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A laser pointer is a portable, pen-sized laser designed to be held in the hand, and most commonly used to project a point of light to highlight items of interest during a presentation. Most laser pointers have low enough power that the projected beam presents a minimal hazard to eyes for incidental exposure. Consequently, beams from laser pointers are generally not visible from the side in normal clear air, but only visible as a point of light where the beam strikes a diffusely reflective surface. Some higher powered laser pointers are faintly visible via Rayleigh scattering when viewed from the side in moderately to dimly lit conditions.
Additional recommended knowledge
Types of laser pointer
The 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 pointer
Green 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.
Nd:YVO4 is replacing Nd:YAG and Nd:YLF due to lower dependency on the exact parameters of the pump diode (therefore allowing for higher tolerances), wider absorption band, lower lasing threshold, higher slope efficiency, linear polarization of output light, and single mode output. For frequency doubling of higher power lasers, LBO is used instead of KTP. Newer lasers use a composite Nd:YVO4/KTP crystal instead of two discrete ones.
Some green lasers operate in pulse or quasi-continuous wave (QCW) mode, to reduce cooling problems and prolong battery life.
Blue laser pointers, which became available around 2006, have the same basic construction as green lasers. They most commonly lase at 473 nm, which is produced by frequency doubling of 946 nm laser radiation from a diode-pumped Nd:YAG or Nd:YVO4 crystal. For high output power BBO crystals are used as frequency doublers, for lower powers KTP is used.
Blue lasers can also be fabricated with InGaN semiconductors. In this case, no frequency doubler is needed. The Japanese company Nichia controls (in 2006) 80% of the market. 
Laser pointers are often used in school and business presentations and visual demonstrations as an eye-catching pointing device. Red laser pointers can be used in almost any indoor or low-light situation where pointing out details by hand may be inconvenient, such as in construction work or interior decorating. Green laser pointers can be used for similar purposes as well as outdoors in daylight or for longer distances.
In pointing applications such as these, natural hand tremor may cause unwanted jittery motion of the laser dot. Future laser pointers may solve this problem by stabilizing the laser beam from unwanted hand tremor.
Laser pointers can be used as toys for pets, especially for cats in play. Some offer a selection of designs for the laser beam to project (e.g. images of butterflies, mice, or flowers), to provide variety. Opinions are divided on the safety of laser pointers used in this way. Some consider laser pointers to be a healthier alternative to the more traditional string for cats because they reduce the risk of choking on the string. Others are concerned that the laser beam may damage pets' eyes, or that the pet will develop frustration problems from not being able to catch the prey. 
Green laser pointers can also be used for skygazing. On a moonless night, a green laser pointer beam can often be clearly seen, allowing someone to accurately point out individual stars to others nearby.
The output of laser pointers is generally limited to 1 mW or 5 mW in order to prevent accidental damage to the retina of human eyes. Usually, pen lasers are class 2 or class 3a lasers, which require extended viewing times to damage the retina severely. There is some debate about whether outputs of 5 mW may damage eyes if viewed through spectacles or contact lenses. The Food and Drug Administration (FDA) has determined that Class 3a lasers could cause injury to the eye if viewed directly for approximately 0.25 seconds, although it has cited evidence that exposure to visible lasers is "usually" limited by the blink reflex of the eye, which they have timed at just under 0.25 seconds.
In the late 1990s, the laser pointer became a fad amongst adolescents as an irritant to be pointed stealthily at a movie theater screen or even at a person's eyes. During late 2004 a man was arrested in USA under terrorist laws when he was identified as pointing a high power green laser pointer into the cockpit of an airplane.
Despite legislation limiting the output of laser pointers in some countries (such as the USA and Australia), higher-power devices are currently produced in other regions (especially China and Hong Kong), and are frequently imported by customers who purchase them directly via internet mail order. The legality of such transactions is not always clear; typically, the lasers are sold as research or OEM devices (which are not subject to the same power restrictions), with a disclaimer that they are not to be used as pointers. DIY videos are also often posted on Internet video sharing sites like YouTube which explain how to make a high-power laser pointer using the diode from an optical disc burner. As the popularity of these hazardous devices soared, many manufacturers (mainly in China) began manufacturing similar high powered pointers. The FDA has published a warning on the dangers of such high powered lasers. Despite the disclaimers, such Lasers are frequently sold in packaging resembling that for laser pointers. Lasers of this type may not include safety features sometimes found on laser modules sold for research purposes.
In Utah it is a class C misdemeanor to point a laser pointer at a law enforcement officer and is an infraction to point a laser pointer at a moving vehicle.
High-powered portable lasers
In recent years, high-powered portable lasers have come on the market from a number of specialized suppliers. These lasers can exceed several hundred mW in output. Because those are Class IIIB lasers (higher than 5 mW output), they are not true "laser pointers" by legal definition, although hobbyists still refer to them as "pointers."
Many Class IIIB portable lasers incorporate safety features and interlocks not found on a standard, low-power laser pointer. These may include a safety dongle and/or safety key, an aperture shutter, a power indicator light, and an emission delay of two or three seconds.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Laser_pointer". A list of authors is available in Wikipedia.|