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On Thu, 3 Aug 1995 UnderhillP@ao*.co* wrote: > In a message dated 95-08-03 14:16:41 EDT, you write: > > >+++++++++ From: Dave Drake +++++++++++++++ > >I am most impressed with an American Cave Light that I'm told has a 3 hour > >burn > >time with a 50 W bulb. ...etc. > >+++++++++ end snip ++++++++++++++++++++++++ > > > >My question is why are so many technical divers interested in dive > >lights, WATTS. WATTS have nothing to do with bightness. > > > >If I remember my college physic's, light brightnes is measured in > >Candle power or Lumins. If a diver what to know how bright a light > >is he and/or she needs to compare the Candle Power and/or Lumin rating. > > > >Can someone help me out here???? > > Anybody with a cheap test meter can calculate the "wattage" of a bulb. To > arrive at a *meaningful* value for lumen output requires very accurate > measurements of voltage and current, a knowledge of colour temperature, > reflector efficiency and a whole host of other things. Generally, high > lumen-per-watt bulbs have a much shorter life expectancy than low l-p-w > bulbs. Also the lumen output can vary dramatically with very small changes > in operating voltage and will vary over the life of the bulb. Basically, > quoting wattages is easier, although I agree that a lumen figure would be > useful. > > Philip > -- > Send mail for the `techdiver' mailing list to `techdiver@terra.net'. > Send subscription/archive requests to `techdiver-request@terra.net'. I agree,one reason wattage ratings are used is that they are easily verifiable with inexpensive test equipment,but i think there's another factor involved here regarding burn times. Given a bulb of known wattage and voltage,you can calculate an approciamate burn time based on the capacity of your battery pack.It's all well and good to have a bright light,but it does one little good if said light quits 20 minutes into a 90 minute dive. Other useful factors include the spread of the beam (measured in degrees), and the color of the light (usually measured in degrees Kelvin).The narrower the beam and the whiter the light generally results in amore usable and subjectively brighter light.This general rule does not apply for specialized purposes such as photography,where a wider angle may be more beneficial. Focal lengths may also come into play,as in the case where a very narrow beam used underwater may first converge and then diverge,rendering it much less useful than anticipated. Finally,lighting requirements for recreational dives may be vastly different than those for explorational or scientific diving.For reacreational diving,my personal choice is the MR16 halogen reflector,inparticular the 21-volt 75 watt bulb overdriven to 24 volts.This DOES shorten the life of the bulb,but produces an intensely white light,and used in conjunction with a 13 AH battery pack gives useful burn times of 90 to 100 minutes.It is also relatively inexpensive,and can be purchased fo 10-15$.
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