Fl632.8 1

The WARP setup was integrated into an inverted microscope (Nikon Eclipse Ti). A schematic of the setup and light path is shown in Supplementary Note 10. Collimated light from two identical red LEDs (625 nm centre wavelength, Thorlabs M625L3) was combined onto the same illumination path using a 50–50 beamsplitter. Narrowband filters centred at 632.8 nm with a FHWM of 1 nm (Thorlabs FL632.8-1) were used to select the desired measurement wavelengths from the LED spectra. A tilt of 15° was introduced to the filter at LED 2 to select 628 nm. The LED wavelengths were measured either with a fibre spectrometer, or by matching interference fringes to those produced under illumination with a monochromator coupled to a halogen light source. An achromatic doublet lens (f = 150 mm) focused the light from the two LEDs to the back focal plane of the microscope objective lens (Nikon S Plan Fluor ELWD 40×), generating a collimated beam incident perpendicular onto the microcavity chip. The resulting reflected interference image from the cavity was captured by an sCMOS camera (Andor Zyla 4.2 or Hamamatsu Orca Flash 4.0). A trigger signal from the camera and an IC 4017-decade counter circuit toggled between which LED was used for illumination.