of 617 Squadron's attack on the German Dams in May 1943. To order your copy go to the Museum Shop.
RAF Museum Shop