23/05/14: Check out Matthias' great video at Woodgears.ca of destructively testing a cheap table which also uses a cardboard honeycomb structure for the top.
We were at IKEA recently to get some furniture and I wondered how they managed to make the thick wooden panels so light (for example, the "Lack" coffee table). After a bit of searching, it turns out that they use a cardboard honeycomb structure, sandwiched between two thin layers of solid wood (well, particle board at least). Here's a photo showing the construction (from http://www.ikeahackers.net/2013/01/lack-arcade-coffee-table.html):
I later found a good video from National Geographic which shows the construction process.
After buying an "Expedit" shelving unit for all of £45, I did what anyone would normally do - take an X-ray photo! These were done with a 30kV tube and a KMC Green Regular intensifying screen. The screen was photographed with an exposure time of 3 minutes. Distance between the tube and the wood was about 1m.
Here are normal-light, x-ray, and combined photos. When I had the X-ray machine set up, I also did one of a VHS tape for fun.
You can clearly see the cardboard honeycomb and reinforcing wood around the edges. In the photo of the corner, you can even see through the edge reinforcing and can make out the veneer layer on the surface!