The wavelength of light is measured using a spectrometer, while the intensity of light is measured using a photometer. The speed of light is measured by measuring the time light takes to traverse a known distance and dividing distance by time.
A spectrometer measures the properties of light from a specific portion of the electromagnetic spectrum. The device measures the intensity and polarization state of the different parts of the target spectrum. Different wavelengths give different photon counts and different wave numbers, enabling the measurement of a discrete or continuous spectrum.
Spectrometers use diffraction gratings to split light into movable slits, isolating specific parts of the spectrum, while a photodetector measures the intensity of that specific spectral region. Spectrometers measure wavelengths from gamma rays to the far infrared.
A photometer measures total light intensity falling on a target surface or passing through a target material. Digital photometers can infer other properties of the incident light, such as illuminance, irradiance, absorption, scattering and fluorescence. Photometers detect light using photoresistors, photodiodes or photomultipliers. A removable filter may be installed on the photometer to isolate light of a specific wavelength for intensity measurements. A combination of a photometer and a spectrometer is called a photospectrometer.