query: What unit of measurement is used to measure light intensity?
passage: Luminous intensity
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela(cd), an SI base unit.
passage: Fluorescence interference contrast microscopy
The observed fluorescence intensity, formula_1, is the product of the excitation probability per unit time, formula_2, and the probability of measuring an emitted photon per unit time, formula_3. Both probabilities are a function of the fluorophore height above the silicon surface, so the observed intensity will also be a function of the fluorophore height. The simplest arrangement to consider is a fluorophore embedded in silicon dioxide (refractive index formula_4) a distance "d" from an interface with silicon (refractive index formula_5). The fluorophore is excited by light of wavelength formula_6 and emits light of wavelength formula_7. The unit vector formula_8 gives the orientation of the transition dipole of excitation of the fluorophore. formula_2 is proportional to the squared projection of the local electric field, formula_10, which includes the effects of interference, on the direction of the transition dipole.