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# How to Tell People Your Hobby is Making and Collecting Lasers

#### Wolfman29

##### New member
I think the question is about how we perceive light though, not about intensity fading with the square of the distance. But yes, assuming that we are talking about any sort of diffuse reflection (be it scattering or just normal reflection), the cause of intensity fading with the square of the distance is just mathematical.

#### scumbagatheist

##### 1
I hear what you're saying gentlemen. But, without testing (yet), I cannot imagine that if I measure one of my lasers with my Ophir @ 1 meter from the diode, slightly over-focused (don't burn the thermopile), and then measure it again at 2 meters (without changing focus), that I am going to get a reading that is 1/4 the output of the first!

I will have to test this tonight when I get back home. And some said it was silly to carry my lasers and LPM all in one case... :crackup:

#### Wolfman29

##### New member
Wait, so now we're talking about distance vs. power?

That's purely mathematical. LPMs basically integrate intensity over the area of the thermal sensor. So, if the intensity stays constant (which is does at a constant distance), it doesn't matter how big the dot is on the sensor. However, note that if you defocus it enough such that the dot is bigger than the sensor, you will note that moving it away WITHOUT changing the focus will decrease the power. That's how our eyes work too: the sensor (in this case, our eyes) stays at (relatively) constant area, and so increasing the distance to sensor will decrease intensity.

So I just deleted a false statement. I thought about it more while showering.

Here is a picture depicting the effect.

So the black line is the detector and the green line is the diameter of the beam at that point Note that the detector stays the same size. If we assume the beam is roughly circular, then the area of the beam at that point is Pi*(D/2)^2, or Pi*D^2/4. If the same amount of photons pass through both cross sectional areas, then the intensity of the beam would correspond to:

I = 4*Power / ( Pi * D^2),

where "Power" is the TOTAL power of the laser. That is, the power of the laser as measured by an LPM where the dot is smaller than the detector. Note that the diameter doubles as you double the distance, which makes sense. So let's call I_1 at the initial distance and I_2 at twice the distance. Then, we know that the diameter at I_1 = D and the diameter at I_2 = 2*D. So, we can calculate the intensity at I_1 and I_2:

I_1 = 4*Power / (Pi * D^2 )
I_2 = 4*Power / (Pi * (2 * D) ^2 )
= Power / (Pi * D^2)
= (1/4) * I_1.

Now, we know that the power read by an LPM is the intensity of the beam integrated over the area of the detector, which corresponds to the intensity of the beam multiplied by the area of the detector. So we can calculate what the power of the beam would read in each of these cases:

P_1 = I_1 * A
P_2 = I_2 * A
= (1/4) * I_1 * A
= (1/4) * P_1.

Here, P_1 and P_2 are not actually the power of the laser because it is not reading the whole beam! That's the discrepancy between "Power" used about and these apparent powers, P_1 and P_2. So, you can see here that the power read from doubling the distance is going to be a fourth of what the power is read at the original distance, assuming the size of the beam is at least as big as the detector at the initial distance.

It results strictly from the math.

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#### DrSid

##### New member
Energy in whole beam will stay more or less constant .. so if you fit whole beam on the sensor in both cases, you should get more or less same reading.
Change to focus so it just fits the sensor at first distance .. then double the distance .. and it's rather clear that now you should fit about 1/4 of the area of the spot on the sensor ..

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#### hakzaw1

##### Well-known member
As I said this is no big deal- i had it wrong for a long time in my thinking that a 400mW laser was twice as 'bright' as a 200mW and was told be someone way more learned thaT it would take an 800mW to see twice the 'brightness'( by eye)from 200mW.

There have been many posts here (from mostly newcomers) saying THINGS LIKE: they were disappointed when their new 100mW was NOT ten times brighter than their 10mW.And we are talking ONLY about lasers of the 'same' wavelength- -- when you start mixing colors that all changes a lot.

Good sharing of info thanks to all.

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#### IsaacT

##### New member
The only 532nm I have had for a long time(since I sold my old Jetlasers Ti-A 250mW almost a year ago) is my 100mW. I cannot WAIT to get my 800mW CNI unit that is on order!!!! It is going to be mindbogglingly bright!

#### CDDLDLSR

##### New member

Worse

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#### DrSid

##### New member
Double post AGAIN ? Could be funnier if it was not for the strange dude in your avatar ..

#### EpicHam

##### New member
Double post AGAIN ? Could be funnier if it was not for the strange dude in your avatar ..
Thats called "Turkish Spam"

Staff member

Worse

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#### Laik

##### New member
I'm know as 'the laser kid' at school. Here is a regular conversation when people just find out i can make lasers:
Curious person: How do you make them?
Me: It's complicated, you don't want to know
Curious person: Yes, i do!
Me: Okay, well you first have to start with what diode you wan't...
Curious person: What's a diode?
Me: The part that excites the photons
Curious person: Ughhh....
Me: The part that makes the actual laser, anyway, then you need to figure out what host you need and what you want your heat sink made out of and what driver and what you want it set to *turns into a big mumble* Don't worry about it, it is too hard to explain quickly to you
Curious person: Okay... So what do you do with them?
Me: Well, they just fascinate me, i just sit their and stare at the beam for ages...
Curious person: That's pretty cool that you can see the beam but it would get boring after a little while and you spend so much money on it
Me: Did i mention i can burn stuff?
Curious person: Really, what can you burn!?
Me: Lot's of stuff. I can engrave plastic, pop balloons, cut tape...
Curious person: Can i burn that kids arm over there?
Me: Go away -_-

#firstworldproblems

#### EpicHam

##### New member
I'm know as 'the laser kid' at school. Here is a regular conversation when people just find out i can make lasers:
Curious person: How do you make them?
Me: It's complicated, you don't want to know
Curious person: Yes, i do!
Me: Okay, well you first have to start with what diode you wan't...
Curious person: What's a diode?
Me: The part that excites the photons
Curious person: Ughhh....
Me: The part that makes the actual laser, anyway, then you need to figure out what host you need and what you want your heat sink made out of and what driver and what you want it set to *turns into a big mumble* Don't worry about it, it is too hard to explain quickly to you
Curious person: Okay... So what do you do with them?
Me: Well, they just fascinate me, i just sit their and stare at the beam for ages...
Curious person: That's pretty cool that you can see the beam but it would get boring after a little while and you spend so much money on it
Me: Did i mention i can burn stuff?
Curious person: Really, what can you burn!?
Me: Lot's of stuff. I can engrave plastic, pop balloons, cut tape...
Curious person: Can i burn that kids arm over there?
Me: Sure you can, but do you have a license?