The best monochromatic microwave sources are those efficient cavity-resonator valves, the klystron and the magnetron. The wavelength they emit is determined by the size of their cavity. These days, silicon microengineering can create and shape objects and cavities with dimensions around a wavelength of light. So Daedalus is now planning micro-klystrons and micro-magnetrons to generate bright monochromatic light.

It should be simple enough to form a proper cavity in silicon, and coat it with low-resistance metal to make it a good resonator. The main problem is making an electron beam fine enough to enter the cavity. A conventional cathode would be far too coarse. Fortunately, point sources for electron beams already exist, for field-ion and electron microscopes. With luck they could be formed by the same photofabrication used to make the cavity.

A single photoklystron would be a wonderful point source of light, but very feeble as a lamp. A DREADCO team is designing an array of many hundreds of photoklystrons sculpted on a single silicon wafer. Integrated circuitry on the same wafer will apply control and power voltages to the klystrons of the array. The whole thing will be sealed in an evacuated glass envelope from which the generated light can escape.

Integrated-circuit mass production is so cheap that the photoklystron lamp could well be competitive with its fluorescent and filament rivals. If the klystrons on the wafer were all different, so that each radiated its own frequency, the resulting lamp could be almost perfectly white, ideal for domestic illumination. Even better, it would be a cold white, with no infrared. And as a resonant device, the lamp should be highly efficient, far brighter per watt than its rivals.

If all the photoklystrons in the lamp were identical, it would be perfectly monochromatic, an ideal spectroscopic source. Like a conventional klystron, it could be tuned over a useful range by varying the voltage applied to its electrodes. A series of such klystron lamps could span the visible and infrared bands seamlessly, right out to the millimetre region. Physicists would at last be released from the inflexible, untunable grip of the laser.

The Further Inventions of Daedalus (Oxford University Press), 148 past Daedalus columns expanded and illustrated, is now on sale. Special Nature offer: m.curtis@nature.com