Re: WL Spyder III Arctic- Full review and videos!
8. Beam Specifications and Visibility
Although 445nm brought cheap blue to the masses, and at incredibly high powers too (with diodes pushing the 2W barrier), the diodes themselves are plagued by horrible beam specifications. Unlike the well-known red and Blu-ray diodes, 445nm diodes do not run single-mode. Instead, they have a significantly larger emitter, and multiple transverse modes are allowed to run in this emitter. The end result is a highly astigmatic beam that is extremely difficult to collimate without the correct optics.
These beams have two different axes to them- the fast axis, and the slow axis. As their name suggests, the fast axis diverges more rapidly than the slow axis. As a result, there are two divergence measurements given- one for the fast axis, and one for the slow axis.
People are often confused by the two axes, especially at the point where the beam leaves the aperture. Contrary to what you may think, the wider axis is actually the slow axis, while the fast axis is usually needle-thin. The axes have differing focal points, and as a result, collimate differently, with one axis diverging more than the other. This explains why the beam profile appears rotated 90 degrees when viewed at a distance. Here are three photos illustrating this- one taken at 10cm, another at the transition point (around 1.25m), and one at 3m.
Divergence measurement was taken at 3 meters; a non-reflective heatsink was used as a beamstop.
At aperture- Fast Axis: 2mm, Slow Axis: 5mm
At 3m- Fast Axis: 7mm, Slow Axis: 3.5mm
Divergence on FA: 1.67mRad (to 2 sig. figs)
Divergence on SA: -0.5mRad (to 2 sig. figs)
Not too bad for a 445nm, especially considering the fact some DIY builds have up to 5mRad on the fast axis. Although not on-par with the 1.5mRad value given on the Wicked website, it still falls reasonably close.
Wondering what happened with the negative divergence? Turns out that this laser isn't focused properly- there should not be a negative divergence value for the slow axis. This indicates that there is a beam waist somewhere along the beam. Another set of measurements taken at 10m showed that the slow axis had expanded to 11mm, which translates to a divergence of 1.1mRad (on par with red and violet single-mode diodes). This may have been intentionally done as it may have provided the best divergence for the fast axis, while not compromising the divergence on the slow axis.
Approaching 800mW, you would have expected that beam visibility would be solid- and it is. Although far away from the peak of the visible spectrum, 445nm, being at the shorter end of the visible spectrum, is affected by Rayleigh scattering much more than green, and is also closer to the peak of blue sensitivity in the human eye. These factors, coupled with the fact that the peak of the visible spectrum shifts towards the blue in darkness, allow 445nm to rival 532nm in terms of visibility in the right conditions.
On low mode (150mW), it is of equal brightness to a 30mW green laser, both in terms of spot and beam visibility.
It takes the full 680mW before the Arctic can finally overpower my 30mW green module.
The beam is extremely visible when viewed in person, even when in a lit environment. Compared to 660nm (which is roughly equivalent from the other end of the spectrum), it is significantly more visible. My only red laser decided to go LED yesterday, so unfortunately, there's no comparison shots. Speaking of the red laser, it's the same one with the mode-hopping issues, and I'm surprised it lasted as long as it did.
However, the beam is visibly divergent, even at shorter ranges. Unlike a green (or even red) laser, the beam can be seen to appear to converge (at the point where both axes are equal) and diverge rapidly from that point.
A quick note here regarding relative visibility and color blending. Although a 30mW green may appear to be significantly brighter than the Arctic on low, the Arctic will completely overpower the green as soon as they are mixed.
And blended... As you can see, the green has disappeared completely.
Because 445nm is near the peak of blue perception in human eyes, the 445nm appears significantly more 'blue'. As a result, 1:1 ratios can be used when mixing green and 445nm- although the blue will be significantly less visible, they will balance out when mixed.