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Lasers & Sources

The dangerous dark companion of bright green lasers

The spectra of inexpensive, green laser pointers exhibit IR components that are 10 times more intense than green light and, consequently, could cause retinal damage.
10 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003328

In December 2009, we purchased three inexpensive, green laser pointers (GLPs) for $15 each. Advertised to produce ‘10mW’ of green light, the common packaging of the devices suggests that they were produced by the same manufacturer, although they carried no traceable trademarks. One of the pointers produced much weaker green light than the other two. We performed quantitative measurements, showing that the weak unit emitted approximately 10 times more invisible IR light than the visible green. Green light activates the eye's blink reflex, which provides a protective mechanism. However, we are completely vulnerable to IR radiation, since exposure to it may only be noticed after significant retinal damage has occurred. After further investigation, we found that this problem is common in low-cost GLPs, although its seriousness varies widely. We propose a simple diagnostic method to detect IR radiation that requires only access to a common Web camera.

Inexpensive GLP operation is based on three key elements, each of which marks a milestone in the history of laser technology. First, a semiconductor 808nm-wavelength diode laser pumps a neodymium-ion oscillator, which produces radiation at 1064nm. That light is then passed through a frequency-doubling crystal to produce green light with a wavelength of 532nm. These functions are now routinely integrated into a pen-sized package (see Figure 1).

Figure 1.Schematic of the operation of a green laser pointer (GLP) based on a multiple-crystal assembly (MCA). The familiar external package contains two AAA batteries that power the unit, a printed circuit board with the pump laser-diode (LD) driving circuitry, and a diode-pumped solid-state (DPSS) laser module. The 808nm pump LD is optically coupled to the Nd:YVO4 (neodymium-doped yttrium vanadate) conversion crystal (violet section of the MCA), which emits 1064nm light into the KTP (potassium titanyl phosphate) frequency-doubling crystal (light-blue section). The 532nm light from the KTP crystal is sent through an expanding and collimating (i.e., nonfocusing) lens assembly to produce a collimated output beam. In this configuration, an IR filter prevents the 808 and 1064nm light from exiting the laser. In the GLP discussed here, no IR filter was present. OC: Output coupler. AR: Antireflectivity. HR: High reflectivity. HT: High transmissivity. (Figure credit: Samuel M. Goldwasser.2)

Recently, GLPs shone at aircraft have caused unscheduled plane landings. In two separate incidents, pilots of a commercial flight and a patrol helicopter were temporarily blinded by GLPs.1 At such large distances, beam divergence prevents permanent retinal damage, but pilot distraction remains a serious concern. These reports imply that consumers who use the devices for pranks and entertainment are oblivious to their dangers.

Class IIIb devices4 refer to lasers that emit between 5 and 500mW of light. Significant eye damage may be incurred at powers well below this level. The human visual response is more sensitive to green than other colors in the visible spectrum (see Figure 2), which is the main reason GLPs are preferred in low-visibility settings, such as large lecture halls. Despite their wide use, few eye-damage incidents involving GLPs have been reported, thanks to the human blink reflex in response to green light.5

Figure 2.Diagram of human visual response showing the wavelength of the GLP line at 532nm and the 650nm wavelength of a typical red laser pointer. The blue curve shows the perceived brightness of light from a source of the same intensity. Thus, a 5mW GLP would appear as bright as a 41.3mW red laser. (Data from the 1931 report of the International Commission on Illumination.3)

What does pose a danger is the low conversion efficiency of the so-called second-harmonic generation from 1064 to 532nm. This may be caused by a manufacturing fault in the beam alignment into the crystal. Normally, as conversion to green light absorbs power, intercavity IR power decreases. However, if the conversion efficiency is low, IR intercavity power may build up to very high levels. Consequently, an IR-blocking filter is essential to prevent leakage. A survey of 100 accidental, nonmedical, laser-induced eye injuries reported in the scientific literature up to 1999 found that neodymium-based lasers with wavelengths of 1060–1064nm were implicated in 49% of all cases, while 532nm lasers accounted for only 7%.6

Figure 3.Experimental setup for determining whether IR radiation is emitted by a GLP. The GLP light passes through a hole in the paper sheet and is diffracted by the black-masked compact disc (CD) on the left. Five green diffraction spots are visible on the paper sheet.

Our experiment consists of the following elements. As shown in Figure 3, ordinary disposable drink cups with V-shaped slots, or stack of books, may be used as unorthodox optical mounts. A small transmission hole is cut from a piece of ink-jet or photocopier paper, which is positioned between the laser and the compact disc (CD). The laser light passes through the hole in the paper and hits the CD nearly perpendicularly. The paper sheet acts as a backlit screen for the light diffracted back from the CD and can be viewed by the eye or photographed from behind the laser. Figure 4 shows the presence of IR power by comparing the diffraction pattern provided by two pictures, the first taken with a digital camera that is not sensitive to IR and the second with an IR-sensitive camera. Figure 5 shows power measurements quantifying the efficiency problems of the GLP that we tested. Total IR radiation from the 808nm pump and 1064nm oscillator is 10 times greater than that from visible 532nm green light.

Figure 4.(top) Ordinary camera photograph of diffraction of green laser light by a masked CD, from the vantage point of the Web camera. The GLP is visible in the foreground. The yellow lines are aligned with the green diffraction spots corresponding to zero-, first-, and second-order diffraction. (bottom) The same vignette, photographed by a Web camera with no IR-blocking filter. Here we see intense diffraction spots—caused by 808nm IR light and not visible to the eye—between the first- and second-order diffraction spots of the green light. Note that the IR light spreads out beyond the green, which could be injurious, for example, to a cat closely chasing a spot of green light.

Figure 5.Power spectrum of the laser under test, showing the ratios of the measured power of each laser line to the measured power of the green line at 532nm.

We found no IR-blocking filter when we disassembled the GLP. Because filters are necessary in cases of low conversion efficiency, and because GLPs are easily accessible, it is very important that these potential hazards be seriously addressed. As an internal response, the National Institute of Standards and Technology is developing a standardized test for all laser pointers used by employees. We hope that our initiative will raise awareness about the hazards of GLPs and inspire people to take more personal responsibility in employing such devices.

Jemellie Galang, Alessandro Restelli, Charles W. Clark
National Institute of Standards and Technology (NIST)
Gaithersburg, MD

Jemellie Galang, born in Manila (Philippines), is a graduate student in chemical physics at the University of Maryland, College Park, and a guest researcher at NIST through the University of Maryland's Joint Quantum Institute. She graduated with a degree in physical sciences from the same university. She is president of the student chapters of both The Optical Society of America and SPIE.

Edward Hagley
Acadia Optronics
Rockville, MD