This Small Business Innovation Research (SBIR) Phase II research project will construct a detector with the capabilities of broadband imaging in the far infrared to terahertz band. The far infrared (FIR) to terahertz (THz) band of the electromagnetic spectrum has recently opened up with the proliferation of sources in this regime. However, the detector systems available on the market for this spectral region are currently expensive and inflexible. The research is centered on the study of a specific material that will convert the thermal imprint of incoming THz radiation into a visible, wavelength dependent signature that is analyzable by a detector and specialized software. A scanning system based on this detector combined with a tunable source will be designed for use as a security/inspection system. The research will incorporate this detector, capable of imaging a wide spectrum of FIR-THz radiation sources with sensitivities better than current technologies at a fraction of the cost, into a scanner system that can scan small parcels, bags and humans to identify hazardous materials or contraband.
As researchers and industries increasingly exploit this previously inaccessible portion of the electromagnetic spectrum, the need for a better imaging diagnostic tool becomes ever more important. A less-expensive, more sensitive imaging detector of FIR-THz sources is necessary before real-world applications, such as in medicine, become widespread. The realization of this particular application will impact the security and non-destructive testing markets.
The principle of operation of T-camera™ is based on the property of thermochromic liquid crystals (TLC) to change color when heated. The molecular structure of TLC changes with temperature, producing a change in the material's optical properties hence, by imaging the TLC surface with a color video camera one can determine the temperature map of the surface. TLC materials have been in use as a CO2 laser diagnostics for decades, but as a very rough gauge of laser beam position, not as a user friendly or calibrated tool. Inspired by the use of the TLC sheets as an alignment tool at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory, RadiaBeam decided to develop the T-camera as a fully featured, broadband beam profiler. The prototype development was funded by an NSF SBIR grant. The T-camera prototype specifications are presented in Table 1. Since the TLC response is purely thermal, there is no fundamental limitation on the wavelength range in which the T-camera is sensitive, and the very same device can be used to image radiation beams in a very broad spectral range, depending only on the limits of the absorber attached to the TLC sheet. Thus, the T-camera can image from millimeter range to UV. Our intended primary application for the T-camera, however, is imaging of FIR, sub-millimeter and millimeter wave sources, since sensitive and inexpensive solid state detectors already exist in the visible to UV range. The main features of T-camera differentiating it from the state-of-the-art pyroelectric viewers optimized for the same spectral range are: • High sensitivity in a pulse regime; • Large field of view (FOV); • Inexpensive, easily replaceable components. The input radiation beam is delivered towards the sensitive layer, which is a thin absorptive material on which a TLC sheet is attached. The other side of the sensitive layer is attached to a thermally stabilized transparent water chamber that allows for back-view geometry. A computer controlled system is used to maintain the temperature within the range where a particular TLC material exhibits the strongest optical response to a minimal change in temperature. By imaging the illuminated TLC sheet with the CCD camera, one can obtain a transverse profile of the incoming radiation beam. The observation of the change in color on the TLC layer of the sensor element is performed with a color CCD camera. The camera currently used was chosen for its small footprint and its IEEE 1394 interface (compatible with most notebooks and computers for ease in image acquisition). To properly interpret and manipulate the image as well as to control the temperature stabilization of the camera, proprietary image analysis and camera control software was developed at RadiaBeam. The RGB image is converted to hue, saturation, intensity (HSI) space. The saturation and intensity channels are discarded and only the hue is converted to temperature; this requires the camera specific calibration polynomial to be executed. Each hue pixel is passed through the calibration polynomial to obtain the temperature. This sophisticated image processing along with ultra high temperature stabilization allows for sensitivity enhancement of two orders of magnitude or more; making this product extremely competitive in the THz detector market. Other features include background subtraction, artificial high/low frequency noise filters, and gamma, gain, black level adjustments. The image extraction and analysis software is self-contained, uses a user-friendly graphical user interface, and has cross-platform compatibility. A new version of the T-cam has been recently developed with collaboration with Cal State LA. The new version of the T-cam was designed to be more compact and user friendly as well as more thermally stable. RadiaBeam is continuing development and working to further improve the design, which includes even greater size reduction, component optimization and extensive thermal simulation. ?
