Methods and apparatus are provided for determining a characteristic of a sample of a material by the interaction of electromagnetic radiation with the sample. The apparatus includes an optical assembly and a protective barrier. The optical assembly sequentially illuminates a plurality of volume elements in the sample with an intensity distribution in the sample that drops off substantially monotonically from a first region in a first optical path and collects electromagnetic radiation emanating from each of the volume elements. The optical assembly collects the electromagnetic radiation emanating from each of the volume elements with a collected distribution that drops off substantially monotonically from a second region in a second optical path. The first and second regions at least partially overlap in each of the volume elements. The optical assembly can be configured as a probe, to be directed to the evaluation of a sample of a biological material. A protective barrier can be disposed between the optical assembly and a body tissue, to prevent contamination of the optical assembly by said body tissue.