On 11 May, he was hospitalised at RAF Rauceby. The exact reason is unknown, but suggestions include a sinus or middle ear problem. He was then sent on two weeks convalescent leave. This absence meant he was unable to participate in Operation Millennium, the "1000 Bomber raids", the first of which was made on Cologne on 30 May 1942. He found this frustrating because this raid saw the introduction of the Bomber stream. This was where the aircraft were concentrated together in an attempt to overwhelm the defences, with each allocated a specific place, height band, and time slot. This period saw the introduction of aiming-point photography. Gibson tried it out and then encouraged all aircrews to become "photo minded". Obtaining good aiming point photographs quickly became a competition both within, and between squadrons.