Gadget Master

Poorly-made green laser pointers can damage your eyesight, and mighty US tech lab NIST has devised a home table top experiment to help separate the dangerous from good.

Very much in the Blue Peter mould, it requires two plastic cups, a CD, and a webcam.

The problem is that some green pointers emit a dangerous level of invisible infra-red alongside their safe level of green. Why?

Well, red pointers simply have a red laser inside. But green lasers are expensive, so many green pointers instead use a low-cost (808nm) infra-red laser and some cunning physics.

The 808nm laser pumps a yttrium orthovanadate crystal doped with neodymium (Nd:YVO4), causing the crystal to lase deeper (1064nm) infra-red.

This beam in turn passes through a potassium titanyl phosphate (KTP) frequency doubling crystal, which halves the wavelength to 532nm - green. All this happens in a tiny little module at the tip of the pointer.

NIST says that if the KTP crystal is misaligned, or some of the internal reflective coatings are too thin, invisible 1064nm escapes.

From one offending "10mW" green pointer, it measured nearly 20mW of infra-red leakage "powerful enough to cause retinal damage to an individual before he or she is aware of the invisible light," said NIST.

Apparently, better green pointers are not only made properly, but include an infra-red just-in-case filter on their output.

The home test uses a webcam, easily modified to be sensitive to infra-red.

The DIY test rig is on page 6 of NIST scientific paper.