An apparatus and a material processing system, the material processing system having an optical access port, the apparatus comprising: an imaging optical source that is configured to produce imaging light that is applied to the material processing system, wherein the material processing system is configured to implement a material modification process and to create a phase change region (PCR) in a material; a material processing beam source that is configured to produce a material processing beam that is applied to the material in the material modification process, wherein the material processing beam creates the PCR in the material; at least one element that is configured to direct the imaging light at a plurality of imaging beam positions (20a-20f, 20i, 20h) proximate the PCR; at least one input-output port that is configured to output a first component of the imaging light to the optical access port of the material processing system and that receives a reflection component of the imaging light; an optical combiner that is configured to combine the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and an interferometry output processor that is configured to process the interferometry output to determine at least one characteristic of the PCR; and characterized by further comprising: a mechanically actuated objective lens (A, 109, 314) that is controlled to keep focus of the material processing beam a certain distance from the sample location, wherein the another path length is a reference path length which is changed in correlation with actuation of the mechanically actuated objective lens. an imaging optical source that is configured to produce imaging light that is applied to the material processing system, wherein the material processing system is configured to implement a material modification process and to create a phase change region (PCR) in a material; a material processing beam source that is configured to produce a material processing beam that is applied to the material in the material modification process, wherein the material processing beam creates the PCR in the material; at least one element that is configured to direct the imaging light at a plurality of imaging beam positions (20a-20f, 20i, 20h) proximate the PCR; at least one input-output port that is configured to output a first component of the imaging light to the optical access port of the material processing system and that receives a reflection component of the imaging light; an optical combiner that is configured to combine the reflection component and at least another component of the imaging light to produce an interferometry output, the interferometry output based on a path length taken by the first component and the reflection component compared to a path length taken by the at least another component of the imaging light; and an interferometry output processor that is configured to process the interferometry output to determine at least one characteristic of the PCR; and characterized by further comprising: a mechanically actuated objective lens (A, 109, 314) that is controlled to keep focus of the material processing beam a certain distance from the sample location, wherein the another path length is a reference path length which is changed in correlation with actuation of the mechanically actuated objective lens. <EOS>