- Millimetre wave radiation can be used to see inside objects and reveal hidden information.
- Improving the performance of millimetre wave illuminators and detectors while keeping the devices compact has proven to be challenging.
- Dr Pranith Byreddy and Dr Kenneth O at The University of Texas at Dallas, USA and Dr Wooyeol Choi at Seoul National University, Korea have developed pixels including an illuminator and detector with improved sensitivities as well as a lensless imager using these pixels.
- They were able to create portable millimetre imaging devices that can see objects as small as 1 mm through a cardboard box.
With the right kind of electromagnetic radiation, it is possible to see the insides of objects as easily as their outer shape. We are all familiar with the use of x-rays to see hard tissues like bone inside our bodies, but other kinds of electromagnetic radiation, like millimetre wave radiation, can be used in a similar way.
Millimetre wave radiation is particularly good at penetrating materials like plastics and building materials, so it can be invaluable in locating objects such as wires, pipes, or wooden beams hidden in walls. With the right imaging device, it is possible to see objects down to 1 mm in size. There are already some applications making use of millimetre wave radiation, such as passenger screening in airports. The resulting images show hidden objects but still maintain the privacy of the person being scanned.
To achieve the current performance, the team increased the signal-to-noise ratio by more than one hundred million over 15 years of research.
There’s another viable application for millimetre wave imaging devices that lies in the realm of security as well. Achieving a 1-mm spatial resolution would mean that it is possible to look at the contents of a package or an envelope without unsealing it and in a way that is completely non-destructive. Furthermore, it is also possible to ‘hide’ authentication markers that are invisible to our eyes but can be seen using a millimetre wave scanner as a way of preventing forgeries and helping to ensure goods are authentic.
While millimetre wave imaging is undoubtedly a useful tool for a variety of different applications, there have been some technical challenges with making compact devices which can rapidly acquire high-quality images that allow us to identify objects. Dr Pranith Byreddy and Dr Kenneth O at The University of Texas at Dallas, USA and Dr Wooyeol Choi at Seoul National University, Korea have been working to improve some of the detector capabilities for millimetre wave imaging devices. Their aim was to make compact imaging devices that do not require lenses or optics.
Seeing clearly
Every imaging detector or camera creates some inherent noise. While this noise is often not noticeable on cameras that work with visible light, such as standard smartphone cameras, the detector noise can be a problem when operating in low-light conditions. In cameras for millimetre wave detection, the signal-to-noise ratio has been poorer than their visible equivalents. Moreover, it has been hard to design imaging devices with a wide antenna bandwidth that can capture a wide range of frequencies.
The team found a way to make a lensless device that can be used for imaging objects that are about 1 cm away.
Byreddy, Choi, and O, alongside other colleagues, have overcome many of these limitations by redesigning the individual pixels that make up a millimetre wave device. The team created tiny 0.5 mm by 0.5 mm pixels, each of which has a signal source, a detector, and an antenna. The output of each pixel can be combined with other pixels to improve the overall image quality.
To achieve the current performance, the team increased the signal-to-noise ratio by more than one hundred million over 15 years of research. Now, the team have created tiny integrated circuits so compact that they can be incorporated into handheld devices like the backs of smartphones.
Part of the challenge the team faced when designing a device that would be thin enough to go into a smartphone was that they could not use any external optics. Many types of cameras use things like lenses to improve the image focusing and capture the smallest visible objects. However, to make a device sufficiently small that it could be integrated into a smartphone, the team did not have enough space to include any optical elements.
Integrated design
As well as making pixels with much better sensitivity that could easily be incorporated into large imaging arrays, the team found a way to make a lensless device that can be used for imaging objects that are about 1 cm away. While it might seem advantageous to have a larger operating distance for a handheld imaging device, for millimetre wave radiation there are privacy and security concerns about making devices that have too large operating ranges.
The team’s device can image objects up to about 2 cm away. The researchers are now working on improvements to further increase the sensitivity and signal to capture images of objects up to 20 cm away. They also aim to design pixels that will function with a slightly higher operating frequency – using shorter wavelengths of electromagnetic radiation – as this will help to improve the resolution even more and make it easier to see even smaller objects.
The team is optimistic that their new technology, which is based on the CMOS technology found in nearly all electronics we use in our daily lives, not only offers better performance but also a route to building robust, affordable devices.
What inspired you to conduct this research?
Kenneth O: I was inspired by Superman’s x-ray vision – his ability to see through physical objects as if they were invisible.
How many pixels do you think you can feasibly join together?
We think we can feasibly join together between 1,000 and 10,000 pixels. This will make it easier to use imaging devices and increase the speed and robustness of image capturing.
What do you think will be the main applications of millimetre wave imaging in the future?
Besides the ones mentioned already, we see much potential in the following applications: automotive radar imaging for safety, detection of people, and medical applications.
How do you propose the privacy concerns of these technologies can be addressed?
In order to maintain privacy, we need to limit the range of imaging devices to ~20 cm by restricting their performance. This would mean that people will be aware of the presence/operation of such devices.
