| CPC G01J 3/513 (2013.01) [G01J 3/0264 (2013.01); G01J 3/0267 (2013.01); G01J 3/0275 (2013.01); G01J 3/10 (2013.01); G01J 3/4406 (2013.01); G01J 2003/468 (2013.01); G01J 2003/516 (2013.01)] | 14 Claims |

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1. An optical stack comprising a test sample disposed on a first optical filter, the test sample configured to convert at least a portion of an incident excitation light having an excitation wavelength to a converted light having at least one converted wavelength different from the excitation wavelength, the first optical filter comprising a plurality of microlayers numbering at least 20 in total, each of the plurality of microlayers having an average thickness of less than about 500 nanometers (nm), such that the plurality of microlayers has:
an optical transmittance T1>20% at the excitation wavelength and at a first incident angle;
an optical transmittance T2>20% at the at least one converted wavelength and at a second incident angle; and
an optical reflectance R1>40% at least one of the excitation wavelength and the at least one converted wavelength and at least one of the first incident angle and the second incident angle;
wherein:
for the at least one of the excitation and the at least one converted wavelength, a corresponding optical transmittance of the first optical filter changes by at least a factor of 2 when an initial incident angle corresponding to the at least one of the excitation and the at least one converted wavelength changes to the initial incident angle corresponding to the other one of the excitation and the at least one converted wavelength; and
for the second incident angle, a microlayer optical transmittance of the plurality of microlayers versus wavelength comprises a lower transmission band separating a first higher transmission band and a second higher transmission band, the first higher transmission band comprising the excitation wavelength and the second higher transmission band comprising the at least one converted wavelength.
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