Integrated optical illumination reference source

An integrated illumination reference source for generating an illumination reference signal comprising:

• an optical element located at a last stage of a sealed optical system having a plurality of optical components, the plurality of optical components located upstream of the optical element, the optical element having a first outer surface and a second outer surface, the first and the second outer surface being substantially opposing, wherein the optical element receives an incident optical signal at the first outer surface and projects the incident optical signal from a first region of the second outer surface onto a surface under reflectivity test that is located external to the sealed optical system, the surface under reflectivity test facing the second outer surface and reflecting a first portion of the projected incident optical signal received from the second outer surface back towards the second outer surface;
• and
• a reflective device that is located on a second region of the second outer surface offset from an optical axis of the optical element, the reflective device having a reflective surface that reflects a second portion of the incident optical signal from the second region of second outer surface back through the first outer surface, the first region of the second outer surface including a lens area for only transmitting the incident optical signal onto the surface under reflectivity test and transmitting the reflected first portion of the projected incident optical signal received form the surface under reflectivity test back towards the plurality optical components located upstream of the optical element, the second region of the second outer surface including a lens area for only reflecting the second portion of the incident optical signal as a reference optical signal,
• wherein the reflective surface of the reflective device is encapsulated between the first outer surface and the second outer surface of the optical element, the encapsulated reflective surface being contamination-free based on the reflective surface being shielded from contaminants released from the surface under reflectivity test, the contamination-free reflective surface used to determine reflectivity calculations that are independent of the released contaminants, and
• wherein fluctuations in the reference optical signal is based on optical power fluctuations caused by the plurality of optical components located upstream of the optical element, the reference optical signal being independent of any contamination associated with the last stage optical element facing the surface under reflectivity test.