High Speed Photography - Video

Video

Early video cameras using tubes (such as the Vidicon) suffered from severe "ghosting" due to the fact that the latent image on the target remained even after the subject had moved. Furthermore, as the system scanned the target, the motion of the scanning relative to the subject resulted in artifacts that compromised the image. The target in Vidicon type camera tubes can be made of various photoconductive chemicals such as antimony sulfide (Sb₂S₃), lead(II) oxide (PbO), and others with various image "stick" properties. The Farnsworth Image Dissector did not suffer from image "stick" of the type Vidicons exhibit, and so related special image converter tubes might be used to capture short frame sequences at very high speed.

The mechanical shutter, invented by Pat Keller, et al., at China Lake in 1979 (US 4171529 ), helped freeze the action and eliminate ghosting. This was a mechanical shutter similar to the one used in high-speed film cameras—a disk with a wedge removed. The opening was synchronized to the frame rate, and the size of the opening was proportional to the integration or shutter time. By making the opening very small, the motion could be stopped.

Despite the resulting improvements in image quality, these systems were still limited to 60 frame/s.

Other Image Converter tube based systems emerged in the 1950s which incorporated a modified GenI image intensifier with additional deflector plates which allowed a photon image to be converted to a photoelectron beam. The image, while in this photoelectron state, could be shuttered on and off as short as a few nanoseconds, and deflected to different areas of the large 70 and 90 mm diameter phosphor screens to produce sequences of up to 20+ frames. In the early 1970s these camera attained speeds up to 600 Million frame/s, with 1 ns exposure times, with up to 15 frames per event. As they were analog devices there were no digital limitations on data rates and pixel transfer rates. However, image resolution was quite limited, due to the inherent repulsion of electrons and the grain of the phosphor screen. Resolutions of 10 lp/mm were typical. Also, the images were inherently monochrome, as wavelength information is lost in the photon-electron-photon conversion process. There was also a fairly steep trade-off between resolution and number of images. All images needed to fall on the output phosphor screen. Therefore, a four image sequence would mean each image occupies one fourth of the screen; a nine image sequence has each image occupying one ninth, etc. Images were projected and held on the tube's phosphor screen for several milliseconds, long enough to be optically, and later fiber optically, coupled to film for image capture. Cameras of this design were made by Hadland Photonics Limited and [http://www.cordin.com Cordin Company. This technology remained state of the art until the mid 1990s when the availability of CCD image capture enabled instant results in digital format.

In addition to framing tubes, these tubes could also be configured with one or two sets of deflector plates in one axis. As light was converted to photoelectrons, these photoelectrons could be swept across the phosphor screen at incredible sweep speeds limited only by the sweep electronics, to generate the first electronic streak cameras. With no moving parts, sweep speeds of up to 10 picoseconds per mm could be attained, thus giving technical time resolution of several picoseconds. As early as the 1973-74 there were commercial streak cameras capable of 3 picosecond time resolution derived from the need to evaluate the ultra short laser pulses which were being developed at that time. Electronic streak cameras are still used today with time resolution as short as sub picoseconds, and are the only true way to measure short optical events in the picosecond time scale.