NEP and coupling efficiency. (a) The NEP at f = 200 Hz (see panel d), as function of absorbed power using the pinhole setup for the same device as in Fig. 3. The black line shows the theoretical curve. The absorbed power is evaluated from the source power using the measured efficiency as indicated in the insert, and the mean value of the points, η_opt = 0.0389 ± 0.004, is given by the line. The statistical error bars are smaller than the dots. (b) The averaged coupling efficiencies for the two experiments. The red dots represent the results for the large radiator, where all detectors couple almost identically to the source as expected; the calculated coupling is given by the line. The black dots give the coupling for the pinhole setup, where only the best aligned detector is expected to reach the theoretical coupling value given by the grey square. We note that, in both cases, the measured coupling exceeds the theoretical value by the same factor of ~1.65. (c) The measured NEP_exp(P_abs, f = 200 Hz) for the pinhole experiment, as displayed in Fig. 2c, together with the dark NEP obtained at the same modulation frequency using a change in bath temperature to obtain the response. (d) The measured NEP_exp(P_abs, f) for the pinhole experiment at the lowest radiator temperature for all measured MKIDs. We observe no dependence of the NEP on aluminium volume. We note that panel a and Fig. 3 are given for the detector with F₀ = 2.74 GHz.