It is often desirable or necessary to input an energy beam at an angle to an optical axis of a focusing objective system. The reflected beam from the sample often needs to be collected. Since reflection occurs according to Snell's Law, the reflected energy must be collected at the same angle to the sample as the angle of incidence. In visual imaging systems, to accurately view detail of a sample plane without distortion, the viewing system must be situated near normal to the sample. These conditions dictate that the incident and the reflected beams co-exist in space such that they are co-axial or near co-axial. In order to separate these beams, a beam splitter is used.
The most simple form of a beam splitter is a refractive beam splitter employing a partially reflective mirror that redirects the beam at substantially a right angle into a focusing system and then to the sample which then transmits part of a reflected beam back through the focusing system, beam splitter and onto a detector. A beam splitter of this type is theoretically limited to an efficiency of 25% because half of the input beam is lost on the initial reflection and half of the remaining beam is lost when the beam is transmitted through the beam splitter. This type of beam splitter also may introduce chromatic aberrations into the reflected beam. The refractive beam splitter, however, has the advantage of having a very simple construction.
Another means for splitting an input energy beam involves off-axis paraboloids, ellipsoids or spherical mirrors. These off-axis systems have means for directing the beam to a focusing mirror which focuses the beam onto a sample. Energy reflected from the sample is focused by a second mirror and directed to a detector through whatever optical arrangement is chosen. These systems have the disadvantage of introducing substantial distortions of the visual image, a phenomenon that is particularly troublesome at high magnifications. Off-axis systems are also not suitable for applications where it is desirable to mask part of the image being sent to the detector.
Finally, complex mirror arrangements may be created that illuminate the sample and recreate an undistorted image of the sample that may be used by a detector. These complex mirror arrangements, while producing an adequate optical path, involve considerable mechanical complexity and expense in manufacture. Moreover, complex systems involve complex problems of alignment of the optical components. Finally, these systems are generally not compact, making for a system that is cumbersome, complex and difficult to maintain in optical alignment.
Present beam splitters generally do not provide an economical and accurate method of redirecting an image beam at an angle to the optical axis of an infrared microscopic imaging system. This problem is particularly acute in connection with obtaining infrared spectra of extremely small size samples where it is necessary to mask the image received by the detector.