Author Topic: Why IR and not visible light?  (Read 155 times)

istarion

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Why IR and not visible light?
« on: June 19, 2018, 05:11:40 AM »
I’m curious why the MilesTag system utilizes IR emitters band sensors rather those that operate in with visible light? Does IR offer less attenuation, better range and/or sensitivity? I ask because it seems from both a playing and safety perspective, visible light emissions would be better.

berk

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Re: Why IR and not visible light?
« Reply #1 on: June 19, 2018, 10:54:01 AM »
Take a look at the attached spectrum of sunlight. You will notice that the sun outputs much less power around the 940nm wavelength which is the one we use. This means the sensors are less likely to be flooded.

http://wtamu.edu/~cbaird/sq/images/sunlight_wavelength.png
« Last Edit: June 19, 2018, 10:56:04 AM by berk »

ezio

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Re: Why IR and not visible light?
« Reply #2 on: June 19, 2018, 11:11:30 AM »
From what I've been able to find, there are possibly several reasons, but I can't give you any hard evidence for any of them without doing a lot more research.  Here are some of the top answers I found:

  • The power required to drive IR LED's is the smallest in the light spectrum.  It may have been selected to preserve batteries.  The more you transition through visible light into the blues and eventually ultraviolet, you're spending the most power.  No sense in wasting energy for no perceived advantage.
  • One source suggested that IR wavelengths are broader and more easily detected by silicone photodiodes, which would be more useful at longer ranges than visible or UV light.
  • It's easier to detect spikes in the infrared wavelengths from ambient levels than it is to identify spikes in visible light from ambient levels.
  • Finally, I think it would be terribly distracting and temporarily blinding to be in a firefight with all these blinking lights in your face.

Those are the best reasons I could find, but again, nothing definitive.  To be clear, however, there were some consumer grade laser tag systems in the 80's that used visible light, according to Wikipedia.
« Last Edit: June 19, 2018, 11:14:48 AM by ezio »

istarion

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Re: Why IR and not visible light?
« Reply #3 on: June 21, 2018, 09:31:00 AM »
Thanks for the insight. Hadn’t thought about the spectral noise to overcome. I was thinking there was just as much or more IR frequency radiation surrounding us during the day, we just couldn’t see it. :-)

Harlequin

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Re: Why IR and not visible light?
« Reply #4 on: June 22, 2018, 09:25:35 PM »
Istarion, did you also post this question on the Lazer Tag Modders Facebook group recently? If not, I'll post Brian 'TagFerret' Farley's input on the subject:
Quote
The reasons are many, and most all of them have been hit on in the above comments.

The main reason IR was used in the first arena tag systems was that these worked by continuously broadcasting weak IR beacons in all directions from each player, and the "guns" each had a single receiver photodiode located in the barrel behind a lens to detect whether or not they were pointed at another player when the trigger was pulled (this is sometimes called a "Reverse IR System"). There are a bunch of highly technical reasons as to why this is a lot easier and cheaper to do and more reliable from a play standpoint than having one very bright LED in the "gun" and multiple of receivers on each player looking out in all directions (what is sometimes called a "Forward IR System"). Anyway, you don't want all your players lit up like Christmas trees in the dark arena until they actually get "hit," so this needed to be IR instead of visible light.

When Worlds Of Wonder developed the first Lazer Tag home system, designed for outdoor play in sunlight, they stuck with IR because the available IR LEDs were more powerful and less expensive than visible LEDs with the same power output. There were also concerns of "flash blinding" if visible light powerful enough to be detected against background sunlight were used in nighttime play.

Since then IR has remained the medium of choice because of that nice "quiet band" in ambient sunlight right around 940 nm wavelength. These days most photodiodes are made with a dark plastic body that blocks the visible light and just lets this narrow band of IR through (the photodiode itself would be just as sensitive to visible light as it is to IR if it were made using a clear plastic body) and the most powerful inexpensive IR LEDs are designed to produce 940nm - 950 nm wavelengths to take advantage of this.

As for the question of "why use IR instead of visible light in TV remotes?"... In actuality, the very first TV wireless remotes in the early 1950's WERE visible-light-based, using a strobe like the "Phaser Force" sets you remember from childhood. The TV receiver had a little photo-eye vacuum tube (good IR photodiodes and LEDs had not yet been invented) which detected visible flashes. Each flash caused a motor or solenoid to automatically advance the tuning switch by one station. The high cost and problems with flashes of sunlight reflecting off passing cars and such caused TV makers to switch to ultrasonic "dog whistles" as remote controls by the end of the 1950's. Eventually the development of cheap semiconductor photo-diodes and LEDs in the 1970's allowed TV makers to switch over to using modulated IR for their remotes, and the development of small microcontrollers in the early 1980's made it possible to encode more complex commands. It is no coincidence that Lazer Tag was developed shortly after TVs started using IR and small microcontrollers: The high volumes demanded by TV manufacturers drove the prices down to where they became palatable for toys. Some of the earliest examples of tag-type toys in my collection are from the late 1970's and early 1980's, and have IR LEDs and photodiodes but no microcontroller yet!

Finally, the green flash you've seen might be intentionally added to the IR LED. Many IR LED makers added a little bit of chemistry that produced visible light to the predominantly-IR chemistry, so that technicians could easily tell if the IR LED was working or if it had burned out, was soldered in backwards, or did not have power. Most of the time this is a dim red tint, but those early Starlytes used an unusual IR LED from Siemens which could have had a green tint. Back then the manufacturers did all sorts of monkeying with the indicator color, plastic color, long/short lead positions relative to the "flat spot" on the side of the plastic body, and so on to identify the specific model of IR LED or IR photodiode.