This is done by using wavelengths of ultraviolet light that are totally absorbed by the ozone layer, yet are transmitted in the Earth's atmosphere.
[2] Ultraviolet illumination can be produced from longer wavelengths using non-linear optical materials.
They must have a suitable birefringence in order to phase match the output frequency doubled UV light.
[3] Research is underway for substances that are very non-linear, have a suitable birefringence, are transparent in the spectrum and have a high degree of resistance to damage from lasers.
It is important to have a high transmittance within the solar-blind spectrum, but to strongly block the other wavelengths.