Lasers are regulated by the US government. The US Food and Drug Administration (FDA) limits the output power of visible lasers that can be sold to “civilians” to no more than 5 milliwatts (mW). The FDA classifies the lasers we allow to purchase as Class IIIA. They are considered “safe for the eyes” because if you happen to stare at a person, the brain will trigger the blink reflex long before the Laser pointer damages the retina. Since infrared lasers are invisible and will not cause blinking reflections, the FDA strictly supervises their sales.
Now, we refer to the “brightness” of the green laser in the opening paragraph. Even if the power of these two IIIA devices reaches 5 mW (or less), it is brighter than the red laser. The reason lies in the chromatic response of our eyes. Our photoreceptors-rods and cones on the retina-are more sensitive to green wavelengths of 520 to 570 nanometers. We think green is brighter because this wavelength is easier to excite our photoreceptors.
The AN/PEQ-2A upgraded version includes a blue removable safety block that physically prevents the mode switch from turning to any high-power mode, which can cause eye damage. Both models are waterproof to 20 meters and use two AA batteries for power. There are other civilian variants too-they can provide more power, visible/IR laser engraver combination or other features.
The core of each laser is located in its cavity, where light is generated and amplified to produce stimulated emission. The cavity is fixed by a pair of mirrors to provide optical feedback-a prerequisite for laser emission. In VCSELs, the cavity is only a few microns thick, hundreds of times thinner than typical cavities in edge emitters (Figure 1a). Such a short cavity allows the device to be opened and closed very quickly, but the disadvantage is that the mirror requires a high degree of reflection to produce laser emission.
To see this more precisely, take a look at the diagram above comparing 1 watt lasers. A green 1 watt blue laser pointer at 88% apparent brightness has a visual hazard distance of 25.5 NM, while a blue 1 watt laser with 3% apparent brightness has a visual hazard distance of 4.8 NM. There is an 88/3 or 29 times difference in the apparent brightnesses, but only a 25.5/4.8 or 5.3 times difference in the visual interference hazard distances. The square root of 29 is 5.4. (The calculation is off by 0.1 because the diagrams use rounded and not exact numbers.)