Spark flash camera and image intensifiers (1974 – 1980)
Around 1974, the spark flash system was designed for the Commission of trials of the Royal Netherlands Army to photograph projectiles in flight. The schematic design of the camera system is shown in the accompanying diagram. The camera C, the flash unit F and the semi-transparent mirror 50/50 form one entity. The flash F with a source diameter of 1 millimetre and a burn time after ignition of 1 microsecond (µs) creates the image in the centre of the camera lens. When ignited, for the camera it looks like the flashlight is coming out of the camera lens. The light reflected by the retro-reflector largely returns to the camera lens (the material property).
The mechanical shutter of the camera is opened during the flash. If a projectile in the flight (perpendicular to the plane of the drawing) passes the camera, the shadow image thereof is displayed together with the shock wave pattern. The projectile is detected with two light gates. An electronic ‘ear’ activated the system when a shot occurs. Several seconds later, the system returns to the idle state.
An additional powerful flash was added afterwards to lighten the surface of the projectile.
The spark flash camera requires scaffolding to shield the sunlight. That disadvantage could be solved by replacing the mechanical shutter with a very fast electronic shutter. In 1977, a prototype switched image intensifier was developed as a shutter. With a detection surface of 18 mm in diameter, photographs were successfully taken of projectiles in flight in full daylight. The lens (F/O) projects the image of the projectile onto the front side of the image intensifier (S). During 1 microsecond, the image is admitted to a microchannel plate (MVP) which can amplify the image up to 50,000 times. That image is presented at the rear of the image intensifier (S). Using a lens group (L), the image is projected on film (C). The image intensification is needed to be able to adequately illuminate the film given the very short exposure time.
The shutter action of 1 microsecond is controlled by an optical detection gate placed under the projectile trajectory.
The image intensifier camera is placed on a flash with a point-shaped light source, just like with the spark flash camera. The additional flash that allows for shadow and shock wave photographs can be removed. The limited resolution, however, was a disadvantage compared to the photos taken with the spark flash camera. A prototype with a 40 mm diameter detection surface gave better results.