I thought I had a simple need: I wanted a small, neutral white, regulated, single AA headlamp which could easily be switched between flood and spot. The Zebralight H51w seemed nearly perfect, except for needing to buy two lights, one for flood, one for spot. So, I built a diffuser:
The diffuser is a Fenix AD401. I cut off 5.4 mm of the diffuser collar, on the side away from the hinge. By sliding the collar snugly against the lens mount of the H51w, the diffuser is centered on the H51w lens, as can be seen in the following photo. When in use, the diffuser is rotated 270 degrees away from what would be its normal diffusing position on a typical flashlight.
Note the diffuser did not sit flat against the outer flange of the H51 lens. To prevent glare from this gap, I glued a piece of thin cord around the outer circumference of the diffuser. I further reduced glare by painting the clear plastic edge of the diffuser.
So far, it's working very well. No problems with glare. The diffuser fits snugly against the lens when in use, but also lies flush against the H51 body when not in use.
The diffusion pattern has more artifacts than are visible in the photograph, but nothing distracting in normal use.
1. The optical quality of the diffuser is only fair. It was also inexpensive, so I didn''t expect stunning quality. I plan to drill out the plastic in the center of the diffuser and glue in a higher-quality diffusing lens, perhaps something from flashlightlens. Can anyone suggest a good source for ground-glass discs?
2. I thought the the diffuser might tend to swing away from the H51 lens, but so far it has been staying in place. Nonetheless, I will probably install some small magnets to hold the diffuser snugly against the lens.
The headband mount, visible in a few pictures, deserves a brief explanation. I removed the elastic headband from the silicone holder, and replaced it with the string retractor portion of a Petzl Zipka headlamp. It's not as comfortable as the elastic band, but it's easier to carry in a pocket. Doesn't bounce as much as the stock version, either.
I'm quite happy with the outcome, and it should be an easy mod for anyone who can't decide between the flood and normal versions of the H51.