A monolayer of graphene (SLG) is nearly invisible in the visible spectral range (usually a value of 97.7% is given). In our group we developed a reversed optical setup using the cross polarizing properties of mica to detect graphenes with a much higher contrast value. Depending on the wavelength, we estimated values between 10 and 20% for SLG. Our measurements at 488 nm gave 12.8+/-1.2%. Maybe this setup is also of use to your research.

Article High contrast optical detection of single graphenes on optic...

Thank you