I would definitely agree with the first statement of your first post on this thread. -“I've discovered that powerrating is not in itself a foolproof indicator of brightness.” However an indication that the laser operates at less than 100% duty cycle doesn’t necessarily mean that it is dimmer. There is a very good reason to operate a portable battery powered laser at less than 100% duty cycle. Solid state lasers (as well as other LEDs) do not have a linear light output increase with increasing input current such as a resistive filament would. In fact a plot of the output versus current input starts out essential flat with no output until the lasing transition occurs and enough electrons are at an excited energy level to sustain the “stimulated emission”. Once you have passed this transition knee the output goes up sharply (and typically fairly linearly through the primary operating range of the diode) with increasing current input. The net result is that to double the output of the laser, considerably less than double the input current is needed. The FDA definition for radiant power is “time-averaged power emitted, transferred or received in the form of radiation, expressed in watts (W).” So operating a diode pumped solid state (DPSS) laser at twice the peak output at a half duty cycle would drain your batteries less and have less heat output to dissipate than 100% duty cycle at the same output power. (In fact duty cycle other than 50% could even be more efficient). Now the FDA emission duration period for a Type IIIa laser is spec’ed at 3.8 x 10-4 sec. So if the modulating frequency of the laser is ~2 KHz or higher you would get pulse durations less than the emission duration period and the time-averaged power would be used for the classification. Unfortunately the vendors of these devices don’t provide enough information for you to know how it is really operating. Is it 5 mW RMS or 5 mW peak? There are a number of other factors that could directly impact the apparent brightness of a laser that you don’t get any insight to with just the stated output rating. For example: the Nd:YVO4 crystal and the KPT doubler-crystal obviously operate at less than 100% efficiency, so remaining 808 and 1064 nM light would contaminate the output power measurement even though they are not visible, so the quality of the IR filter would have a direct bearing on the apparent brightness. There are a number of other factors related to the collimation and collimation optics quality that effect beam divergence etc. that could also have a direct effect on apparent visibility. I have discussed this subject with some of my professional colleges that are “real” optical engineers/scientists and we came to the conclusion that a lot of these factors could be difficult to quantify analytically and probably the best comparative method would be to just get several different green lasers together and shine them at the same time and see which looks brighter. I for one would be interested in seeing if we could do that at an upcoming Starparty or after a SLAS general meeting. etc. As to lasers advertised at higher than 5 mW outputs – I think it is very likely that these very same laser diodes could well be driven at outputs listed by these sellers at StarPhaser, WickedLaser, etc. by simply changing resistors in the drive electronics. The IR laser diodes we use as a light source for our Fiber Optic Gyros we usually run at 7-10 mW in their application (generaly at currents of <100 mA). On Friday I had occasion to have one tested. At 350 mA it had a measured output of 84 mW. Now our laser diode modules have an integrated thermal electric cooler (TEC) and a temperature sensor and control loop to stabilize the temperature to a desired set point. Obviously these “high powered pointers” wouldn’t have such elaborate measures (or the electricity to drive them). I really wonder what provisions they have to conduct heat away from the semiconductor die. Elevated junction temperatures are among the leading contributors to unreliability in semiconductor devices. I would not be the least bit surprised to find these devices prone to fail very quickly. As Patrick has pointed to, there are also some serious concerns associated with laser outputs exceeding legal limits for pointers. The FDA made those rules for a reason. In addition to the legal issues, I for one would not want to be in any way responsible for damaging someone’s eyesight. Something that people would be particularly vulnerable to with dark-adapted eyesight. I would highly recommend anyone considering the purchase of a laser at outputs greater than Class IIIa to at least read the FDA warning at http://www.fda.gov/cdrh/comp/internetlasers.html. I for one am pleased with my Z-Bolt BTG-6 laser (rated at 4.9 mW). It seems to be easily visible in all but the most transparent conditions. Why don't we get as many lasers together as we can sometime and see what the differences really look like. Ivo --- Chuck Hards <chuckhards@yahoo.com> wrote:

Well, it looks like for a hand-held "pointer" type laser, 5mw is the legal limit in the USA. So, to get the brightest possible unit, you would want one that is:

A. Actually optimized to put out a true 5mw.

-AND-

B. Continuous-duty cycle.

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