UV vis is a device that used to measure the concentration of the target compound at maximun absorbance wavelenght based on the direct relationship of absorbance value with the compound concentration.......so the decrement of the absorbance initial and after ....indicate the reduction of concentration of the compound... where uv in typical photocatalytic reaction also use UV range light (could be uv-A, uv-C) but for the different purpose which is to degrade the pollutant...