Unlike neon voltage stabilisers the glow modulator concentrates the discharge within a small diameter hollow cathode. The construction typically has a disc anode with a small hole in the centre facing the top of the envelope with the hollow cathode below.
The first mechanical television receivers used an Osglim type of discharge tube. In this design the glow spreads evenly over the front surface of the cathode and the intensity varies with the modulation. The side of the cathode facing the anode is insulated with a mica sheet forcing the discharge to the opposite side - the viewing side. No optics were required but the image was feint.
For a brighter image the crater lamp was designed and with some optics would produce a bigger image, possibly up to 100 mm high. The Neotron 25 is the first such lamp we have found.
The Neotron 25 with a UX4 base.
The text of the advert for the above tube reads: A most intense concentrated source of 'cold light' which may be readily modulated. This neon lamp is intended for use in systems where the transmitted image is projected by optical means on a screen. Practically entire light is emitted from a point about 2 mm in diameter. Tube is viewed end on.
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The WirePhoto system explained
The Wirephoto systems of the late 1930s used this type of lamp to expose the photographic paper in the receiver and probably this was the main market for the Neotron 25.
The development from the Wirephoto systems was the fax machine where the recording mechanism was thermal imaging or xerography. The TMH-2 was designed for fax machines and has a maximum current of 15 mA.
The Russian TMH-2 image shows the modern construction with an anode above the crater whereas the Neotron 25 looks to have a anode that is a coil of wire around the insulated cathode.
The text of the A2/DC/S exhibit refers to the use of such a lamp for through the air point to point communication. The author's web site details experiments in the early 2000's with low power lasers for modulated light communication where a distance of 76.1 km was recorded.
Optical communications these days would naturally be considered as fibre-optic links but free space communication using LED's for transmission are the descendants of the glow modulator and there have also been experiments with ceiling based optical Li-Fi for Internet etc.