Ultra-high-speed framing cameras have long been the workhorse for acquiring elusive data in scientific fields where very fast phenomena are studied, such as high-voltage discharge, crack propagation, detonics, and impact studies. Today these cameras are also increasingly used in the fields of biomedicine, nanotechnology, and space research, where the ability to “freeze” the motion using very short exposure times (typically rates of 500,000 frames per second or more) gives access to valuable new information.
With all of these applications, the quest for detailed analysis means that the images must be of the highest quality and resolution. Once considered temperamental technology, the high-speed camera has been developed into a simple "plug and play" system that can be used by non-experts as well as dedicated users. In this article, we will review the technology behind ultra-high-speed cameras and provide examples of how the tools can be best used.
Technical Developments: A Simplified Optical Path
Traditionally, multiple-channel fast framing cameras have used rotating mirrors, plate glass beamsplitters, or pupil split optical systems to direct the light either to film or individual image sensors. Each of these optical system designs came with disadvantages such as vignetting, parallax, astigmatism, and large instrument size. In the last five years, demand from the scientific community has driven companies such as Specialised Imaging Ltd. to develop a new set of multiple-channel fast framing cameras with technological innovations that address previous limitations.
Using sophisticated optical design tools enabled Specialised Imaging Ltd., for example, to create an optical design where the optical path of its SIM series camera was folded, while maintaining equal transit times on all 16 viewports, resulting in a compact model. As it is not possible to predict the wavelength of light that enters into the beam splitter, special attention was given to the lens design so that the focal plane is coincident for all visible wavelengths (350-900nm).
In the SIM optical design, each of the viewports is populated by a sensor to capture the image. With some fast framing cameras, the sensors are conventional interline transfer charged-couple devices (CCDs), which can be electronically shuttered down to a few hundred nanoseconds. Electronic shutters, however, have limitations in so far as the shuttering signals have to be synchronized to the readout of the sensor, which can result in long delays from an asynchronous trigger to the point at which an image is captured. The very nature of electronic shuttering also means that each sensor can only acquire one short-exposure image. If a second short-exposure image is required, it can only take place after the first has been fully read out from the sensor, which can be up to 200 milliseconds later.
Another high-speed imaging method fits each viewport on the beamsplitter with an 18mm high resolution image intensifier coupled to an interline transfer CCD sensor. Intensified CCD (ICCD) sensors can have a 6μm pitch micro-channel plate (MCP) (see Figure 1), which provides a staggering 50 line pairs per millimeter of spatial resolution. The sensors are also capable of exposures as short as 3 nanoseconds.
ICCDs overcome the timing limitations of interline CCDs because the sensor remains in darkness while the intensifier is gated off, thereby allowing the sensor readout to be re-synchronized as soon as a trigger is received. This arrangement allows the first short-exposure image to be taken as soon as 50 nanoseconds after an asynchronous trigger. Similarly, because there is no ambient light between exposures, the image intensifier can be gated a second time as soon as the charge has been transferred into the interline readout registers to allow two images to be taken in quick succession by each ICCD. In the SIM type of cameras, this time has been reduced to around 500 nanoseconds.