This article describes how to motivate senior school pupils using environmental experiments with a luxmeter. A luxmeter is a portable device for measuring illuminance E (the SI unit is lux, lx) at industrial sites, offices, hospitals, schools etc. to check the light intensity of illuminated devices or workplaces, in order to fit valid standards. The article also shows how pupils "discover" the photometrical law in class. This type of teaching can help to show the connection between physics and daily life, and students can enjoy the process of investigating science. (Contains 10 figures.)