10.1117/2.1201208.004406 Optical up-conversion enables capture of millimeter-wave video with an IR camera Richard Martin, Christopher Schuetz, Thomas Dillon, Daniel Mackrides, Peng Yao, Kevin Shreve, Charles Harrity, Alicia Zablocki, Brock Overmiller, Petersen Curt, James Bonnett, Andrew Wright, John Wilson, Shouyaun Shi, and Dennis Prather Combining a distributed aperture with optical up-conversion generates a video-rate, passive millimeter-wave imaging system that provides situational awareness in degraded visual environments. Millimeter waves (mmWs) are electromagnetic signals with frequencies between 30GHz (10mm wavelength) and 300GHz (1mm wavelength). They are longer in wavelength than IR and terahertz signals but shorter than radio waves. Most objects naturally emit weak amounts of mmWs, much like IR radiation or heat. In addition, the atmosphere provides high thermal con- trast for reflective objects at mmWs in all weather conditions, day and night. Signals at these long wavelengths are able to penetrate many materials, including clothing, most building materials, and atmospheric conditions such as fog (see Figure 1). However, unlike x-rays mmWs are completely safe and non- ionizing to human tissue. These advantages have been driving the desire to develop a real-time imaging system that operates at mmWs for surveillance and security applications. 1 Our eyes cannot ‘see’ mmW signals. To detect them, we at Phase Sensitive Innovations (PSI) Inc. 3 and the University of Delaware (UD) have developed sensitive imaging systems capa- ble of capturing imagery in the Q-band (33–50GHz) and W-band (75–110GHz) mmW regimes. These systems can be used for all- weather navigation, situational awareness in degraded visual environments, and stand-off detection of contraband and im- provised explosive devices. As an example, Figure 2 shows a picture of a UD laboratory taken with a W-band, single-pixel scanning imager. The imagery is intuitive and the modality can be completely covert, unlike radar or laser detection and ranging images. Figure 1. Atmospheric attenuation curves from 10GHz to 1THz un- der various levels of relative humidity (RH) and fog. Plot was gener- ated using atmospheric codes developed for the North Atlantic Treaty Organization and notes some of the atmospheric ‘windows’ used for imaging. 2 The diffraction-limited resolution of an imaging system is proportional to /D, where  is the wavelength of the radia- tion collected and D is the width of the imaging aperture. In mmW imaging, the wavelength of the radiation is more than a thousand times larger than that of visible light. Therefore, the imager aperture needs to be larger than a traditional camera to obtain suitable resolution. Traditional imaging approaches use optics and a focal plane array (FPA) to produce the im- agery. An FPA requires a lens, larger volume (particularly with the larger aperture size needed for an mmW imager), and an expensive mmW detector for each pixel. In contrast, the new approach we developed at PSI uses a distributed aperture imaging system to capture the complex (amplitude and phase) mmW signal at discrete points. These complex signals are then Continued on next page