The term photometry refers to measurement of quantities for optical radiation as evaluated according to a standardized human eye response, and therefore, is limited to the visible spectral region. The primary aim of photometry is to measure light in such a way that the results correlate with what the visual sensation is to a normal human observer exposed to that radiation. Photometry uses either optical radiation detectors constructed to mimic the spectral response of the eye or spectroradiometry coupled with appropriate calculations for weighting by the spectral response of the eye. In photometry, luminance is an important parameter in a sense that it represents how bright objects look to the human eyes. The official definition of radiometry is measurement of the quantities associated with radiant energy. Photometry and colorimetry are essential for evaluation of light sources used for lighting, signaling, displays and other applications where light is seen by the human eye.