In photo-reflectance, the sample's internal electric field is modulated by the photo-injection of electron-hole pairs (thus reducing the latent field). In order to achieve photo-injection, the energy of photons in the pump beam must exceed the band gap of material within the sample. Furthermore, semiconductors with little or no electric field will exhibit little or no electro-reflectance response. While this situation is not common, this point makes clear the importance of maintaining the probe intensity at a minimum, since any photo-injection of electron-hole pairs from the probe will necessarily offset the sample baseline condition by reducing the latent field. (Likewise, any CW component of the pump is undesirable.) Conversely, if the probe intensity is too low, detection may not be possible with conventional photodiodes. A further consideration is that phase-locked detection is a practical necessity due to the small size of the experimental signals (~ppm) and the unique ability of phase-locked detection methods to reject noise outside a narrow bandwidth centered on the modulation frequency.