Reflectance difference spectroscopy (RDS) is a spectroscopic technique which measures the difference in reflectance of two beams of light that are shone in normal incident on a surface with different linear polarizations.^[1] It is also known as reflectance anisotropy spectroscopy (RAS).^[2]

It is calculated as:

${\displaystyle RDS=2{\frac {r_{\alpha }-r_{\beta )){r_{\alpha }+r_{\beta ))},r\in \mathbb {C} }$

${\displaystyle r_{\alpha ))$ and ${\displaystyle r_{\beta ))$ are the reflectance in two different polarizations.

The method was introduced in 1985 for the study optical properties of the cubic semiconductors silicon and germanium.^[3] Due to its high surface sensitivity and independence of ultra-high vacuum, its use has been expanded to in situ monitoring of epitaxial growth^[4] or the interaction of surfaces with adsorbates.^[5] To assign specific features in the signal to their origin in morphology and electronic structure, theoretical modelling by density functional theory is required.