| 摘要 |
A path of travel for radiation extends to one optical element, then to another optical element, and then away from the latter. One of the optical elements is respectively reflective and non-reflective to radiation above and below a first wavelength, and the other is respectively reflective and non-reflective to radiation below and above a second wavelength. According to a different aspect, a path of travel for radiation extends to one of first and second optical elements, then to the other optical element, and then away from the latter. The first optical element is reflective and non-reflective to radiation on respective sides of a first wavelength, and the second optical element is reflective and non-reflective to radiation on respective sides of a second wavelength. The first optical element can tilt in relation to the path of travel to change the first wavelength.
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