Across West Africa, schools, homes, cookhouses, stores and workplaces alike are typically constructed from mud brick, and roofed with either grass or corrugated metal. Windows are small, because otherwise the walls will fall apart.
Very little light gets in.
Solar Bottle Bulbs aren't the only way to solve these problems. But for daytime lighting…
Kerosene was the prior generation of off-grid lighting in West Africa, now replaced with battery-driven LED torchlights. And good riddance!
Kerosene is expensive and dangerous to store and use. It emits dirty and unhealthy smoke and fumes, and is terrible for the environment.
Kerosene lanterns don't even produce much light! And they're hot!
They won't be missed.
LED torchlights are current in-place tech. Cheap, high-powered LEDs extend the life of available batteries, driving their operating costs below that of kerosene.
However… The batteries used in torchlights are terrible.
They're a recurring expense, taxing the resources of rural African families. Used batteries are often left on the ground or, at best, buried; they contain strong acids and heavy metals and both can leach into the water supply.
Thanks to rural electrification programs, grid electricity is increasingly available across The Gambia and West Africa. Where available, it's an improvement over kerosene and batteries, and can produce sufficeient light to illuminate a classroom.
But it comes with significant downsides:
There's no question about it: Solar Bottle Bulbs are safer, cheaper, cleaner, and often brighter than other available solutions.