Colorimeter
An instrument that measures colour through three filters approximating human vision, used mainly for calibrating displays.
In depth
A colorimeter is fast, affordable and accurate on emissive sources, which makes it the standard tool for monitor calibration. It cannot measure reflective surfaces such as prints, and because its filters are fixed it can be inaccurate on displays whose backlight technology differs from the one it was designed around.
Examples in practice
A colorimeter run monthly keeps a studio monitor within a small colour error of its calibration target.