Improved image resolution for high-speed photography

As the old saying goes, there are many ways to skin a cat. Similarly, there are a plethora of ways to perform high-speed photography, many of which could no doubt provide detailed information on the most rapid and effective means of liberating that proverbial cat of its furry pelt. A new concept in high-speed photography provides both high speed and high image quality. The technique, recently demonstrated by researchers in Jordan and the United States, relies on the precise timing of dual-cavity lasers and dual-frame cameras, and the ability of each camera to record images at only selected wavelengths. Their system can record multiple, high resolution frames at rates up to 200 MHz¹.

One method of recording high-speed images of an event is to move the image rapidly across the recording medium. This can be accomplished by using a rotating mirror to move the projected image across a strip of photographic film. Alternatively, the film itself can be moved at high speed while the image remains fixed in space. Another variation is to use multiple cameras positioned so that they all observe the event along the same axis. Each camera is triggered in turn, thereby recording a history of the event. Along with the requirement for precise timing of the cameras, the duration of each exposure must be sufficiently short to capture the image without it blurring due to motion.

One of the fastest electronic "shutter" techniques is provided by image intensifiers, which incorporate micro-channel plates capable of exposure times as short as 1.5 ns. Image intensifiers are a necessary requirement in many applications because of low light levels during the brief time they record. The problem with intensifiers, however, is their limited resolution. These researchers wanted the superior image resolution provided by multiple, non-intensified cameras, therefore they needed to find an alternative method of controlling the exposure times.

The approach they took was to use a laser to produce brief flashes of illumination for each camera to take a snapshot in time of a high-speed event. By using a rapid succession of laser pulses and multiple cameras, it is possible to record a sequence of images of an event over time, with inter-frame separation dictated by the time between pulses. However, without any means of shuttering the cameras at the high speeds possible with an image intensifier, a camera might be exposed more than once by multiple laser pulses. They therefore used a train of laser pulses, each with a different wavelength, together with wavelength selective optics so that each camera was only sensitive to one of those wavelengths. In this way, light from one pulse would expose only one camera and no other.

They used a dual-cavity Nd:YAG laser with harmonic generation and an optical parametric amplifier (OPO), to produce 5 ns duration pulses at one of four unique wavelengths. The cameras they used were dual-frame CCD (charge coupled devices) cameras. These are commonly used for particle image velocimetry (PIV), in which they record two frames in quick succession. They were selected for this work not only because it is a mature technology, but because they also enabled the frame rate to be doubled. Four, full resolution image frames could be recorded at 200 MHz or eight image frames could be recorded at 8 MHz.

1) A novel ultra-high speed camera for digital image processing applications, A Hijazi and V Madhavan 2008 Meas. Sci. Technol. 19 085503 (11pp) doi: 10.1088/0957-0233/19/8/085503