Optical coupling to VISIR MKIDs efficiently over a wide wavelength range presents a greater challenge than typical for long-wavelength MKIDs. The large variety of coupling schemes developed for long-wavelength MKIDS fall into two categories: (1) transmission line coupled, and (2) direct absorption coupled. In the first category, radiation is collected by an antenna and guided to the MKID by a superconducting transmission line (made of a higher gap superconductor than the MKID). The MKID is designed to act as a resistive termination (at frequencies above its gap) that matches the characteristic impedance of the optical input line. For VISIR MKIDs, the optical frequencies are above the gap of any superconductor, so this approach cannot be used. In the second category, the MKID material is directly illuminated by means of lenses or placement inside a waveguide. In the long-wavelength case, the optical frequency is well above the superconducting gap frequency, but far below the inverse of the Drude scattering time. The thin MKID film then acts as a sheet resistor with a real-surface impedance equal to the DC value (typically tens of ohms/square) seen in the normal (nonsuperconducting) state. By appropriate choice of the index of refraction of the (transparent) substrate and use of a back-short, highly efficient optical coupling to the MKID can be achieved over a fractional bandwidth of 30% or more. At the much higher frequencies in the VISIR case, MKID materials exhibit a more complex dielectric function. Figure 4 shows the surface impedance at VISIR frequencies for two examples of MKID films, molybdenum nitride and thin aluminum, which we have used at NASA Goddard. The real part of the impedance is not frequency independent, and the imaginary part is not small. This is typical of all MKID materials, including TiN, NbTiN, PtSi, and WSi. An optical efficiency near 100% can be designed in some narrow frequency range by forming an optical cavity involving the MKID layer, its substrate, and auxiliary metal or dielectric films; however, it seems a complex task to achieve efficiency simultaneously over 0.4 to . Additional complications are (1) the MKID films are not necessarily thin compared to the optical wavelength, (2) one of the favored substrates, single crystal silicon, has its semiconducting gap in the frequency range of interest, and (3) amorphous dielectrics associated with TLS may add noise. The TiN MKIDs in ARCONS absorb 70% of the light at , but only 30% at , and microlens arrays are used to focus the light onto the small inductors.35 More than one MKID design may be needed in biosignature characterization focal planes to efficiently couple photons from 400 nm to . For LUVOIR, this may not be a significant penalty because the coronagraph itself will have limited bandpass, perhaps 10%, as a consequence of needing to achieve a starlight suppression ratio. Nevertheless, decreasing MKID inductor size (to increase spectral resolution) and improving absorption efficiency are important challenges for VISIR MKID development.