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Old 02-01-2013, 03:00 PM   #1
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Arrow How to determine the Intensity or power of laser beam...?

How to determine the Intensity or power of laser beam knowing only the beam divergence and beam diameter?

I have a 200mW laser pointer since it's China made I have to recalculate. The I have a beam divergence equation =2arctan(d'-d/2L) and beam diameter experimentally (measured manually) So from there how do I calculate for intensity or even the power of the laser? many thanks

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Old 02-01-2013, 03:23 PM   #2
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Default Re: How to determine the Intensity or power of laser beam...?

I don't see that equation to be of any use in calculating a laser's power. Use an LPM to measure the power.
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Old 02-01-2013, 03:30 PM   #3
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Default Re: How to determine the Intensity or power of laser beam...?

what's an LPM? yes me too i don't see any help with the equation either.. have any ideas? I have no proper materials to use but i also need theoretically to measure new power by new divergence...
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Old 02-01-2013, 05:50 PM   #4
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Default Re: How to determine the Intensity or power of laser beam...?

LPM = Light Power Meter

It measures optical intensity of light.

Beam diameter and divergence are independent of output power. You can have a weak-ass beam that is tight, or broad, and a strong beam that is likewise, tight or broad.

An LPM contains a light sensor and a meter of some sort. You point the laser into the sensor and you take your reading on the accompanying gauge.

The LaserBee products mentioned on this forum, and found on ePay are a good entry level LPM. But expect to pay $200 or more for one.

Last edited by pschlosser; 02-01-2013 at 05:52 PM.
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Old 02-02-2013, 08:47 AM   #5
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Default Re: How to determine the Intensity or power of laser beam...?

Quote:
Originally Posted by pschlosser View Post
Beam diameter and divergence are independent of output power.
I agree.

Quote:
Originally Posted by pschlosser View Post
An LPM contains a light sensor and a meter of some sort.
LPMs have a TEC which converts heat to electrical current(also works the other way) and which is then interpreted by the circuitry and displayed in terms of mW/W.

Quote:
Originally Posted by pschlosser View Post
LPM = Light Power Meter
Laser Power Meter

Last edited by Zeebit; 02-02-2013 at 08:48 AM.
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Old 02-03-2013, 10:07 AM   #6
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Default Re: How to determine the Intensity or power of laser beam...?

I don't think you guys understood what OP wants to do here.

If I understood it ( ),

We're going with assumption that total optical power is known, divergence is known and we want power DENSITY (not total power) for a given distance.

And LPM's do not measure "optical intensity of light". They measure the power of light hitting their sensor area. 'Tis all.

Here's a calculator for quickly determining the beam diameter:
pseudonomen137's JScript Diameter Calculator

Now, I do not know the equation for power density so you're going to have to look that up. Sorry.
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Old 02-06-2013, 07:32 AM   #7
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Default Re: How to determine the Intensity or power of laser beam...?

okay please hear me out. what i did is a preliminary test using 1mW laser pointer. I do not know the exact power of the laser since it's china made, I calculated the beam diameter and divergence using a ruler and white paper. I attached the white paper onto a wall and the ruler onto the white paper then my distance is 3m and the aperture diameter is 1.75mm and the secondary diameter (in 3m away) is about 5mm so using the formula for beam divergence i got 1.08mrad. with knowing these can't I just use some improvised stuff or maybe formulas for theoretical approach to know the intensity or just the power? I don't think they sell those LPM stuff around in my place i'm not from US so it's hard to find those stuff and it doesn't come cheap if they are any here plus the shipping is a major issue i need the data 2 days from now. TO MAKE THE STORY SHORT: Any improvised methods for LPM? ANY SUGGESTIONS? I appreciate all of your comments. *sorry for the bad english*
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Last edited by Elektr0n3Ro9000; 02-06-2013 at 07:36 AM.
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Old 02-06-2013, 01:27 PM   #8
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Default Re: How to determine the Intensity or power of laser beam...?

What is it that you want to find out? Output power or power density?
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Old 02-06-2013, 02:13 PM   #9
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Default Re: How to determine the Intensity or power of laser beam...?

can I do both? but basically the output power given divergence and diameter. to get the intensity
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Old 02-06-2013, 03:49 PM   #10
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Default Re: How to determine the Intensity or power of laser beam...?

Sorry I do not know the formula for optical power density. Just do a google search.

However, I do not see any way to calculate output power if given with divergence and diameter. Beam characteristics are independent of output power (as far as I know). You can have 445nm (blue) lasers that have the same divergence and diameter but of different power.

Last edited by Zeebit; 02-06-2013 at 03:54 PM.
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Old 02-07-2013, 12:13 AM   #11
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Default Re: How to determine the Intensity or power of laser beam...?

Quote:
Originally Posted by Elektr0n3Ro9000 View Post
okay please hear me out. what i did is a preliminary test using 1mW laser pointer. I do not know the exact power of the laser since it's china made, I calculated the beam diameter and divergence using a ruler and white paper. I attached the white paper onto a wall and the ruler onto the white paper then my distance is 3m and the aperture diameter is 1.75mm and the secondary diameter (in 3m away) is about 5mm so using the formula for beam divergence i got 1.08mrad. with knowing these can't I just use some improvised stuff or maybe formulas for theoretical approach to know the intensity or just the power? I don't think they sell those LPM stuff around in my place i'm not from US so it's hard to find those stuff and it doesn't come cheap if they are any here plus the shipping is a major issue i need the data 2 days from now. TO MAKE THE STORY SHORT: Any improvised methods for LPM? ANY SUGGESTIONS? I appreciate all of your comments. *sorry for the bad english*
So... with your eyes open you see a dot on the wall.

If you use a 100mw Laser that has a beam diameter of 1.75mm
at the aperture
and a beam diameter of 5mm at a distance of
3 meters
what type of formula do you think you would need to
get 100 times the value as when you measured the 1mW Laser..

The physical dimensions of a beam will not give you a Laser
Power measurement and even if you wanted to know the
Power Density of the Laser you would STILL need to know
the actual Power output of the beam.

That can only be done with a calibrated Laser Power Meter...


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Last edited by lasersbee; 02-07-2013 at 12:15 AM.
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Old 02-07-2013, 02:32 AM   #12
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Default Re: How to determine the Intensity or power of laser beam...?

i get it now, so my second question is can you do some improvised LPM? went to some shops there's no available.
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Old 02-07-2013, 02:52 AM   #13
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Default Re: How to determine the Intensity or power of laser beam...?

There are some threads here about DIY LPMs. Just search them up because I'm to lazy to do that for you.

You must calibrate it with a laser with a know stable output.
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Old 02-07-2013, 02:57 AM   #14
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Default Re: How to determine the Intensity or power of laser beam...?

Figuring out how strong or bright a laser is outputting is hard to do without a LPM. The LPM really makes it easy and simple. Now If... and this is a big if. If you have other laser pointers the same color, and you know what their output is, you can compare them side by side and get an approximation of how strong the unknown laser is.

Your original question had something to do with intensity AND power. Power, you cannot measure without some measuring equipment, like an LPM. But, if you have a bunch of lasers that are already measured, then you can approximate the unknown laser's power by comparing them to each other.

Intensity sounds a little like density. If we know that rock #1 is more dense than rock #2, we know that for the same volume (an example is a cubic centimeter) rock #1 is heaver than rock #2. The same can be applied to the intensity of light. If I take a 100mW laser, and focus the beam wide, so it makes a one meter spot on the wall 10 meters away, the light will be much less intense, than if we focus the same 100mW laser down to a 1mm spot, 10m away. So the same laser can produce light that is more dense, more intense by making a smaller spot. It is more intense, per square centimeter, right?

Just as we cannot determine which rock is more dense, without a method to weigh the rock and measure it's volume, we cannot determine how intense a laser is, or the density of its light, without a way to measure the light output. That is why we need an LPM. We can measure how much space the spot takes up on the wall, but without knowing its output from an LPM, it is only part of the information we need.

Measuring a spot size, at difference distances from the laser, we can, as you already know, figure out it's divergence, and predict what the spot size will be at any given distance. But without knowing the measured power using an LPM, we only know part of the information we need.

There are things like burning black tape, popping black balloons and lighting matches that have been done so many times by people with LPMs, they begin to know how much output power it takes using the different colors (wavelengths) to do these things. If your laser cannot do any of these things, all we know is that it is below the power it takes to do them. We do not know by how much power it is below.
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Old 02-07-2013, 04:21 AM   #15
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Default Re: How to determine the Intensity or power of laser beam...?

so it's really really needed huh.. just one last question/ favor can you please tell me the power range of a regular <1mW laser pointer? im pretty sure you guys already tested one since you guys have one of those LPM. im planning to order online (in US) but it's going to take about 3-5 months! thanks so much appreciate all the help.
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Old 02-07-2013, 04:38 AM   #16
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Default Re: How to determine the Intensity or power of laser beam...?

The power range? Since it's less than 1mW, the power range is 0-1mW Is taht what you mean?

Or how far it will shine?

Why not buy one from a forum member already in the EU?
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