Incorrect IMBRO.  The light from the sun peaks in brightness in the yellow-green part of the spectrum.  Humans' eyes are tuned to be most receptive to this bandwith of light.  That's why roadsigns are green, hazzard signs are yellow, and firetrucks and tennisballs are a mix of yellow green. Everything else being equal, the human eye is most receptive to yellow-green. RED is good because photons of red light carry the LEAST ammount of energy (blue-purple, on the other end of the rainbow, carries the most).  Red light has the least effect on vision at night (red tail lights on cars) and is used in darkrooms to avoid exposing the film to unwanted light energy.  Red lasers are good because it allows for the highest intensity of the beam with the lest ammount of energy.  In other words, you get the brightest dot with the least damage to your eyes.  For the same brightness, a green laser would carry much more energy and present a greater hazzard.  I'll keep the red lasers.   Although a green laser might be useful in my physics class...  

Very informative. You wouldn't happen to be Wilson from Home Improvement would you?

[:D]

Okay Robert, you seem to know more about this than I do.  I teach high school physics, but kinematics is my pet, not optics.  I'm at home and all of my reference books are at school, so if I ask a question that can be easily found in a reference book, please forgive me.  For two lasers with the same wattage rating, 5mW for example, 635 and 532 nm per your example, you say that the 532 nm would be 3.8 times brighter. How is that possible if the power limits are the same?  If the 532 nm photon has more energy than the 635 nm, does the green laser simply emit less photons per second than the red to maintain the same output wattage?

If we want to keep the light energy entering the eye below a certain threshold wattage level to avoid damage, wouldn't a red laser be more advantageous, because if each photon of red light carries less energy than the green it would allow a greater number of photons per second to enter the eye than the green laser without crossing the threshold.  If light sensitivity in the eye is directly related to the number of photons received (and please correct me if I'm wrong), the red light WOULD be brighter than the green at the same wattage level, because you get more photons into the eye for the same amount of energy.

Photon for photon, green is brighter because it carries more energy, but wattage for wattage (Watts being energy per second) red is brighter becaue you get more photons for your money.

Okay, blow me out of the watt-er (HA!) if I'm wrong, but it's late and I need to figure out how I'm going to explain vector addition to my sophomores tomorrow.  Pray for me.