According to Grum luminescence is “the phenomenon of the emission by matter of electromagnetic radiation that for certain wavelengths or restricted regions of the spectrum is in excess of that due to the thermal radiation from the material at the same temperature.” We encounter several different types of luminescence in everyday life such as fluorescence, phosphorence, cathodoluminescence and chemiluminescence. Fluorescence occurs when light absorbed in one wavelength band generates emissions in longer wavelength bands. This shift towards longer wavelengths is called Stoke's shift.Fluorescence is specially prevalent in printed materials. It is commonly associated with the optical brightners used to whiten paper and the dyes used in the printing inks. Paper has a strong tendency to absorb in the blue region of the visible spectrum (that is why grocery bags are brown.) Optical brightners work by absorbing in the near UV region, where we do not perceive the decrease in reflectance, and re-emit the energy as blue light, which offsets the paper natural yellowishness. The most common fluorescent dyes are red and magenta. They work by absorbing radiation in the blue green regions, which makes them look more saturated, and re-emit energy in the red end of the spectrum, further increasing the colorimetric purity. Fluorescent yellow dyes are also very common.