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I do storage, so I do a lot of fiber. I am also unlucky enough to have a patch panel with single strands, so when someone else does patching for me, it's usually a mystery which of the two strands I'm holding will end up being the tx (send) and which will be the rx (receive). We have a color code: the yellow one is supposed to be "hot". It's followed, accidentally, just under half the time.

I know that the vast majority of active cables in my datacenter are sending visible, harmless light. That's why I've seen a lot of people looking into one or the other cable, knowing that it's safe. The problem is I also have very few very dangerous lasers that I wouldn't recommend shooting into someone's hand, let alone their eye. One of them is outside the visible wavelength and strong enough to be read 80KM away.

A light meter will tell you whether a strand is lit up, but we only have one, and it's usually being used by someone else. Shining it on a paper or a hand is another technique, but I have trouble seeing it. I've seen little cards some people carry around, but I don't have one and don't want to carry something around I only need once a week.

What's the best way to tell whether a cable has light on it without requiring special equipment?

  • 13
    "Do not stare into fiber with remaining eye." – EEAA Dec 2 '14 at 21:29
  • 2
    Get some management backing to beat, er, educate your co-workers into following your company standards. – mfinni Dec 3 '14 at 14:15
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    I've seen the datacenter manager do it ;) – Basil Dec 3 '14 at 14:25
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    Exactly. Ever see what happens to those old CRT tubes when you don't de-energize them and happen to touch exposed parts? – Avery Payne Dec 4 '14 at 19:37
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    Murphy's law, @warren :) If I reverse the standard, they'll drift the other way. – Basil Dec 9 '14 at 1:39
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Something everyone carries around with them, a camera phone, can be a massive help. Even if the light is so faint that you can barely make it out, you should be able to tell the difference with a camera phone.

Here's an example of a rx cable (with no light coming from it):

Cable without light

Here's an example of a tx cable (with light coming from it):

Cable with light

It's really blurry because most camera phones won't focus a eighth of an inch from the lens, but it gets the job done without risking your eyes, and uses a tool that most everyone has.

  • Shining 1000BASE-ZX (the 120km/75mi variety) on a cell phone's camera is pretty likely to damage the camera, though I'd rather fry my camera than my eye (especially since I still couldn't see the light at that frequency). Also, high end cell phone cameras might filter more of the NIR light than others, so it might not work with all cameras. Interesting note, the light is almost always red to infrared, but IR typically shows up as blue or purple in unfiltered camera sensors. – Chris S Dec 4 '14 at 18:54
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    I'm probably in a minority- I have a cheap-ass phone I don't give a damn about. If it stops being able to see light from cables, I'll have it replaced. It's still cheaper than a new light tester or a new set of eyes. – Basil Dec 4 '14 at 21:48
  • Could you shine it on a diffuse surface and photograph that, or is that still too unsafe? – user253751 Apr 24 '17 at 9:49
  • I don't know if I'd be better able to see the light on a card or something with a phone than my eyes. – Basil Apr 25 '17 at 11:06

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