A millimetre wave ‘camera’ is bigger than a normal camera. Since the wavelength is typically 6000 times larger than the light your eyes see, the imager needs to focus the light with a much larger lens or mirror.

Millimetre wave cameras typically only have a few detecting elements which are very expensive. these must be scanned back and forth across the scene to build up a full picture. This is difficult to do quickly - imagine that your digital camera could only record one pixel at a time: how long would it take you to take a photo?

In order to record millimetre wave video less time can be spent looking at each pixel in the image and the temperature reading is less accurate. This is why the video images don’t look as ‘clean’ as the still images.

TYPES OF IMAGING
VISIBLE:
almost entirely reflected light
(unless observing light source directly)

INFRARED:
mostly emitted heat + some reflected heat

MILLIMETRE WAVE:
some emitted heat + mostly reflected heat

PASSIVE IMAGING:
recording naturally occurring signals from the scene

ACTIVE IMAGING:
illuminating the scene artificially and comparing the reflected signal with a copy of the illuminating signal