In a lasing medium when a photon encounters an atom at an excited state, it stimulates the emission of another photon. Laser light can be focused to very small spots. Light-emitting diodes (LEDs) are semiconductor devices that emit light at a selective wavelength band. The typical spectral band of single color LEDs is 15 to 50 nm. For components that are often used for lighting, it is common to find them in photometric quantities, such as in lumens (lm) or lux. Radiometric quantities are physical quantities, whereas photometric quantities reference to the spectral response of the human eye. To convert from photometric quantities to radiometric quantities for a source that falls within the visible part of the spectrum, the reverse process is used from the one described in radiometric to photometric conversion. Thermal sources are light sources that emit in the visible and infrared wavelengths.