Reflectance spectroscopy is used for samples that are difficult to analyze by transmission. The
samples can usually be analyzed as is without the need for preparation or any modification.
Reflectance measurements can be divided into two basic categories – internal or external
reflectance. The technique of Attenuated Total Reflection (ATR) is employed for internal
reflectance measurements as the beam of infrared radiation passes through an ATR element
(crystal) in contact with the sample. For external reflectance measurements, the infrared beam of
radiation is reflected directly from the sample surface. This type of external reflectance
measurement can also be divided into two types – Diffuse or Specular.