A method of performing a measurement of properties of a sample, by directing a first beam of light at the sample, where a combination of the wavelength, energy, and length of time is sufficient to cause temporary damage to the sample. The first beam is reflected from the sample. The properties of the reflected beam are sensed to create a signal. A length of time is waited, sufficient for the damage to substantially heal, before a second beam of light is directed at the sample, where a combination of the wavelength, energy, and length of time is sufficient to cause temporary damage to the sample. The second beam is reflected from the sample. The properties of the reflected beam are sensed to create a signal. The first and second electrical signals are analyzed to determine the properties of the sample.