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Old 08-20-2011, 09:46 PM #1
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Default about CW power measurements...

And how LPM's actually calculate the CW power.

Obviously with a pulsed configuration you can accurately say "it produces X for X period of time" and thats it..it delivered X energy to the target. Simple.

But with continuous wave it is a cumulative effect...so if I wanted to determine how much energy is imparted into a target with say 2-300mW continuous wave over a period of many seconds ..it would not be accurate to just say "Oh thats 2-300mW of energy". an LPM would tell me the entire time..the target is only be effected by 300mW. But not really..it has the energy the 300mW gave it a fraction of a second ago...and the fraction of a second after that...and after that...and after that...giving the target much more energy than the LPM says the beam is generating

Is there a standard "tick rate" for LPM's that gives you this "consistent" power reading by breaking it up into individual pulses and giving you the average??



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Old 08-20-2011, 11:30 PM #2
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Default Re: about CW power measurements...

There is a rather large difference between power and energy. Watts are a measure of power, not energy.
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Old 08-21-2011, 01:38 AM #3
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Default Re: about CW power measurements...

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Originally Posted by Cyparagon View Post
There is a rather large difference between power and energy. Watts are a measure of power, not energy.
Ahhhh....I severely overcomplicated the matter. In my idiocy I failed to understand watts can inherently be translated into joules per SECOND. Which I suppose answers my own question.

I would then assume a LPM measures watts by dividing the joules over a given interval.

Is that correct?

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Old 08-21-2011, 01:52 AM #4
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Default Re: about CW power measurements...

This simple revelation also helped me come up with the calculation that a 250mW laser is = to 15 watts per minute. Or 15 joule/minute. Or 15 watt seconds over the course of a minute

Now I need to figure out a way to calculate how much of this energy is absorbed by a medium..to do work..vs radiated.

Would it be adequate to come up with a method to determine wasted "power" not absorbed by the surface by measuring ambient intensity? and applying that value over a given area of a room

Is there a device that can pickup ambient laser radiation or light and not a directed beam?

This just has me so curious ^_^. Just for the problem solving aspect! id love to be able to know exactly how much energy is readily convertable into useful work, vs wasteful radiation on a variety of surfaces.

I suppose you could establish a baseline with a surface containing a theoretical "100% absoption" rate..or something with a known value ...and get a "control" reading to judge all subsequent tests off of with different materials.

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Old 08-21-2011, 06:30 AM #5
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Default Re: about CW power measurements...

The thing is, a watt per second, or even a watt per minute, is a whole different thing from 1W. So 1W is one joule per second, whereas 15 watts per minute is the change of joules per minute, every minute.

Power is strictly time-independent, i.e. no matter how long you apply a certain amount of power, the energy applied will be the same.

250mW is NOT 15 jW/min. 250mW is going to be .25J/second. Which, you are right, turns out to be 15 joules per minute. So you are doing 15 units of work every minute.

But it's good that you are thinking outside of the box!
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Old 08-21-2011, 02:17 PM #6
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Default Re: about CW power measurements...

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Originally Posted by ChukiDori View Post
I would then assume a LPM measures watts by dividing the joules over a given interval.

Is that correct?
Thats what it boils down to. In simplest terms a thermal lpm is a target heated up by your laser, and cooled down by coduction, convection and thermal radiation. The reading is only accurate when this situation in balanced, i.e. the temperature is constant and the laser heating is equal to the cooling by the other factors.

In a practical meter the time constant to reach equilibrium may be in the order of 1 second to much longer, perhaps 1 minute.

By this this mechanism you can also measure the -average- power produced by a pulsed laser, as long as the pulses are (much) faster than the equilibrium time.
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Old 08-21-2011, 03:10 PM #7
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Default Re: about CW power measurements...

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Originally Posted by Wolfman29 View Post

250mW is NOT 15 jW/min.
Indeed not! 15 Jigga watts would be more tha na bolt of lightning and then I could sell it to the government
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Old 08-21-2011, 06:50 PM #8
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Default Re: about CW power measurements...

You know what I meant.
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