Lux is Latin for 'light' and is also the preferred SI unit (International System of Units, or Système international d'unités in French, thus 'SI') of measuring luminescence, the less favoured being the lumen.
It's also a powerful hardware-driven rendering program, LuxRender, and something I've been mucking about with recently. It's open source (and thus free) and has a number of extremely cool features. But it's also very slow. Have a look at these two pictures I rendered up today...