Quantitative analysis by TLC is most frequently carried out by the visual comparison of the intensity of the spot of interest to that of a standard. This, for example, would allow the sample concentration to be compared with that of a legal standard to ensure compliance with regulatory limits. For more accurate work, spot scanning techniques must be used. In general, it is the optical properties of the separated material that are used for the quantitative assessment of a thin layer spot. If the spot adsorbs in the visible range of wavelengths, then both reflected and transmitted light can be measured. If, however, the substance in the spot only adsorbs in the UV range of wavelengths, then only reflected light can be measured, as the silica base of the plate itself will adsorb UV light. If the substance fluoresces, then the fluorescent light can be directly measured as emitted light, or, alternatively, if a fluorescent plate is used, the quenching due to presence of the spot can be measured. Unfortunately, there is no simple linear relationship between spot transmittance or reflectance, and calibration curves must be obtained.