Fluorescence Suppressor. When a sample is observed to have lost colour or have changed colour during washing, this may be due to the effect of washing or the effect of the presence of optical brightener on the sample. This can be assessed using the fluorescence suppressor. Both the tested and untested samples are placed side by side. The fluorescence suppressor is then placed over the 2 samples. If the observed difference is not present when viewed through the fluorescence suppressor, the change in shade or colour loss is due to the effect of optical brightener.