Millimeter wave camera: single-element prototype as demonstration of the technology

Passive Millimeter Wave Imaging System
with White Noise Illumination for Concealed Weapons Detection

Introducing passive mm-wave camera with additional incoherent illumination

While first commercial millimeter wavelength imaging security camera prototypes already come to the market, there are some obstacles on their way. Most important of them which come behind of the cost of the scanners, is relatively long exposure intervals required to get good image on the scanner screen. Why a long exposure required? It is easy to understand the answer if you can consider mm-wave scanner as a usual security camera, but only working in mm-wave band of spectrum. The long exposure is the effect of the fact that a human body emits very light mm-wave radiation while most of it comes at visible and infrared bands of spectrum. So to get a good picture in mm-wave band, you have to wait some time – exactly like first photographers had to wait for seconds to make pictures at the early days of photography.

By analogy with photography, to shorten an exposure in mm-wave band of spectrum, you can just light up an object that you shoot. This obvious solution has the only drawback, - it is not easy to design mm-wave sources of light (so called noise sources) that are true incoherent. Beams are incoherent if the phase relationship changes rapidly and randomly. Otherwise, when lighten up by coherent noise sources, an interference pattern consisting of dark and bright fringes may be formed resulting in practically unintelligible image on the screen of millimeter wavelength imaging security scanner.

To respond to this requirement of incoherent "white noise" mm-wave noise sources, ELVA's pulse noise sources deliver the real incoherent radiation within 5 GHz bandwidth. They employ specially designed silicon IMPATT diodes, operating in a pulse mode. Average 50mW level of output power allows using the device for illumination of targets for the improvement of sensitivity of passive imaging system. The sources are available for all frequencies from 26 to 140 GHz; the typical pulse duration is about 100ns.

Project Description:

ELVA-1 proposes a project on design of passive millimeter wave imaging system (also called “mm-wave camera”) equipped with additional mm-wave spectrum illumination for concealed weapons detection. The camera will work in 94GHz frequency band and be able to remotely scan groups of individuals for metallic and nonmetallic (dielectric) weapons using passive millimeter wave detectors. The 94GHz frequency was chosen because it’s known as atmosphere transparency window in mm-wave band and therefore this frequency is well researched and has many mm-wave components already designed for. 94GHz imaging system allows get image resolution as high as 3-5mm. The imaging system intended to monitor in the mm-wave spectrum the body of an individual and objects that he/she might carry.

Unlike prototypes proposed by other companies, ELVA-1 camera will be equipped with low-powered mm-wave spectrum light sources of ELVA-1 proprietary design. The additional illumination is intended to boost up the quality of obtained images and significantly shorter the time required to create the image. Therefore, it will make much shorter the time to spend on checking of an each individual compare to other existed camera prototypes. The light sources have output power comparable with the power of usual mobile phones to be safe enough for using in public areas with no health risks. The contrasts in mm-wave reflection from objects can be captured in images providing a visual representation of a human body and concealed objects if any of them exist under the clothes.
 

Project Status (May 2004):

ELVA-1 successfully designed a single-element prototype as demonstration of the technology. The prototype includes two mm-wave spectrum light incoherent sources (so called “white noise sources”) and one receiver with scanning antenna. The antenna has a mechanical scanning drive, which allows building a picture of an object in mm-wave spectrum by scanning it in X- and Y- axis. The lab prototype intended to prove an ability to create a mm-wave spectrum picture based on ELVA’s receiver and noise sources (lighting elements).

The principal distinguish of lab prototype from a real operational mm-wave imaging system prototype, which is intended to work on security check points -  is the number of channels (mm-wave receivers). The lab prototype has only one channel, while an operational mm-wave imaging system prototype will have to have a matrix or receivers. The number of receivers will be determined by picture resolution and the technique for creating pictures. In case of still graphic imaging system a matrix of receiver required (exactly like in photo digital cameras) where a one receiver represents a one pixel of the image. For example, for a picture of 100x100 pixels the total number of receivers will be 100x100=10,000. Same time using scanning antenna it could be possible to use just 1-dimensional antenna, and in this example it will be 100 receivers only. The advantage of matrix receiver is image creation speed (like in digital camera, you need just press “Shoot” button and see the picture). The advantage of scanning antenna is its cost as the number of required receivers decreased tremendously.

Current efforts seek to improve the resolution of the system and to develop a functional prototype (with phased array or scanning antenna) suitable for operational evaluation. Delivery of this prototype should occur in 6-8 month after project will receive financing. 

Below there are lab prototype specs, and block-diagrams that illustrate the principle of operation for ELVA mm-wave imaging system lab prototype.

SPECIFICATIONS

Diagram 1. How mm-wave imaging system lab prototype works

Diagram 2. Mm-wave imaging system lab prototype block diagram
 

Photo. A sample target (aluminum foil balls on cocktail tubules) and its mm-wave image