Measuring brightness with a grease spot photometer
30 min · by Peter Bohacek · Updated Oct 7, 2026
A piece of white paper stained with oil or grease allows students to compare the intensity of light sources. The device takes seconds to make: just rub a small amount of oil or grease onto a piece of paper. Then place one light source on either side of the paper and observe whether the grease spot looks light or dark.
Learning goals
Students can design and conduct an experiment to compare the intensity of light sources, and explore how distance affects intensity.
Equipment needed
a piece of white paper
some oil or grease such as vegetable oil, butter or margarine, mineral oil
access to some sources of light, such as a window, cell phone, or flashlight
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