Lenslok was a copy prevention mechanism found in some computer games and other software on the Commodore 64, Sinclair ZX Spectrum and Amstrad CPC. The most famous game to use it was Elite for the ZX Spectrum.
The Lenslok device was essentially a row of prisms arranged vertically in a plastic holder. Before the game started, a two-letter code was displayed on the screen, but it was corrupted by being split into vertical bands which were then rearranged on screen. By viewing these bands through the Lenslok they were restored to their correct order and the code could be read allowing access to the game. The device was small enough when folded flat to fit in a standard audio cassette case.
In order for the Lenslok to work correctly the displayed image has to be the correct size. This meant that before each use the software needed to be calibrated to take account of the size of the display. Users found this setup particularly annoying, at least in part due to the poor instructions that were initially shipped. Additionally, the device could not be calibrated at all for very large and very small televisions, and some games shipped with the mismatched Lensloks that prevented the code from being correctly descrambled. The Lenslok system was not used in later releases of Elite.
(Credits: Above text is from Wikipedia)
Like all copy protection devices, the Lenslok was entirely ineffective against anyone with a modicum of skill in Z80 assembly language. Finding the Lenslok code was not hard; looking for the instructions the Lenslok code printed on the screen would narrow it down. Techniques to defeat the Lenslok code varied from using hardware devices like the Multiface to examine the memory location where the code you were supposed to type lay, to defeating it entirely by modifying the code in a number of ways, for example finding the address called on a successful Lenslok decode and changing the loader to call that address instead of the Lenslok code. The spread of pirated versions of Elite was relatively slow in the days of the ZX Spectrum compared to the spread of pirated modern games on the Internet, however usually within days of a game release, someone would have a tape of a hacked copy.
The overall assessment of the Lenslok was like every other DRM system that has come since: it caused undue annoyance to legitimate users whilst not actually preventing piracy.