Fluorescent Lamp

Inman describes converting "ultra-violet rays emitted by the discharge into visible light rays". Two stages: make UV efficiently, then convert it. Write both down.

Never open, cut or break a fluorescent tube. Handle it by the ends, keep it in its sleeve when not in use, and read the final step before you start.

Run an incandescent lamp for two minutes. Record light output at 300 mm and feel the heat from 100 mm away. Most of the input is leaving as that heat.

Same distance, same meter, similar rated power. Record light and surface warmth. More light, far less heat.

Examine the switched-off tube. The dull white lining is the phosphor. The glass is clear — the whiteness is the converter.

View the incandescent lamp: a smooth continuous rainbow. View the fluorescent tube: a broad band with bright discrete lines superimposed. Two different mechanisms, visible directly.

The sharp lines are mercury atoms emitting at fixed wavelengths; the smooth background is the phosphor. You are seeing both stages at once.

In a dark room, shine the UV LED on plain paper. It glows far brighter than the LED alone should allow. That is fluorescence — the second stage on its own.

Try tonic water, a highlighter mark, a banknote and a plain rock. Some glow strongly, some not at all. Fluorescence is a property of the specific substance.

UV in, visible out — never the reverse. Some energy is always lost as heat in the conversion, so the light that comes back is always longer wavelength than the light that went in.

Using powder from a mercury-free UV LED lamp only, sprinkle a little on card and light it with the UV LED. It glows white — this is the coating in Inman's tube.

Shine the UV LED on the switched-off tube's white lining. It glows. The tube's phosphor works on any ultraviolet — the mercury discharge is just the supply.

View two colour temperatures through the spectroscope. The mercury lines are identical; the broad band differs. Colour is chosen by phosphor, not by the discharge.

The patent. US 2,259,040, "Electric Discharge Lamp", granted 14 October 1941 to George E. Inman of East Cleveland, Ohio, assignor to General Electric. The document states that it "is a division of my application Serial No. 75,772, filed April 22, 1936, and now U.S. Patent Number 2,146,579, granted February 7, 1939". So the work dates from 1936, and this is a later slice of an earlier filing — which is why quoting 1941 as the invention date is misleading.

The mechanism, in Inman's words. The object is "fluorescent lamps... having high luminous efficiencies with a long useful life", achieved "by employing a low pressure discharge through a gaseous atmosphere, preferably mercury vapor, which produces ultra-violet radiation with very high efficiency", with the fluorescent material "applying a coating of the material to the inner surface of the lamp container or envelope". Every part of that sentence is testable, and steps 6 to 12 test them separately.

The unglamorous engineering is in the electrodes. A large part of the specification concerns metal discs sealed to each end of the tube, dish-shaped so they can be thin and still hold vacuum, made of "an iron alloy containing about twenty-nine per cent of chromium". It also credits a colleague's separate application — Harold D. Blake, Serial No. 3,334 of 24 January 1935 — for that seal. Fluorescent lighting is routinely credited to Inman alone, and the patent itself is more generous than that: he led a GE team, the seal came from a colleague, and the underlying idea has a long European lineage running through Becquerel's fluorescent-coated tubes and the Moore and Küch lamps. The contribution here is making it manufacturable and commercially viable, which is a real achievement and a different one from inventing the principle.

⚠ The mercury awareness note, and it is for everyone. Every fluorescent tube and compact fluorescent lamp contains a small quantity of mercury, which is genuinely toxic and is why this build never opens one. If a tube breaks indoors: get people and pets out, ventilate for fifteen minutes, do not vacuum (a vacuum disperses mercury vapour and contaminates the machine), pick up fragments and powder with stiff card and sticky tape, seal it all in a jar, and take it to household hazardous waste. Intact tubes are safe to handle. Spent tubes are not general rubbish and should go to a recycling point. This warning is deliberately open to all readers — the people most likely to break a tube are the people who need it.

What replaced it, and why the physics survived. The LED has now taken over general lighting, being more efficient still, mercury-free and instantly dimmable. But the phosphor idea did not go away: a white LED is a blue LED with a phosphor coating that converts part of the blue to yellow, which is the same trick — an efficient short-wavelength source plus a converter — that Inman's tube used with ultraviolet. The lamp is obsolete; step 8 is not.