The spectrum of VR devices
As with most technologies, there is a spectrum of products for virtual reality ranging from the simplest and least expensive to the very advanced.
Old fashioned stereoscopes
Cardboard is at the low end of the VR device spectrum. Well, you could even go lower if you consider the ViewMaster that you may have played with as a child, or even the historic stereoscope viewer from 1876 (B.W. Kilborn & Co, Littleton, New Hampshire), as shown in the following image:
In these old fashioned viewers, a pair of photographs display two separate views for the left and right eyes that are slightly offset to create parallax. This fools the ...