Shining a light on intensified imaging
Keely Portway investigates how the latest developments in specialised intensifier cameras have allowed opportunities for a number of exciting specialist applications in low-light scenarios
Image intensifiers help to increase the intensity of the available light in a system, which allows better image reproduction in low-light scenarios. This specialist technology is used in a range of applications, including biotechnology, industry, research and astronomy.
Image intensification works thanks to intensifier tubes that use a loop wave. These convert photons into electrons, amplify the electron signal and then reconvert the electrons back into a photonic signal.
The technology is not a new concept – in fact, one of the first documented references of an image tube dates back to 1928, although at that point no product was developed. More recently, and – importantly – successfully, Dr Emil Ott developed the PCO advanced imaging concept in the early 1980s while conducting research at the Technical University of Munich. This led to the development and launch of PCO (now part of Excelitas Technologies) as a business and the introduction of its first image-intensified camera.
Image intensification based on intensifier tubes became the go-to way to extract valuable information out of just a few emitted photons – until fairly recently it was the only method available to do so. But, with advances in camera technologies, other methods of achieving this have come to the forefront, such as EMCCD and sCMOS, which have proven popular thanks to their high image quality and resolution, and ease of handling.
But there are still applications for which intensification is the only viable technology, says Gerhard Holst, senior imaging product and application scientist at Excelitas. He said: ‘Intensified cameras are still the only cameras that allow for photon detection in a few nanoseconds, as EMCCD and sCMOS cannot cover this range.’
High-speed imaging of an exploding titanium wire by means of a hsfc pro (predecessor of the pco.dicam c4); the purpose was the estimation of the temporal evolution of the plasma
These applications are particularly specialist, often niche applications, for which a great deal of thought must go into the kind of technology required. Holst elaborated: ‘We consider our cameras to be not only cameras, but also measuring instruments. Modern scientific cameras such as the pco.dicam series all have different features and we try to explain these features so that our customers can select exactly the camera they need for their particular application.’
The pco.dicam family of nine MCP image-intensifier camera systems offers extremely short exposure/shutter times down to 4ns. The appropriate spectral behaviour can be selected by the photocathode material of the MCP tube.
Journey into space
One example of a particular application for intensified imaging is the Ernst-Mach-Institute for Short Time Physics, based in Freiburg, which was commissioned by the European Space Agency to investigate the security shields of an unmanned person carrier for the International Space Station. Holst explained: ‘There are small emergency modules from which people could leave the station and return to Earth. They have to cross a layer of space debris that is flying around, and since it is outer space, it is flying pretty fast because it is not delayed by any atmosphere. So these carriers have a two-layer security shield construction in case they get hit. They can do simulations of what happens with the energy generated if such a very fast moving metal piece hits a shield, but they have to verify this.’
Impact of space debris on the shields of the unmanned emergency carrier of the ISS (hsfc pro, predecessor of the pco.dicam c4). Image credit: Ernst-Mach-Institute for Short Time Physics, Freiburg
The researchers therefore constructed an experiment using a gas gun to shoot an accelerated metal sphere onto the shield construction, and then imaging what was going on. Holst continued: ‘Because the object hits the shield construction at such a high speed, the exposure times need to be down there in the nanosecond range, which is extremely short. Then, even with a four channel system, they could obtain a slow motion sequence with nanosecond separation, and they could prove that the distribution of this energy happens the way they have simulated it.’
Another example of such a specialist application surrounds electric power lines that occasionally, for various reasons, may need to be switched on or off. ‘The problem,’ said Holst, ‘is that if you want to switch it on or off, the energy flowing is larger than the free air resistance. So, even if you open the switch, the current is still flowing, because the air is not sufficient as an insulator to interrupt that.’
Because of this, special gases are blown using an air pump to extend the arc, to help reduce and break down the energy. ‘So the behaviour and the control that the switches perform needs to be investigated with intensified imaging cameras, so people can safely control what they’ve done to interrupt or stop the current,’ explained Holst.
Likewise, Holst has seen situations with its research facility customers that deal with high-power fuses that are fast acting to protect semiconductors from melting if something goes wrong. ‘They have a similar problem,’ he said. ‘They need to know if the design of the fuse does its work properly, and since the melting process is extremely fast, they need a fast or short exposure.’
Also in research facilities, where it is common practice to drill holes or cut metal with a high energy laser, researchers want to know what happens in the cutting process because there is the possibility of ionised gases escaping out of the hole. ‘If they want to know what is going on,’ said Holst, ‘they need an extremely fast exposure time to get a snapshot of it. There are so many different needs and applications.’
Looking to the future, will intensified imaging cameras develop further and open up more use cases? Holst believes that yes, the technology can still improve, but that it would only be minor improvements. ‘But for some applications,’ he said, ‘the minor improvements would have a major impact. It might be the difference between “yes, I can measure” and “no, I cannot.” As an example, with the current pco.dicam series of image intensifier-based cameras, we also offer the use of the smaller microchannel plate image intensifier, whereas previous models had a 25mm diameter photocathode. And the larger area in general, electrically speaking, in most cases has a larger capacitance, which means it gets slower – we are talking about nanoseconds, so it’s still pretty fast, but if a system has a larger photocathode, it’s naturally a little bit slower. Whereas the smaller image intensifiers can be operated faster. We have some good data that with the 18mm, we can offer a two nanosecond exposure time. So you might ask “well, three or two, what difference does that make?” For some applications it is the difference between yes or no.’
Photon Lines has been the official channel partner of PCO since 2003, when its French parent company, Photon Lines SAS, extended its operation to include the UK and Ireland. David Gibson, managing director of Photon Lines, stated: ‘The relationship has been very successful over the years (including for our French office, who first introduced us), and from those early beginnings we have steadily added complementary photonic products to form the extensive portfolio we have today. This includes cameras operating from the UV, through visible, extending right out to 11 microns in the thermal infrared region. These days we even offer hyperspectral imaging solutions across this entire wavelength range. We will always be indebted to PCO for the confidence they have shown from the early days, the experience has been a stimulating and extremely enjoyable one.’
Find out more about the pco.dicam series of intensified imaging cameras and help in selecting the right camera for the right specialist application by downlaoding the latest white paper.