Tweaking light output

These posts are somewhat out of order.. so sue me ;)

On my orange solar jar (dual LED, dual AA battery), I decided to see if I could boost the light output by swapping in something other than the generic oem Chinese rechargeable AA batteries that come with it (which are 600mAh). A pack of four Energizer rechargeable 2450mAh was $14 at CVS drugstore (raising the cost of this jar from ~$14 to $21).

Adding the 2450 mAh batteries boosted light output significantly. The LEDs are very "spot", and are pointed straight down, so the brightest area is actually under the jar. I will be working on redirecting this light in the future. My light readings (taken with a Sekonic L558-C) indicate between 1 and 3 footcandles (at exterior surface of glass) around the sides of the jar, when fitted with the oem Chinese batteries. These readings were taken with the hemispherical dome extended, in a completely darkened room, meter in incident FC mode. The bottom of the jar read 11 footcandles (a BIG different between the bottom and sides!). After swapping for the Energizer batteries, I metered again, and now found between 1.5 and 4fc on the sides, and 30fc on the bottom!

Given the way light works (inverse square laws and such), I would expect a doubling or trippling of light at the bottom to have an non-linearly reduced affect on the sides of the jar, hence the 25% increase at sides. I believe this can be shifted in favor of light output by making the bottom of the jar more reflective, as it is currently probably much worse than standard 18% gray, let alone something designed for such a purpose (for instance, panda film). (I believe the light mostly passes through the bottom of the jar, being diffused at least 1-2 stops by the frosted glass, bounces off whatever is underneath, goes back through the glass yielding another reduction in light, and then scatters inside the jar).

Research

My initial research into building solar jars found several "DIY" and "HOWTO" type web sites (1, 2). These sites in turn had suggestions for how to actually assemble the thing, as well as where to source components.

The main issues are:

The electronics (battery, LED, light sensor, photovoltaic panel, etc)

• The electronics can best be sourced in one place by canabalizing a solar driveway light (or similar). A friend found Lowe's offers a 12pack for $30. I regret this purchase (more on that later, but basically.. they are too dim, too bad I didn't see this page first, as I think this jar uses the exact same guts). Second, I sourced a single "Fence Light" from Hampton Bay (a Home Depot house brand), for $9.xx. This light has 2 amber LEDs, and two 600MaH batteries. I looked at a couple other lights, and discovered the PV panel wouldn't fit in the mouth of my Slom jars, so I would recommend buying your jar(s) first and bringing to 'eyeball' the fit before buying electronics.

The jar

• I started googling to try and find a source for Mason jars, and quickly realized that the "latch top" jars that work best are not made by Mason. Mason jars have a tin lid. Searching for "hermetic jars" seemed to be the best way to find the right type of jar. Ebay and Amazon both have options, but they are slow, involve shipping costs, and generally offer just a single jar. Ultimately I went for the IKEA Slom, as it's cheap, and even though most IKEA products are a little, um, crappy, I figured it's hard to screwup glass+silicone.

The only issue with the IKEA Slom jar is that it isn't frosted. I assumed I would have to do this myself. Some web sites suggest various methods. Basically, any of the following would work:

1. Rustoleum "frosted glass" aerosol spray
2. Beer + Epsom salts
3. Glass Etching liquid/cream/etc. BE CAREFUL.
4. Sandblasting

I went with #1, because the jar didn't need to contain anything else (so the propensity of the spray to flake off or chip wouldn't be an issue). If the jar needed to contain stuff, especially anything that might eventually be eaten, I'd have gone for the glass etching liquid (which is not only dangerous, but also expensive-- although it can be reused, so the cost isn't that bad over say, 10+ jars).

The Beginning

You may or may not already be aware of the SunJar designed by Tobias Wong. I've been a fan of the piece since seeing it online a year or two ago, and when a friend of mine who tutors received one as a holiday gift from the parents of one of her students, I started thinking about building a sun jar. I like tinkering, and well aware that I could probably build several for the $50 cost of just buying the retail version, I decided to go for it.

This blog is a place to aggregate my thoughts/experiences, and perhaps assist others who want to tinker rather than just outright purchase a retail product.