WWII era IR Sensor?

While kicking around more time-traveller-to-1935 stuff, I got a new idea.

The time traveller brought some transistors and stuff with him. By some point in the 1940’s I presume they were backward engineered to “Solid Valves.”

OK, so I am trying to aim American V-1 missiles at Japanese cities, but I have an accuracy problem. What if I fitted such weapons with some sort of heat detector? The missile would e launched from a B-17. When it crossed the coast, the temperature of the sea and shore would be quite different. Perhaps the city itself would have a distinctive heat signature.

Could a heat (IR?) sensor be made with such technologies? Could they have performed the task I outlined?

(Why yes, I am having a slow day, why do you ask?)

Wiki article on the AIM-9 Sidewinder missile, which uses IR:

So quite doable technically? Excellent.

While the coastline would be easy to spot with an early sensor; would a Japanese city be noticeable?

I would think that someone with 1930’s electrical knowledge would be able to understand that a transistor is just an electrically controllable resistor. It’s fundamentally different enough that I don’t think they’d call it a “solid valve”. A lot of the applications of replacing a valve with a transistor to reduce size, power consumption, etc. might not be immediately obvious to the guy examining the transistor.

Go on, I am listening. The thing about writing fiction is we can change it after we write it.

But then again, we are drifting into cafe Society territory.

Well, after American aircraft visited it, a Japanese city’s IR signature would be very noticeable.

How that could apply to your scenario: in WWII tactics, “Pathfinder” aircraft with really well-trained aircrews sometimes were used to go in first and drop incendiaries to “light the way.”

If you want to make a guided V1 style rocket, you might have better luck using radio beams to guide it. The Germans used radio beams for bomber navigation with some success in Europe, until the British figured out ways of jamming the signals. The Germans would use two beams. The bombers would ride one beam until they intersected the second beam, at which point they would drop their bombs. Your rocket could essentially do the same thing by riding one beam and cutting off its engine when it detects the second beam.

Getting back to IR:

Playing around with a transistor or diode that some time traveler brought back is one thing. Manufacturing your own is something else. How much knowledge does your time traveler have? Modern semiconductors are “grown” not fabricated (they are crystals). The way you make transistors and diodes and such is that you intentionally contaminate your growing crystals with specific materials while they are growing. Contaminating silicon with boron, aluminum, gallium, or indium makes P type silicon, and contaminating it with arsenic, phosphorus, or antimony makes N type silicon (hint - all of the N types are in one column on the periodic table and all the P types are in another column). If you grow a layer of N followed by a layer of P, you get a diode. NPN or PNP makes a transistor. Getting the engineering details of this with 1930’s technology might be a bit tricky, and of course someone has to explain the basic concepts to a 1930’s scientist, who I’m sure is going to say “you want me to grow a crystal out of molten silicon while the chamber is filled with arsenic vapors? Are you nuts?”

But here’s the thing. If you can make a transistor in 1940, you’re going to discover (maybe by accident) that if the base of the transistor isn’t covered, it acts like a phototransistor, whether you want it to or not. It will respond to visible light as well as IR to some degree. If you cover the base with exposed photographic film it will block the visible light and the transistor will only respond to IR.

If you are bringing back transistor technology, you are bringing back IR detector technology too, you just might not realize it.

Just finished “Inside the Third Reich” and Speer mentions one mistake was pursuing development of the V-2s when the rocket guys were also developing a SAM that used heat sensors to guide themselves to the engines of bombers. So they didn’t even need transistors, just support.

Related to this, both the US and Germany evidently fielded IR night vision equipment during WW2:



A combination of radio beams and that basic IR sensor. The radio beams for rough guidance and the IR sensor to ensure that it goes to the center of the city or wherever the most heat is.

Another issue that I can think of the IR sensor alone is its sensitivity at range, you’d need a lens assembly. Normal glass transmits IR fine especially the glass of that day as it was so primitive --almost no special formulations or coatings. So it is totally possible that a device that resembles a fly eye could be made. It would consist of a group of long focal length lenses that project onto the lead-sulfide sensors which would then guide the missile in.

You could only do this at night though as the ocean and ground will heat up during the day decreasing the temperature differential plus you can’t forget the Sun itself! Probably, the best time would be calculated by determining the max altitude of the missile, it’s max line of sight at that altitude so it won’t see the sun accidentally, and, the longest time after sunset so the ocean and ground around the city has had the most time to cool-off making the city the hot spot.

But, also, you could have it launched with the calculations regarding the rotation of the earth and have it get really close to the target. Once, it’s on a downward trajectory the sensors will turn on as they will be facing downward, which will alleviate the issue with the Sun a bit. Then, it can fine tune it’s trajectory and go for the center of the city.

Some technical issues with that technology of that time:
IR sensors, probably really slow to react which may be a good thing as it will smooth out any erroneous hot spots or flares.

Lenses: unless you have a lens hood on them you can get stray sunlight into them. In reality, you could design the missile to have a lens hood type assembly up front but you would have to compensate for how that hood would affect its aerodynamics. But, the missile does not need to be going Mach 10. Probably be sub-sonic any ways considering the technology of the time. Stabilizers up front and in the rear could keep it on track. Be odd looking though.

Electronics: could be primitive transistor based! According to wikipedia:

So, maybe some dark ops going on there made a few batches of transistorized missiles? But, valves/tubes are also practical and were far more developed in general. The tubes would be the heavier option due to the typical power requirements of heating them up and their grid voltages. But, they did have miniaturized versions and it only needs to last so long so longevity is not an issue.


IR-detecting pilotless aircraft were being played with in the '30s. One of the problems was that they promptly headed for the Sun, which is a far bigger IR emitter than anything else.

How do modern IR tracking systems account for the sun?

What range are you thinking of? In the range used for taking IR spectra, glass is essentially opaque. For that reason, we are required to use sodium chloride plates instead of glass. In the Mythbusters episode about avoiding security systems they demonstrated that a sheet of glass blocked the IR in the range of the camera sensors. I’m pretty sure glass does not transmit IR very well.

A tube is also an electrically controlled resistor, triodes much more so than bipolar transistors (which are more of an electically controlled current source, like a tetrode) When I started learning electronics, most of the textbooks I had access to were still from the tube era, and transistors were mentioned in an appendix or final chapter if at all. (the books were obsolete, but school districts and libraries were slow to replace them). The early transistors were sold expressly as tube replacements. Most of the circuits were analogous…the transistor is just a triode with slightly different behaviour. Most of the problems are due to the fact that transistors have a much wider parameter spread (production, and over temperature) than tubes. The early engineering texts on transistors treated them as analogous to tubes.
It took a decade or so for engineers to learn to design to accomidate this wide spread, mostly through negative feedback techniques. If they had done so sooner, replacement of “weak” tubes would have been much less frequent, but tubes were expensive, so you tried to get all the gain you could from each one…transistors are cheap now, so you just throw in an extra stage or two. Part of the learning curve was that early transistors were not so cheap, and supply could be unreliable.
By the way, the most sensitive night vision stuff is still photomultiplier tube based…or was ten years ago anyway. Seems to me you aught to be able to integrate phototransistor drivers onto a CMOS or CCD array, but if that is being done it is still under wraps AFAIK.

Transistors may not be totally necessary. Some of the earliest printed circuits appeared during WWII. They were part of the Secret VT proximity fuses used in mortar shells. They were created to address the problem of withstanding the stress of being fired out of a mortar. IIRC, the circuits were printed onto a ceramic disk baseplate at the back of the fuse.

The part about printed circuits I remember from an article in Invention and Technology. But HERE is an article about VT fuses in general.

I vaguely recall the British referring to transistors as “electronic valves”.

Here is the Wikipedia article on the German WWII SAM Wasserfall/Waterfall that I mentioned above. Note that it was only partially developed and the infra-red targetting is just briefly mentioned.

Reflecting more on the OP, as far as attacking Japanese cities for area bombing, this would have been a waste of resources. Japanese air defenses were inadequate for defending cities so regular bombing was easy to do, more effective and far cheaper. They only concentrated fighter defenses in places like the Kure naval base. And such a poorly guided missile aimed at Kure would have been nearly useless.

Whoops, you are right, I was thinking of IR photography vice thermal imaging. You would need special glass for thermal imaging. But, he’s writing fiction so he can fudge it. Maybe a pinhole lens calculated for the thermal range?


Well, I have sent the B-29 to Europe so the B-17s are off to Japan. The Allies have taken the Northern Route and have some tiny bases in the North Pacific. Even then the B-17 does not have the range to smite the main Japanese cities. Unless they were carrying buzz bombs.

While inaccurate, they could launched at almost no risk. Increasing the accuracy to be able to hit a city-sized target is exactly the issue my fevered brain is dealing with.

I think the radio method mentioned earlier is very plausible.

A long range air craft could drop via parachute a radio transmitter that the missile would then home in on once in range. If the drop was done at night with a dark parachute it may go undetected long enough for the missile to find it. Maybe do the whole operation at once:

  1. Long range plane drops transmitter, pilot radio’s back to confirm.
  2. Missile launched