Ballistic measurement systems: spark flash camera and image intensifiers (1974 – 1980)

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 millimeter and a burn time after ignition of 1 microsecond creates the image in the centre of the camera lens. When it flashes, it looks like the flashlight is coming out of the camera. The light reflected by the retro-reflector largely returns to the camera lens.

The principle of the sparking flash camera
The principle of the sparking flash camera

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 returned to idle state.
An additional powerful flash was added afterwards to lighten the surface of the projectile.

Vonkenflitscamera (prototype)
Sparking flash camera (prototype)
Spark flash camera
Spark flash camera
Photo made of an APDS (Armour-piercing discarding sabot) projectile in flight with the spark-flash camera
Photo made of an APDS (Armour-piercing discarding sabot) projectile in flight with the spark-flash camera
Photo made of an APDS-sabot - the carrier of the projectile has come off - with the spark-flash camera setup
Photo made of an APDS-sabot – the carrier of the projectile has come off – with the spark-flash camera setup

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.

Schematic layout of the image intensifier camera
Schematic layout of the image intensifier camera

 
Image intensifier camera 18 mm (1977)
Prototype switched image intensifier camera 18 mm (1977)

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.  

The 40 mm image intensifier camera developed for the CoP
The 40 mm image intensifier camera developed for the CvP (1978)

 

Photo of a 155 SC projectile made with the image intensifier camera
Photo of a 155 SC projectile made with the image intensifier camera