Time-of-Flight Microwave Camera

Abstract

Microwaves can penetrate many obstructions that are opaque at visible wavelengths, however microwave imaging is challenging due to resolution limits associated with relatively small apertures and unrecoverable "stealth" regions due to the specularity of most objects at microwave frequencies. We demonstrate a multispectral time-of-flight microwave imaging system which overcomes these challenges with a large passive aperture to improve lateral resolution, multiple illumination points with a data fusion method to reduce stealth regions, and a frequency modulated continuous wave (FMCW) receiver to achieve depth resolution. The camera captures images with a resolution of 1.5 degrees, multispectral images across the X frequency band (8 GHz-12 GHz), and a time resolution of 200 ps (6 cm optical path in free space). Images are taken of objects in free space as well as behind drywall and plywood. This architecture allows "camera-like" behavior from a microwave imaging system and is practical for imaging everyday objects in the microwave spectrum.

Figure 1. Can we recover diffuse-like images at microwave wavelengths?

Combining images from multiple illumination sources creates microwave images with fewer stealth regions. In (A) we see a visible-light image (A1) and a microwave image of an unobscured mannequin (A2, A3) generated by projective recombination of illumination images (B1–4). In (C) the transmitters are on the left and right of the parabolic reflector. Incident rays from TX4 reflect off of P1 and P3, and never return to the camera; however, the reflection from P2 does return and is visible to the camera. Introducing other illumination points allows P1 and P3 to be visible to the camera. In (D) the reflectance lobes for short wavelengths are wider than the reflectance lobes at long wavelengths, thus the multi-spectral images of the scene provide additional information depending on the size of features in the scene. In (E) each image is broken down into the energy received from three spectral bands, leading to diversity in reflectance properties.

Figure 2. The microwave camera can image at wavelengths which easily penetrate drywall and plywood.

In (A–C) an image is taken of a mannequin wrapped in aluminum foil in free-space, placed behind 12.7 mm thick dry-wall, and behind 11.9 mm thick plywood. The mannequin is wrapped in foil in order to approximate the strong reflectivity of the human body. The recovered 41 pixel by 41 pixel microwave-photographs are shown below each visible-light image (D–F).

Figure 3. Two practical applications of a time-of-flight microwave camera are demonstrated.

In (A) we visualize the propagation of microwaves across a metal peacock ornament as a color-coded time sequence, similar to a “light in flight” movie. Here a grayscale, visible-light image is overlaid with color-coded data from the microwave camera. The red channel is the response at an early reference time, the green channel is the response at an additional 588.2 ps, and the blue channel is the response 1078.4 ps after the reference time. One can see the curve of the microwave as it crosses the scene and reflects off of features. In (B) the microwave camera is used to inspect the contents of a box to ensure proper packaging. A set of push pins are placed on three pieces of styrofoam inside of a shipping box in the shape of the letters “M”, “I”, and “T”. By separating the images in time, it is possible to see the three different layers. In (C) the average intensity at 700 ps around each center point is shown.

Figure 4. The illumination bandwidth can be exploited to generate multispectral microwave images.

The reflectance properties of five sub-wavelength wire resonators of decreasing length are shown. In (A) there are five wires of decreasing length (L to R: 17.5 mm, 15 mm, 12.5 mm, 10 mm, and 7.5 mm) placed vertically in styrofoam (which is transparent in the X-band). In (B) a gray scale linear intensity image is shown (full 5 GHz bandwidth). In (C) a multi-spectral image is shown where the primary colors red, green, and blue represent the lower, middle, and upper frequency bands of illumination, respectively. The smaller wires are not as reflective of the longer wavelengths, causing them to appear bluer. The individual frequency band images are shown in (D–F).