Lego Sherlock Holmes figure focuses a light beam on a Lego criminal with a small magnifying glass.

LEGO Counterfeit Detection

The currency of many nations uses ultraviolet features in bank notes to foil attempts at counterfeiting. US currency has strips imbedded in the paper that glow in various colors when illuminated with UV light. A LEGO invention can be used to see this glowing strip, as in the video above of a $50 bill. US bank notes in denominations above $1 have an embedded strip in a different location and of a different color for each denomination. The strip glows yellow on the $50 bill. The video was taken in a dark room so that the strip shows up well. The LEGO device is a modified Light Brick, as described in Chapter 2 of “LEGO Optics.” Visible LED colors were used in “LEGO Optics,” whereas here an ultraviolet wavelength LED is installed. Ultraviolet LEDs are available in the appropriate size package at mouser.com and lighthouseleds.com. The LED I used above had a wavelength centered at 375-nm. Ultraviolet wavelengths can be hazardous to the eye, so care must be taken in the use of such light sources.