From the outside, it looks like an ordinary plastic bottle stuck through a roof. Inside a room without windows, it glows like a light bulb in the middle of the day, with no wire and no switch. The idea was born in Brazil, and El Cronista reports it’s now used in more than 30 countries. Here’s how it works and why it spread.
In short
- A clear plastic bottle filled with water and a little bleach is fitted through a hole in the roof, half outside, half inside.
- The water refracts sunlight and spreads it through the room.
- The light is roughly equivalent to a 40 to 60 watt incandescent bulb, during the day only.
- The hole must be sealed properly, and the roof checked before drilling.
What it’s for
The technique was “developed in the early 2000s by Alfredo Moser, a mechanic living in the city of Uberaba, Brazil,” during a period of power outages, according to El Cronista. The principle is pure physics: the top half of the bottle catches the sun, and the water inside bends that light and scatters it into the room below, “in a 360° angle.”
The result is daytime light “equivalent to a 40- to 60-watt bulb,” the newspaper says. Wikipedia’s entry on Liter of Light gives the same range, adding that it depends on how much sun there is. The small dose of bleach is there to stop algae from growing in the water.
Why it’s used in more than 30 countries
The idea went global in 2011, when the MyShelter Foundation adopted it for its Liter of Light program in the Philippines. According to Wikipedia, founder Illac Diaz’s team installed 15,000 bottle bulbs across twenty Philippine cities within months, and more than 200,000 worldwide within the first year. El Cronista attributes the figure of “more than 30 countries” to UNESCO, which highlighted the project. Wikipedia describes projects in more than 20 countries.
How it’s installed
- Use a clear plastic bottle and fill it with water.
- Add a small amount of bleach to slow the growth of algae.
- Fit it through a hole in the roof, with the upper half exposed to the sun and the lower half inside the room.
- Seal the edges with a waterproof product to keep rain and moisture out.
A properly installed bottle can last up to about five years, according to Wikipedia.
The limit
The bottle “depends entirely on the solar radiation present,” El Cronista warns: it lights a room during the day and does nothing at night. Liter of Light uses separate solar products for that. Drilling a roof also calls for a structural check and a look at local building codes first, and Wikipedia notes that finding a sealant that stays waterproof in the sun is the hardest part, since some make the plastic brittle over time. In a home with electricity, it makes the most sense in a shed, a workshop or an off-grid cabin.
Lighting is changing in other ways too: see why some say goodbye to LED bulbs. A plain bottle has saved more than a room: here’s the sailor who survived 24 days at sea thanks to one. And for the power bill, read how one homeowner cut his heat pump’s electricity use by 35 percent.
One bottle, a little bleach, a well-sealed hole and the sun. That’s the trick.
