Transparent ‘perfect’ mirrors — one-way mirrors that transmit or reflect light completely depending on the direction of view — are useful for security, privacy and camouflage purposes. However, current designs are not perfectly reflective. Now, Ali Jahromi and colleagues from the USA and Finland have demonstrated a new design based on a non-Hermitian configuration — an active optical cavity — that may overcome this limitation. At a critical value of prelasing gain that is termed Poynting’s threshold, all remnants of the cavity’s structural resonances disappear in the reflected signal. At this point, the reflection becomes spectrally flat and light incident on the cavity is 100%-reflected at all wavelengths continuously across the gain bandwidth independently of the reflectivities of the cavity mirrors. Thus, the device at Poynting’s threshold becomes indistinguishable from a perfect mirror. The researchers have confirmed these predictions in an integrated on-chip active semiconductor waveguide device and in an all-optical-fibre system. They note that Poynting’s threshold is, however, dependent on polarization and incidence angle, and that observing the reflection of coherent pulses may reveal the cavity structure via its decay time. Since the concept of Poynting’s threshold is a universal wave phenomenon, it can be exploited in many areas including microwaves, electronics, acoustics, phononics and electron beams.