Laser pointer

A laser pointer is a small handheld laser, powered by one or two batteries, that projects a visible, nearly collimated beam. Its output is typically 1–5 mW at one of a few standard wavelengths: red at 635–650 nm, green at 532 nm, and blue or violet at 445–450 nm or 405 nm. The beam has a diameter of a millimeter or two at the aperture and a divergence of the order of a milliradian, so it remains a small spot across a lecture hall and is still only centimeters wide at 100 m.

How the colors are produced

Red pointers use a laser diode of AlGaInP emitting directly at 635–650 nm, with a small collimating lens; the 650 nm diodes are the least expensive, and 635 nm diodes look about twice as bright at the same power, for the reason given below. Blue (445–450 nm) and violet (405 nm) pointers use InGaN diodes on gallium nitride, the same technology as projector and Blu-ray sources.

Most green pointers are diode-pumped solid-state (DPSS) lasers. An 808 nm diode pumps a small Nd:YVO₄ crystal lasing at 1064 nm, and a KTP crystal converts this by second-harmonic generation to 532 nm. Direct-emitting InGaN green diodes near 520 nm are also used. A DPSS pointer should contain an infrared filter to block the residual 808 nm and 1064 nm light; measurements at NIST on inexpensive green pointers found some emitting more infrared than green, and a power meter reading taken through a green filter does not show it.

Power classes

Classification sets the power a product may emit, and the rules are regional.

United States. The FDA regulates laser products under 21 CFR 1040.10 and 1040.11 and limits laser pointers to Class IIIa, which for visible light is 5 mW. Class II is up to 1 mW visible. Aiming a laser at an aircraft is a federal crime under 18 U.S.C. § 39A. The FDA also accepts classification under IEC 60825-1.

IEC 60825-1 (international). The classes are 1, 1M, 2, 2M, 3R, 3B and 4. For a visible continuous-wave beam:

Class 2 relies on the aversion response, including the blink reflex, to limit exposure to 0.25 s. The numbers are consistent: the maximum permissible exposure for a 0.25 s visible exposure, $18\,t^{0.75}$ J/m², is 25.5 W/m², while 1 mW through the 7 mm pupil aperture used for classification is 26.0 W/m². Many jurisdictions restrict consumer pointers to Class 2 or below; Australia, for example, controls the import of laser pointers above 1 mW.

Why green looks brighter

The eye's daylight (photopic) sensitivity $V(\lambda)$ peaks at 555 nm. From the CIE 1924 table, $V(650) = 0.107$ and $V(635) = 0.217$, while linear interpolation between $V(530) = 0.862$ and $V(535) = 0.915$ gives $V(532) \approx 0.883$. The same optical power therefore looks about

$$\frac{V(532)}{V(650)} = \frac{0.883}{0.107} \approx 8.3$$

times brighter in green than at 650 nm, and about 4.1 times brighter than at 635 nm. In luminous flux, 1 mW at 532 nm is 0.60 lm and 1 mW at 650 nm is 0.073 lm. Violet at 405 nm is the opposite case: $V$ there is below 0.001, so a 405 nm pointer looks dim while delivering the same power, and the higher photon energy, 3.06 eV against 1.91 eV at 650 nm, excites fluorescence in paper and many plastics.

The visible beam path in air is a separate effect, due to scattering by air molecules and dust; the molecular part is Rayleigh scattering, which scales as $\lambda^{-4}$ and is $(650/532)^4 = 2.2$ times stronger at 532 nm than at 650 nm.

Hazards and practical use

A 5 mW beam entering the eye delivers five times the maximum permissible exposure over the 0.25 s blink time, and prolonged staring can cause retinal injury; Class 3B and Class 4 beams can injure the retina before the blink reflex acts. Pointers are useful for quick alignment and for demonstrating beam divergence, but their wavelength and power are poorly specified, and a beam reflected from a mirror carries the same hazard as the direct beam. Protective eyewear is specified by optical density at the exact wavelength, and a filter rated at 532 nm may offer no protection at 1064 nm.

Common questions

What is the legal power limit for a laser pointer?

In the United States, 5 mW (Class IIIa) for pointers. Under IEC 60825-1, 1 mW corresponds to Class 2 and 5 mW to Class 3R for visible CW light, and several countries restrict consumer sale to Class 2.

Is a 5 mW laser pointer dangerous?

A momentary accidental exposure is unlikely to cause lasting injury, but 5 mW is five times the Class 2 limit, so deliberate or prolonged viewing is hazardous.

References: US Code of Federal Regulations, 21 CFR 1040.10 and 1040.11; IEC 60825-1:2014, Safety of laser products, Part 1; CIE, photopic luminous efficiency function V(λ) (CIE 1924, tabulated in CIE 018:2019); J. Galang, A. Restelli, E. W. Hagley, C. W. Clark, A Green Laser Pointer Hazard, NIST Technical Note 1668 (2010); A. E. Siegman, Lasers (University Science Books, 1986).