Power versus brightness linearity

[Doc]

Does the apparent brightness of a laser increase at a 1:1 ratio with output power?

I know that apparent audio power is delineated in a logrythmic scale (presumably due to the compressibility of air) but didn't know if I was missing something with regards to laser power.

[mixedgas]

Does the apparent brightness of a laser increase at a 1:1 ratio with output power?

I know that apparent audio power is delineated in a logrythmic scale (presumably due to the compressibility of air) but didn't know if I was missing something with regards to laser power.

NO. Its very complex. Depends if the eye is set up for night or day, other sources in the viewing field, wavelength of the laser, contrast ratio etc.

Steve

[gottaluvlasers]

i keep going back to a simplistic ratio i remember learning. it DOES NOT take into account many variables. however it is a *ROUGH IDEA* of P:B

4X power = 2X the brightness

i am sure there are many factors like steve said that affect that. wavelength, medium passing through, angle of view...etc...etc

it is a pretty good indication though.

-Marc

[White-Light]

Tockets Chroma utility is quite good for playing around with Doc as it shows the luminance value for not only different power combinations but also different wavelengths.

Its in this thread: http://www.photonlexicon.com/forums/...ead.php?t=5783

That said there appears to be a bug at the moment so it might not run for you. I believe Tockets having a look at it now.

[Doc]

Cheers guys, I'll give it a try

[daedal]

NO. Its very complex. Depends if the eye is set up for night or day, other sources in the viewing field, wavelength of the laser, contrast ratio etc.

Steve

That is very true... all kinds of variables affect the apparent brightness of a laser.

I would be interested in knowing if there is a straight-shot formula for calculating this... For example:
AB = ((i * wavelength) + (j * power) + (k * particle count)) - ((l * viewing angle) + (m * ambient brightness) + (n * other same-direction light sources))

AB is Apparent brightness
i, j, k, l, m, n are constant modifiers
All others are constants

At the same time, I would assume that the function is truly a second derivative as the residual image is also part of the perceived brightness... so the rate of change in any of the above values would make a difference, while a stationary single-color laser, and constants for the other values would constantly give the same apparent brightness...

My brain hurts... We need Tocket's input on this one

--DDL

[allthatwhichis]

Reading that made my brain hurt...

[tocket]

Luminance is directly proportional to the radiant power of the laser, but the perceived brightness is not. Calculating the luminance of a light source is fairly easy. However, calculating the luminance of a laser beam in fog/haze at a certain viewing angle is not so easy, as it involves Mie theory. It would indeed be nice to have a single equation to relate the perceived brightness of lasers under different conditions, but such an equation would be complex and require a very large number of parameters.

As for the relation between luminance and perceived brightness one can use Stevens' power law. It is used to describe the relation between the magnitude of a physical stimulus and its perceived intensity. The equation is simply:



The question here is, what is the exponent a? According to Stevens himself, it is 0.33 for a 5° target in dark, 0.5 for a point source and 1 for a briefly flashed point source. Which value of a is most appropriate for our application? I don't know. Probably somewhere in the 0.33-0.5 region.

If a is 0.33 the perceived brightness is proportional to the third root of the luminance, which means that an 8-fold increase in luminance is required to double the brightness. In the case of 0.5 it is instead proportional to the square root, thus only a 4-fold increase in luminance is required.

Of course, it is not really this simple (as always). Even if Stevens' power law would hold for monochromatic light sources, it certainly does not for polychromatic. For example, green-blue mixtures appear brighter than the sum of their components, whereas red-green mixtures appear dimmer.

I have some trouble with this field however, because it involves sensations. How can you quantify a sensation? That's psychophysics...

[allthatwhichis]

I guess the only way to truly answer this is to experiment... Sounds like a LEM function as there is easy access to multiple projectors, meters, and an abundance of crack heads like us who give a shitte about this...

The question here is, what is the exponent a?

Wouldn't you interpolate it between the 0.33-0.5 region based on the time of the briefly flashed point source?

Of course, it is not really this simple (as always). Even if Stevens' power law would hold for monochromatic light sources, it certainly does not for polychromatic. For example, green-blue mixtures appear brighter than the sum of their components, whereas red-green mixtures appear dimmer.

Couldn't you average the two and factor in our eye's sensitivity to the wavelength some how? Be interesting if it worked out similar to the monochromatic wavelength of the same color; 488 for blue green, 594 for red green.

[daedal]

Luminance is directly proportional to the radiant power of the laser, but the perceived brightness is not. Calculating the luminance of a light source is fairly easy. However, calculating the luminance of a laser beam in fog/haze at a certain viewing angle is not so easy, as it involves Mie theory. It would indeed be nice to have a single equation to relate the perceived brightness of lasers under different conditions, but such an equation would be complex and require a very large number of parameters.

As for the relation between luminance and perceived brightness one can use Stevens' power law. It is used to describe the relation between the magnitude of a physical stimulus and its perceived intensity. The equation is simply:



The question here is, what is the exponent a? According to Stevens himself, it is 0.33 for a 5° target in dark, 0.5 for a point source and 1 for a briefly flashed point source. Which value of a is most appropriate for our application? I don't know. Probably somewhere in the 0.33-0.5 region.

If a is 0.33 the perceived brightness is proportional to the third root of the luminance, which means that an 8-fold increase in luminance is required to double the brightness. In the case of 0.5 it is instead proportional to the square root, thus only a 4-fold increase in luminance is required.

Of course, it is not really this simple (as always). Even if Stevens' power law would hold for monochromatic light sources, it certainly does not for polychromatic. For example, green-blue mixtures appear brighter than the sum of their components, whereas red-green mixtures appear dimmer.

I have some trouble with this field however, because it involves sensations. How can you quantify a sensation? That's psychophysics...

That is really interesting... it seems as though I should be getting 2 items that would help with this...
1. a luminance meter (ones used for photography)
2. a particle counter (quantify the number of 'fog' particles in the air)

I am not quite sure how the luminance meters work for different wavelengths, but I do have some Opal Diffusing Glass.

So, I can use that with a photocell with a known response curve, strobe the beam, change the wavelength, change the mixture, change the power, change the angle... etc. This should give a nice idea of how this will compute... I'm not saying it's easy, and I'm not saying I'll do it today... but maybe in some months when I clear a few projects off my plate

I would be interested in knowing if anyone has any experience with calculating luminance...

--DDL

P.S. Tocket, does polarization have anything to do with perceived brightness? I would assume that it has something to do with how the light reflects off a surface, right?