Determining the droplets' PSF and MTF

The droplet lenses' PSF and MTF provide quantitative measures of the two-point resolution and line pattern contrast limits that can be achieved when the lenses are employed in imaging applications. To get an estimate of the PSF and MTF we expose individual droplets to white light, which originates from an optical fibre with a 50 μm diameter core and is collimated by a spherical lens (f=150 mm). We capture the droplet's PSF using a custom-build microscope composed of a × 50 Olympus objective (NA=0.5), a Thorlabs tube lens (f=200 mm) and an Allied Vision ProSilica GT3300C camera. We only use the camera's red channel (maximum quantum efficiency at 620 nm), expecting that the droplets' resolution would be at least comparable or better for smaller wavelengths. We obtain the MTF by Fourier-transforming the PSF after subtraction of background noise (which is legitimate because we do not expect diffuse scattering from the clear droplet), removal of ‘salt and pepper' noise (due to hot pixels) using a median filter in a 3 × 3 pixel neighbourhood area, and averaging over angular slices of the imaged Airy disk pattern. The resulting PSF and MTF curve must be seen as a conservative estimate as the recorded PSF still possesses residual blur from the microscope optics.