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Most laser pointers have low enough power that the projected beam is not visible from the side in normal clear air, but is only visible as a point of light where the beam strikes a diffusely reflective surface. Some higher powered laser pointers are visible via Rayleigh scattering when viewed from the side in moderately to dimly lit conditions.
Types of laser pointerThe least expensive laser pointers use a deep red laser diode near the 670/650 nanometer (nm) wavelength. Slightly more expensive ones use a red-orange 635 nm diode, making them more easily visible than their 670 nm counterparts due to the greater sensitivity of the human eye at 635 nm. Other colors are possible too, with the 532 nm green laser being the most common alternative. In the past few years, yellow-orange laser pointers, at 593.5 nm, have been made available. In September 2005, handheld blue laser pointers at 473 nm have also become available. As of October 2007, violet (405 nm) lasers are also starting to appear. The apparent brightness of a spot from a laser beam depends not only on the optical power of the laser and the reflectivity of the surface, but also on the color response of the human eye. For the same optical power, the green laser will seem brighter than other colors because the human eye is most sensitive in the green area of spectrum (for low light levels), with sensitivity decreasing as colors become redder or bluer. The output power of a laser pointer is measured in milliwatts (mW). In Europe/UK the legal requirement is that laser pointer output not exceed 1 mW; in USA this output is limited to 5 mW for presentation lasers. All laser products offered in commerce in the US must be registered with the US FDA, regardless of output power.
Green laser pointerGreen laser pointers appeared on the market circa 2000, and are the most common type of DPSS lasers (also called DPSSFD, diode pumped solid state frequency-doubled). They are much more complicated than standard red laser pointers, because laser diodes are not commonly available in this wavelength range. The green light is generated in an indirect process, beginning with a high-power (typically 100-300 mW) infrared AlGaAs laser diode operating at 808 nm. The 808 nm light pumps a crystal of neodymium-doped vanadate (or Nd:YAG or less common Nd:YLF), which lases deeper in the infrared at 1064 nm. The vanadate crystal is coated on the diode side with a dielectric mirror that reflects at 1064 nm and transmits at 808 nm. The crystal is mounted on a copper block, acting as a heatsink; its 1064 nm output is fed into a crystal of potassium titanyl phosphate (KTP), mounted on a heatsink in the laser cavity resonator. The orientation of the crystals must be matched, as they are both anisotropic and the Nd:YVO4 outputs polarized light. This unit acts as a frequency doubler, and halves the wavelength to the desired 532 nm. The resonant cavity is terminated by a dielectric mirror that reflects at 1064 nm and transmits at 532 nm. An infrared filter behind the mirror removes IR radiation from the output beam, and the assembly ends in a collimator lens. The output power of most green laser pointers is on the order of 5 mW. In 2007, the Guinness Book of World Records published a new world record called "Most Powerful Handheld Laser" which was awarded to Shanghai-based Wicked Lasers for their 200-300 mW Spyder series lasers.
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