again the material refractive indices of prisms 12 and 52 are selected to be sufficiently close to one another at the design wavelengths to provide very low reflection coefficients at the interface without additional optical coatings for example fused silica and clearceram u2122 materials may be used as in the previous examples a different embodiment 70 of the invention is illustrated in side view in fig4 a conventional pentaprism 12 is combined with an external reflective element such as a mirror 72 to constitute the filter assembly the pentaprism may be made of any appropriate optical material with both side faces 20 and 22 coated for anti reflection to deliver a safe level of irradiance as desired in the beam i to the observer the highly reflective mirror 72 is appropriately positioned to retro reflect the fraction of light t transmitted through the face 20 of the pentaprism thereby producing a rejected beam r that returns towards the object through the pentaprism while this embodiment of the invention is not preferred because of the spectral non uniformities in the filter produced by the additional coatings it is still useful within more limited spectral bands such as visible light therefore because of its simplicity and thermal stability this embodiment is preferable to prior art filters in yet another embodiment 80 of the invention the filter assembly consists of the combination of three prisms as shown in side view in fig5 two right angle prisms 82 and 84 are optically adjoined along their respective hypotenuse faces 86 88 thereby forming a beam splitter cube with a beam splitting plane 90 the hypotenuse.