Place a drop of liquid paraffin onto the finger to reduce surface reflections. Make sure that the liquid paraffin is at skin temperature to avoid any unwanted physiological disturbances to blood flow. Although some workers have used clear nail varnish this has been found to give a strong surface reflection of the laser. Also the cooling effect of the evaporating solvent could have an effect on capillary flow. The disadvantage of the paraffin oil is that it seems to be absorbed by the skin, and needs replenishing during a long measurement.
This surface preparation is only for the benefit of the video image, it is not necessary for the CBV measurement. If there is a lot of noise during a measurement, try adding more oil to give a thicker layer so that surface reflections are further from the focal point (i.e. weaker).