Help understanding mRad

Was wondering if someone could explain what is actually being measured in divergence.

In simple terms, I'm understand divergence is an increase in the beam width, so a 3mm beam at apperture compared to a 4mm beam at a point of 10m has X mRad divergence, thus the lower the divergence the more 'true' the beam stays for longer. (think thats correct at least)

I've been reading wikipedia for Radian and assume the measurement is effectively the increase in the radius of the beam width, but I'm not certain thats right, mainly due to measurements indicating angles within a circle.

Not really trying to get to complicated, but just looking for a clearer understanding of how it is measured and what measurements are taken.

Thx

in the most basic of terms, divergence is the amount the beam expands over distance. mrad is the calculation of millimeters of divergence per meter.

if a 1mm beam expands to 10mm over a distance of 10M, the mrad value is .9. 10(far field size in mm) - 1(near field size in mm) / 10 (distance in meters).

if a 1mm beam expands to 10mm over a distance of 1M, the mrad value is 9.

this assumes a reasonably gaussian (round) beam.

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Originally Posted by skot

Was wondering if someone could explain what is actually being measured in divergence.

In simple terms, I'm understand divergence is an increase in the beam width, so a 3mm beam at apperture compared to a 4mm beam at a point of 10m has X mRad divergence, thus the lower the divergence the more 'true' the beam stays for longer. (think thats correct at least)

I've been reading wikipedia for Radian and assume the measurement is effectively the increase in the radius of the beam width, but I'm not certain thats right, mainly due to measurements indicating angles within a circle.

Not really trying to get to complicated, but just looking for a clearer understanding of how it is measured and what measurements are taken.

Thx

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Thank you Swami.

Just remember that radians are a unit of measurement for angles, just like degrees. Think triangles... if you take two legs of a triangle and keep them close together, you have a small angle and a small third side. If you pull those puppies apart, you have a wide angle and a large third side. Just like a flashlight, er, a mitsubishi diode. ;)

For the record, there are 360 degrees in a circle, and there are 2*Pi (or about 6.3) radians in a circle.

A milliradian is just 1/1000 of a radian, just like a millimeter is 1/1000 of a meter, and a milligram is 1/1000 of a gram.

here have some radian

http://i.imgur.com/AQUrYb1.gif

Yeah thats the gif I've seen on wikipedia.

Whats getting me is one radian is measured along the circumference where the distance between two points is equal to the radius, with the angle that is created is within the circle.

Might be I'm confusing the circle diagram as the beam spot?
i.e. the black dot shown at the centre is actually the beam souce and the radius would be the distance to the projected spot (in other words its a side view, rather than cross section of the beam spot).
Does that sound right?

Yes, the center of the circle is the beam origin, and the radius is the distance to the wall you're projecting on. The wall is flat, not an arc, but the central angle is still the same.

Cool, thx for clearing this up. All the pieces fall into place.

:cool:

Just something to bare in mind. Divergence is measured in mrd but in FULL ANGLE so when you see 0.8mrd half angle its actually a 1.6mrd divergence. Anything above 1mrd is fine for beamshows but not great for graphics. Half the divergence means double the brightness. That,s why i like low divergence, better graphics and you only need half the power......;)

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Half the divergence means double the brightness

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Half the divergence means four times the irradiance @ equal power.

Perceived brightness is a very complicated topic, and contains many more variables than divergence alone.