The special thing about microwaves is that they go right through many things that we can’t “see through” with our eyes. So, holograms made with microwaves can show things hidden in walls or buried in the ground. In a way, a microwave hologram is kind of like Superman’s X-Ray vision! We can shine microwaves into the ground, and record the interference pattern from buried objects that reflect the waves.
With light, we could then shine light through the interference pattern and make a phantom 3-D image of the object. But, here’s where we run into a problem. Human eyes can’t see microwaves. So, even if we did shine microwaves through the radar interference pattern to make the 3-D phantom, it would be completely invisible to us humans. So, our holographic radar has to “translate” or shift the interference pattern from one portion of the electromagnetic spectrum to another – into the portion that we can see. To do that, it does “color by numbers” – assigning a color (usually a shade of gray) to each different value in the interference pattern, and “voila!”, now we can see the interference pattern!
However, we still can’t shine light through the interference pattern to see the 3-D phantom (because we did not use light in the first place). But because of the way that microwaves behave in the ground - some are “lost” or converted to heat (just like in the popcorn) - the interference pattern itself looks a lot like the buried object, even without reconstructing the phantom image. You can see for yourself in these pictures.
Here are photos of some objects before they are hidden: