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# Wrong beam divergence calculation?

#### XrayX

##### New member
Hi I buy my first green laser pointer.
And for fun I calculate the divergence .
But the calculator give me 0.34 mRad but I think I didn't make the calcul with good measure for the 2 points needed.

Anybody can help me?
Thank you very much
In the forum i see 0.78mRad for green in general

#### Cyparagon

##### Well-known member
Use metric (or at least convert to metric)
Measure the diameter of the dot in mm at a distance of at least 10m, or the furthest you are able. Longer distance means more accuracy.
subtract 1 (for initial beam diameter)
divide by the distance to the dot in meters

Example:
dot: 33/64 inches at 36 feet
dot: 13mm at 11m

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#### XrayX

##### New member
Thank you but I do not all inclusive.
Can you explain all the ways of another?

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#### Cyparagon

##### Well-known member

Um... wat? I sense a language barrier here.

#### XrayX

##### New member
oh Sorry for the last message.
I didn't understand what you try to explain to me.

#### steve001

##### Well-known member
oh Sorry for the last message.
I didn't understand what you try to explain to me.
In metric units measure the beam diameter at 30 meters. Then if you want too, measure the beams diameter at distances greater than that for more accuracy. One way to do it is to use a piece of paper draw an + on it. Then mark the + in millimeter increments on all four sides. Now measure a length of 30 meters from the laser pointers position. Then position the + at that distance and center the beam on the +. A piece of window glass can be used to cut off the bloom created by shining on a white paper. Any color paper can be used instead. It's important to know the beams diameter as it exits the laser pointer.

#### lazeristasUVISIR

##### Active member
Hi I buy my first green laser pointer.
And for fun I calculate the divergence .
But the calculator give me 0.34 mRad but I think I didn't make the calcul with good measure for the 2 points needed.

Anybody can help me?
Thank you very much
In the forum i see 0.78mRad for green in general
I took a picture from a Wikipedia about Gaussian beams - Gaussian beam - Wikipedia, the free encyclopedia

A big theta is a full divergence angle. Wiki calls it a "total angular spread". Gaussian optics in various formulas uses just halve of it (a small theta).

A divergence measurement should be done at two locations at a far distance. How far? It depends on the initial beam diameter. Probably yours is ~1 mm, and it corresponds to zR (a Rayleigh range) ~1.5 m.
Thus start your first beam diameter measurement at 2 m and the second farther.
You'll get DiameterOne at LocationOne, and DiameterTwo at LocationTwo.

Full Divergence = (DiameterTwo - DiameterOne ) / DistanceBetweenTwoMeasurements.

For a better accuracy try to go as far as possible.

Compare your result with the theory:

theta = wavelength / pi / InitialRadius

wavelength = 532 * nm = 532 e-9 m

##### Well-known member
Also read up on this page so you can understand why some laser beams have better divergence parameters than others.

#### XrayX

##### New member
wow thanks every body.I go calculate my divergence.
But last night i see my beam is like that ->

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#### lazeristasUVISIR

##### Active member
wow thanks every body.I go calculate my divergence.
But last night i see my beam is like that ->
Looks like that your beam is focused. Can you access and rotate the front lens?

No i have this

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#### XrayX

##### New member
Yes its removable.
Do you know how to extract the head? (the part with the "coper" ring)

#### XrayX

##### New member
I took a picture from a Wikipedia about Gaussian beams - Gaussian beam - Wikipedia, the free encyclopedia

A big theta is a full divergence angle. Wiki calls it a "total angular spread". Gaussian optics in various formulas uses just halve of it (a small theta).

A divergence measurement should be done at two locations at a far distance. How far? It depends on the initial beam diameter. Probably yours is ~1 mm, and it corresponds to zR (a Rayleigh range) ~1.5 m.
Thus start your first beam diameter measurement at 2 m and the second farther.
You'll get DiameterOne at LocationOne, and DiameterTwo at LocationTwo.

Full Divergence = (DiameterTwo - DiameterOne ) / DistanceBetweenTwoMeasurements.

For a better accuracy try to go as far as possible.

Compare your result with the theory:

theta = wavelength / pi / InitialRadius

wavelength = 532 * nm = 532 e-9 m
Thank you to all the people who have responded.
I calculate a divergeance of 0.71875mRad.
There is a difference good or very bad?
It is true that my other income (0.34mRad)

#### lazeristasUVISIR

##### Active member
Thank you to all the people who have responded.
I calculate a divergeance of 0.71875mRad.
There is a difference good or very bad?
It is true that my other income (0.34mRad)
If you observer that a beam is focusing, the laser divergence can be improved. Just need to adjust a lens, if such possibility exists.