Crooke's Solar Radiometer, Short Clear Stem, Blue GlobeSolar radiometers are also known as light mills because light moves their blades as opposed to wind for windmills. They consist of an airtight bulb containing a partial vacuum. Inside the glass bulb is a fine vertical spindle with a set of vanes mounted at the top. When the glass bulb is exposed to light, the vanes are propelled around, the speed of propulsion increases with the intensity of light, providing a qualitative way of measuring