I've got a few laser pointers: a red one, and a green one. The green laser pointer is much stronger than the red one, it's the type that you can seem the beam if it's dark enough (it's 5mW).
What level of laser output is safe to use for playing with a cat?
This article from Princeton University deals with laser pointer use and human effect, and it mentions that use of laser pointers that are higher than 5 mW are dangerous and should not be used, so it seems like the green one might be safe, although they recommend that the lower the output the better, as they will be safer.
Like with any laser, avoid pointing it at your cat's eyes, or at reflective surfaces that might reflect the laser into their eyes, and that can cause problems, the same way it can for humans.
Further, though, beyond just the output, another concern is that since the cats cannot "kill" the laser, they are not able to come down from the "high" of hunting, and may redirect that energy in negative ways (excessive grooming, aggression, etc), so it is suggested that at the end of a laser pointer play session, you give the cat something to "kill" such as a stuffed toy or something similar.
As low as you can get away with. The issue isn't so much with the output, as it is with keeping the laser beam away from the cat's eyes and minimizing the damage in the event that you accidentally shine the laser in the cat's eyes.
Same reason behind why it's illegal to shine a laser at a passing airplane, it has the possibility of blinding the pilot.
Use the red. All lasers can be dangerous if they hit the eye of a cat or a human, but red lasers are usually much less powerful and still very visible to cats. I'm a little torn on this topic because I've used red lasers as a cat toy lots of times, but it really isn't intended as a cat toy and it's worth remembering that their eyes are far more sensitive than ours. If you do use a laser, just exercise some additional caution.
Color has nothing to do with intensity. No color is inherently "safer". However, the color green is more apparent to the human eye - so a 150mw red laser will appear dimmer than a 150mw green laser. This does not mean it's any weaker or any safer, it has the exact same burning potential.
Also beware, most of the lasers floating around are somewhat lacking in quality control, and often MUCH stronger than advertised. While it's true that a 5mw laser would normally be fairly safe around an animal, I've measured lasers marked as <5mw well over 20mw. This is not in the least bit safe.
My recommendation, if you wish to use a laser with your animal, would be to look for a <1mw (Class II) red (650nm). It will be more than bright enough for indoor use, it's on a spectrum that animals can easily see and even taking into account poor manufacture it should be at a low enough power you won't have to worry.
Most laser pointers are either Class II or Class IIIa lasers in the US. In many other parts of the world, home laser pointers are restricted to Class II lasers, which are safer.
According to the Wikipedia article on laser safety,
Class II: The blink reflex of the human eye (aversion response) will prevent eye damage, unless the person deliberately stares into the beam for an extended period. Output power may be up to 1 mW.
Class IIIa: Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter or power density, though even without optical instrument enhancement direct contact with the eye for over two minutes may cause serious damage to the retina. Output power does not exceed 5 mW. Beam power density may not exceed 2.5 mW/cm2 if the device is not labeled with a "caution" warning label, otherwise a "danger" warning label is required.
From these definitions, we can see that laser toys can be harmful if a cat looks into the beam for a long period of time. Laser toy play typically involves a lot of movement so that the likelihood of your cat staring into the beam for over two minutes is low (you would notice and move the beam). However, when I used to play with laser toys with my cats, one cat was smart enough to stop chasing the dot and would just stare at my hand. I do not know how he would react if the beam was pointed directly at him (if perhaps I wasn't paying attention, or was watching another cat).
There are instructions online to make items such as CD players into stronger laser pointers than you can buy. Since these lasers are of an unknown power, I would not recommend using these for play. Only use commercially available laser pointers so that you can be sure that they are not too dangerous.
Personally, I do not use laser toys because I believe that structured play time is a bonding experience between me and my cats. I also believe that play should be structured around meal times to enforce the hunt-eat-groom-sleep cycle that cats naturally fall into. While occassional hunts may fail, I believe a cat should not be constantly frustrated by hunting sessions that never give the cat the satisfaction of a "kill".
