Illumination

Passive imaging means the reflection from the target and background comes for free, from natural incoherent illumination and emission of the target itself. Every object generates EM radiations at all wavelengths, the intensity of the radiation is proportional to the product of the target physical temperature and its emissivity. The sum of emissivity and reflectivity for a target is 1. The proportion of emissivity and reflectivity varies as wavelength changes, for example, the human body has an emissivity of 65 percent at 100 GHz, increasing to about 95 percent at 600 GHz [7], so it will appear warmer than metals.

The fundamental difference between the passive imaging system and the active imaging system lies in the fact that there is an illumination source in the active imaging system, and because of this, active imaging has more techniques to manip- ulate than the passive imaging system. There is a variety of wave form selections. They can be CW, FMCW or pulse, etc.

2.4.1.1 Considerations for use

While natural illumination has the advantage of coming for free, there is one major issue that hinders its use in the indoor environment, i.e. the natural illu- mination would be very weak and the imaging would be mainly based on the emissivity contrast between the target and its background. This could be diffi- cult if the physical temperature of the target and its background approaches the same. However there are advantages of using passive imaging, it poses no radiation hazard to individuals, and in the case of military applications, the imager has no emitted signature. In active systems the thermal contrast (between wanted and

unwanted targets) constraint does not exist.

2.4.2

Imaging formation mechanism

The passive imaging system is analogous to the optics camera, in which the image is formed directly point by point, with each of them representing the spectrum intensity of a point in the scene. Since MMW naturally emitted by the targets is weak, the system dynamic range is generally low.

Note must be taken that the 2-D image formation mechanism for the passive imaging is different from the active microwave holography approach, in which the system records the scattered EM waves from the target over a planar aperture, with both amplitudes and phases associated with the coordinates being recorded. From this amplitude and phase information and by applying the holographic processing algorithm, the field distribution on the target surface can be calculated, there- fore, 2D image is reconstructed. To record the amplitudes and phases across the aperture, a coherent system is required. Compared to the 2-D image produced by the passive system, the microwave holography approach offers high dynamic range because of the coherent summation. Furthermore, it does not require a focusing reflector/lens, so the system design can be simplified. The schematic diagrams of these two types of imaging systems are shown in Fig 2.1.

But if a collimating device is used, the focal plane imaging array would be much smaller than the pupil plane imaging array. A focal plane array approach or its variations are usually adopted. A quasi-optical collimating device is usually used to focus the weak natural emission/reflection signal from the target. The focal plane array is a staring array (distinct from scanning array in that it image the

Figure 2.1: Sketches of a focal plane array based passive imaging system (upper) and a microwave holography approach based active imaging system (lower) desired field of view without scanning), differences between responses of individ- ual channels introduce noise to the image [4] and need to be calibrated prior to deployment.

2.4.2.1 Considerations for use

As targets RCS could vary greatly, for effective detection, a high system dynamic range is required. This can be achieved in the active, coherent microwave holog- raphy approach.

2.4.3

Direct receiving vs heterodyne

The receiving systems for passive and active imaging can be either direct or heterodyne. In [15], two W band direct detecting receivers were reported. The first one was based on a high gain (48 dB), low noise (NF 6.0 dB) amplifier and a Schottky barrier diode waveguide detector, and the second one used two 17 dB gain MMIC LNAs and a MMIC preamplified detector. Direct receiving has a few advantages such as low system temperature, no LO requirement, fewer parts, high yield and low cost, which make it suitable for the focal plane receiving array systems. Compared to direct receiving detectors, the heterodyne receiving requires a LO signal to mix with the received RF signal, the output is IF which is easier to process than the RF signal in the case of direct receiving. The Noise Figure of these systems are determined by the thermal noise, and it is always present. The lower is the system noise floor, the higher is the system sensitivity.

For the active imaging system, both the transmitter and receiver can be either be coherent and incoherent, and coherent receiving architecture is generally chosen for coherent transmission, incoherent receiving for incoherent transmission. For holographic imaging heterodyne detection is required to obtain the phase informa- tion.

2.4.3.1 Considerations for use

S/N of target and its background is another figure of interest besides reflectivity contrast between the target and its background. This is especially true for the passive imaging systems as S/N figures are generally very low in these cases which implies sensitivity requirement of detectors is high. As target RCS could vary

greatly, for effective detection, a high system dynamic range is required. This can be easily achieved in the active, coherent imaging system.

Direct receiving has advantages such as low system temperature, no LO require- ment, fewer parts, high yield, but the processing for the received high frequencies is difficult. For heterodyne receiving, the LO signal is required to mix with the received RF, but the advantage is the easily processed IF output.