It occurred to me that it ought to be possible to make a vacuum tube which, instead of a heater, uses an intense light source, such as a laser, to kick electrons off of the cathode through the photoelectric effect instead of them being excited using a heated filament. I am aware that two terminal evacuated tubes have been made, using the photoelectric effect to sense light and its wavelength too (as in Einstein's experiment). However, this device would have one or more grids, a plate (anode), and a cathode just like a typical amplifying vacuum tube, the only difference being a light source in place of a heater. Would this:

A. Be possible?

B. Be practical? (at least to the extent to which a vacuum tube is practical at all)


C. Has it been done?

I cannot find anything about this using Google, some lab equipment is taking precedence over the results I am looking for, if they exist.


Here is a functional diagram:Laser Excited Photoelectric Effect Vacuum Tube Amplifier Functional Diagram

Here are the first drawings I made of the device. The scans aren't so great, but it shows my original concept and a proposed Class B amplifier, as an example of an application which almost certainly would never work. Connections points A and B are an incoming audio signal. C is a connection to voltage supply. D is power for the lasers. I suspect that the laser diodes ought not be connected in parallel as is shown, but I didn't think of that while drawing this. Perhaps it is okay.Drawings of Photoelectric Effect Amplifier Tubes and a Class B Amplifier

  • \$\begingroup\$ Photomultiplier tubes are similar. \$\endgroup\$
    – Phil Frost
    Commented Jan 15, 2014 at 16:26
  • \$\begingroup\$ Possibly, but I can't see it making economic or practical sense: let's build an obsolete technology, with more complication and lasers! \$\endgroup\$
    – pjc50
    Commented Jan 15, 2014 at 16:27
  • 3
    \$\begingroup\$ Vacuum tubes are hardly obsolete. Photomultipliers are still the best way to do what they do. Tubes still see widespread use in high-power radio transmitters. And, don't forget that "warm tube sound" in audio amps that some people love. And, anything with lasers is better. \$\endgroup\$
    – Phil Frost
    Commented Jan 15, 2014 at 16:30
  • \$\begingroup\$ Normal vacuum tubes have the cathode in the center, surrounded by the grids, and outside the grids, the solid cylindrical plate - with that construction, it would be very hard to get enough light to the cathode for your idea to work. \$\endgroup\$ Commented Jan 15, 2014 at 17:18
  • \$\begingroup\$ Peter: Well, I was thinking of a more planar design, with flat grid, flat plate, and a somewhat curved cathode. I can send you a drawing I made today if you like. Phil: "And, anything with lasers is better." Precisely \$\endgroup\$
    – Void Star
    Commented Jan 15, 2014 at 20:59

1 Answer 1


These were common before the transistor and called "Phototubes". Every 16mm sound projector in the US schools had one to read the sound track on the film.

With higher voltage and multiple plates - one designed to emit electron when struck by light. The others are meant to kick out more electrons when they are hit by an energetic electron and you get a cascade. These are "Photomultipliers" and are still used extensively for photon counting and any very low light application like fluorescence spectroscopy or phosphorescence lifetime measurement.

There is a very fast and sensitive star tracking application that uses a spinning mask in the optical path to a photomultiplier. I have not seen one in decades even though they have a much faster response than CCD based tracking.

Hamamatsu is one maker of both devices in many forms.

  • 1
    \$\begingroup\$ I think the OP is asking if the photoelectric effect has ever been exploited in a vacuum tube not to sense the light, but just to get the cathode to the point where it emits electrons easily. In other words, have there been vacuum tubes where light was used to replace the function normally served by the heater. I am no aware of any. A heater seems a lot more practical. The concentric cylinder configuration makes sense and is efficient and allows for a heater easily, but getting light onto the cathode of one would be difficult. \$\endgroup\$ Commented Jan 15, 2014 at 18:15
  • \$\begingroup\$ I see. Photoelectric tubes were used to demonstrate voice over light beam and similar ideas which makes the tube act like a receiving or amplifying tube. But flooding a cathode with monochromatic light with energy just below the work function (or just above?) is not the same as producing the energy distribution from heat and the effect on thermionic emission. Hitting electrodes in vacuum with some UV has long been a way to start a cascade. Field effect emission and light --- I'll have to look at some field effect x-ray tubes. \$\endgroup\$ Commented Jan 15, 2014 at 19:58
  • \$\begingroup\$ Yes, Olin has the idea. I was aware of photomultipliers, but your answer gives some details I certainly was not aware of. It isn't "relevant" to the question per se, but I still think it's got some useful information, so please don't delete it. Someone else may find this information interesting at some point. \$\endgroup\$
    – Void Star
    Commented Jan 15, 2014 at 21:06

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