Thus, if the path length and molar absorptivity are known and the absorbance is measured, the concentration of the substance (or number density of absorbers) can be deduced. Commonly, both parameters are constant for given set of experiments, thus, a plot of the sample absorbance against the concentration of the absorbing substance should be a straight line. In practice, a calibration curve is obtained by plotting the measured absorbance of series of standard samples as a function of their concentration. If the absorbance of an unknow sample is then measured, the concentration of the absorbing component can be determined from this graph.