Beam thickness and divergence relation to Brightness

[yoonie]

Hi guys,

Tl;dr :
How important is it to consider beam thickness and divergence for a similar powered laser if we want lots of brightness/visibility.

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More context :
I'm new to single diode laser, and searching I'm trying to understand more.
With DPSS 532, at some point most laser were offering pretty thin beam and low divergence, now single diode seem to have a wide range of choice, which is making thing more complicated to choose a diode.
I assume I know the basics about brightness regarding wavelenghts, human vision, atmospheric condition, output non linear ratio to brightness.

Now for the question :
let's say we have 2 setup, both have their diode and driver are calibrated to deliver 100mW of 520nm,
one will have a 0.8 mrad div and very thin beam
the other would have a 2.5 mrad div and wide beam over 2mm

I assume this hypothesis would work as the DPM would be close enough that the sensor would still get all the output power and not be affected by divergence (you can correct me if I'm wrong there)
But from a few meters away, I would assume it could make a real difference in brightness as having same power trying to make their impact on air molecules for one the area it will have to lit up would be considerbly larger, hence a loss of brightness.

(sub question in there is, wouldn't a 100mw 532 dpss be brighter than an hypothetical single diode 532nm as DPSS still seem to offer best beam profile? (not even talking in the loss from artifact which seem greater in single mode)

Sorry for the long question which the answer is probably scattered through post already made, but couldn't find them.

cheers

 

[Encap]

There are no 532nm single diode laser , only DPSS.

DPSS laser have better beam quality than single diode lasers/ Single mode diode laser have better beam quality than multimode diode lasers

You are mixing physics terms and subjective visual perceptions terms.
Not sure what you want to know but yes the thinner beam will appear visually brighter than a wide beam of the same mW power and the a smaller spot at a given distance created by a beam with lower divergence beam would be visually brighter than a larger spot given both lasers are the same 100mW output. Is just a question of energy density per area usually expressed in mW/cm squared.

What is it you actually want to do or accomplish or know beyond the above.

See for discussion of single mode vs multimode diodes: https://laserpointerforums.com/threads/single-mode-and-multi-mode-laser-diodes.53695/

 

[yoonie]

Hi,
thanks for your reply.

Indeed, that is why I stated "hypothetical 532nm single diode"
I don't think I am mixing physics, as Brightness is still a physical specificity that can be measured as illuminance.
I was not asking about subjective perceptions there, I think you understood this as part of your answer is helping for reformuming the question :
the energy density is what I am talking about, and how can we relate that to a beam's brigthness.

My point was to ask how wrong or right are we when considering power output of a laser as a main indicator of the said laser's brightness.

I think you could add this to the experiment I suggested above :
so you take the different two lasers (same diode, same wavelenght, same output, but different beam profile), you focus them to infinity, and point them to the same angle.
Would a measuring them with a lux sensor located at the same angle from where each laser shoot, would help comparing? (that is were I'm getting lost)

With a dot on a wall at a certain distance, I'm sure energy density would definitely make a difference in brightness, but for a beam to infinity, I am still wondering...

 

[Encap]

Hi,
thanks for your reply.

Indeed, that is why I stated "hypothetical 532nm single diode"
I don't think I am mixing physics, as Brightness is still a physical specificity that can be measured as illuminance.
I was not asking about subjective perceptions there, I think you understood this as part of your answer is helping for reformuming the question :
the energy density is what I am talking about, and how can we relate that to a beam's brigthness.

My point was to ask how wrong or right are we when considering power output of a laser as a main indicator of the said laser's brightness.

"My point was to ask how wrong or right are we when considering power output of a laser as a main indicator of the said laser's brightness."
Generally speaking mW power output is the main indicator of relative visual brightness for same wavelength, common sense and experience wise for visible spectrum lasers--obviously not for invisible spectrum lasers that have no "brightness" eg. mid and above IR.
All laser output is measured in W or mW. The power output of a laser is measured by its wattage, that is, the number of joules of energy it radiates per second (1 W = 1 J ). Is not about human visual perception of a reflected beam.

"Brightness" is the term for the subjective impression of the objective luminance measurement standard i.e. the perception elicited by the luminance of a visual target. It is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed. Brightness should never be used for quantitative description, only for nonquantitative references to physiological sensations and perceptions of light.

If you want to mix and match descriptive terms and measurement systems see: http://www.unitconversion.org/unit_converter/illumination.html
Also see: https://andor.oxinst.com/learning/view/article/radiometry-photometry

There is no chart of or graph of "beam visibility" for real reasons ---whatever you come up with is wrong as you are missing fundamentally why a "beam " is visible at all. You are mixing apples and oranges -- leaving out the fundamental reality.
It is not a matter of output power or wavelength. A "laser beam" is invisible in a vacuum regardless of wavelength or power.
You never actually see the laser beam. What you see is the reflections from particles in the air called aerosols having a diameter significantly less than the wavelength of the light. That causes a beam to become visible. The more of these minute particles there are in the air the more visible" brighter" "the beam" appears to be.
Rayleigh scattering causes photons to be scattered in a roughly spherical manner around these particles. Some of the light is scattered forward (in the direction of the beam), a lesser amount is scattered to the sides and about the same amount that is scattered forward is scattered backwards towards the light source. This backwards scattering is why the beam is more visible to people standing near the a laser, than people standing some distance to the side.

What is it you actually want to know beyond common sense observation all things being equal a higher output/energy density laser beam over the same area would appear visually "brighter" than lower output/energy density laser beam over the same area'
Same for the spot however apparent visual brightness of a spot is greater than for the beam causing it.--difference in reflective surface and area?
Intensity is = Power/ Area ---simple.

What do you want to do with lasers in the real world?

Re: your original question: "How important is it to consider beam thickness and divergence for a similar powered laser if we want lots of brightness/visibility. " Minor differences of smaller beam diameter and/or less divergence would be visually brighter over a smaller area but not a dramatically. Practically speaking most important is wavelength, 532nm is most visible -- get as powerful a 532nm laser with the best beam quality you can afford for maximum possible visual brightness.
Keep in mind:
It takes 4X the output power to double the visual brightness---so twice the brightness of a 100mW 532nmnm laser is about 400mW.
532nm green is about 10X visually brighter than 445nm--a 400mW 532nm green laser is about as bright as a 4W 445nm
Beam visibility to an observer is complicated real world phenomenon of light, eye sensitivity, and reflection from particle/aerosols in the air the details of which are not intuitively obvious as discussed above.

 

[yoonie]

Thanks Encap,
I understand my confusion between brightness and illuminance,
and I'll add that my assumption of being able to find out the brightness and be able to standardise perception is way too simplified here, as angle and distance of the visible beam would be the most important thing to have a formula that would work.