A general rule of thumb often cited is that for the same laser beam dimeter and divergence, you need 4x the power to achieve double (2x) the brightness.
If you put a 5W, 10W, 15W and 20W laser side an output a single static full intensity beam it’s true – doubling your power does not double brightness. Yes it’s brighter, but not “double“.
Why is this?
Simple linear / logarithmic answer
Our eyes react to light logarithmically, not linearly. This allows us to have an incredible dynamic range (we can see very dark things and very bright things, sometimes in the same scene). As light gets more powerful, it takes “more” of it for our eyes to perceive a change.
The inverse square law helps explain this too.
If you were to experiment with a properly calibrated irradiance meter and a laser at different distances/ beam diameter you’d see this correlation in the irradiance. Wavelength also plays into perceived brightness, our eyes are much more sensitive to green for instance than blue (hence full colour lasers needing to use much high power blue laser diodes).
Wider beams
Beyond a certain number of laser diodes for a given wavelength, the combining method is to stack up the beams next to each other rather than overlay with combining optics like PBS cubes (reds often use both techniques, they stack them then combine). This means that larger powers get progressively larger beams, which leads to ever decreasing power densities.
Larger (wider) beams will appear less bright, as the power is spread across a wider area. The devil is in the detail of the specs and construction in terms of brightness. They are not all created equal.
Ways you can work with this in your displays
Just because you need a 20W laser say to do impressive things in a massive venue or outdoors, doesn’t mean that your 5W lasers, say, are no use. For instance, you can have a 5W laser doing solid colour hot beams along side a 20-30W laser doing wide sheets or sinewaves and out in the audience area they may actually look like they’re all the same power lasers.
Another interesting effect is if you have two identical lasers, scanning the same cue and you slow the scan speed of one of the lasers down to say 12kpps while leaving the other laser scanning at 30kpps, say, you will notice that the one scanning slower looks brighter. This is because for any given point, the laser is there for a longer period because it is scanning slower, therefore more “power“ is being delivered to that point.