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.

Quote:

---

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

---

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......;)

Quote:

---

Half the divergence means double the brightness

---

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.

Nice to know but its better to be a little reserved when promoting that my products are more bright then others. If people compare and don,t experiance the 4x brightness while promoting them like and i have to deal with the complaints and the headache :p. So i prefer to be a little on the safe side. But one thing is for sure lower divergence helps allot to get a higher brightness:cool:

I have always resented Kavant's use of 1/2 angle divergence. They are about the only manufacturer I have come across that does this. This is a big deal. A beam that is twice as tight is much harder to manufacture and when equally powerful will be much brighter. Furthermore, a tighter beam allows a smaller beam at the scanner mirrors, small mirrors and therefore faster scanning. Whenever possible compare lasers using a "beam product". Multiply the exit diameter by the divergence. The smaller this # the better the beam.

Been meaning to say Thx Edison for pointing that out. I had noticed the Kvant divergence numbers and had no idea about the 1/2 divergence. Thought they were just good numbers. I guess its a bit of marketing in play, and is good to know.

Eric are you saying use straight multiplication? e.g. 3mm x 0.7 divergence (1/2 angle)
Wouldn't this still give a better result that using the full divergence? In this case 3 x 1.4
I'm clearly misunderstanding your calculation. I'm assuming the exit diameter to be beam size of the out put (3mm in my example used above).

Can you please elaborate on what you mean.

The beam product that I suggest is the product of the exit beam diameter x the divergence and the divergence should always be according to the same standard. So, if you are comparing 1/2 angle divergence then always use 1/2 angle divergence. If you are using full angle divergence (the much more common standard...I mean who the hell looks at 2 cm diameter far field spot on a screen and says "that's a great 1 cm diameter half of the spot") then always use the full angle specification. For Kavant, just double their divergence spec and then compare it to the rest of the world.

Ahh gotcha. .

Been meaning to say Thx Edison for pointing that out. I had noticed the Kvant divergence numbers and had no idea about the 1/2 divergence. Thought they were just good numbers. I guess its a bit of marketing in play, and is good to know.

Yep its a marketing thing, most people look at the numbers and say oh great its only 0.8mrd. Imagine when a guy bought it and someone points it out afterworths, i,m sure he feels screwed......

Look at the specs of a new car, it says that its super efficient when it comes to miles to the gallon. Do the math when you have used a full tank of petrol after normal usage.........;)