The first-order difference in reflectivity of crystals surfaces due to the adsorption of a (sub)monolayer is calculated for crystals transparent to the incident radiation. Conditions are derived for optimizing experimental studies of both strongly and weakly absorbing surfaces and/or adsorbates. The large and small absorption bandwidths involved correspond respectively to (i) electronic transitions to/from surface, interface and adsorbate states, and, (ii) vibrational excitations of adsorbates. The variable parameters are the angle of the incident light beam and the state of the back surface. It is found that the sensitivity varies with angle of incidence, depending on the type of absorbing monolayer.