123Splat:
A picture might help.

123Splat:
Explain directly polarized and inversy (?) polarized. so we are all on the same page.
Directly polarized is, as in every electronic component: anode connected to positive of the power supply and cathode connected to the negative. Inverse polarized is the opposite. Perhaps I didn´t choose the correct expression because I´m not english.
A diode, either IR or Visible Light must not let pass any current if inverse polarized, and must have a 1.3 - 2-5 voltage drop if directly polarized.
123Splat:
You did not say if light exposure made any difference during the diode mode test.
You are right, I forgot. I just tested and there are no differences
123Splat:
5V @270 Ohm is about 18mA. some LED's take about 20mA to light up.
Right again. I tested with 3V3 at 0.20 mA. Multimeter measures 0,46 directly polarized, 0,60 inverse polarized. Still not light on.
123Splat:
If it is/was IR, how do you propose to know if it light up?
Grumpy_Mike:
How are you determining this?
You will not be able to see the light from an IR emitter. You may be able to see it through a camera viewfinder but only dim.
What you really want is a camera without any IR filter. The NOIR Raspberry Pi is one such camera.
I use my mobile camera. It sees perfectly all the other IE LEDs I have, including a KY-005 module that has a LED that seems equal that the one I can´t identify, and all my IR remotes, including Arduino ones and TV one. The camera shows me the IR LEDs with a well differenced purple-pink light.
Grumpy_Mike:
The voltage required to light an IR emitter is in the order of 0.7 to 1.4V so you don't need a higher one.
Tried again, with 1V5. Nothing
Grumpy_Mike:
It wont, if it is a detector there will be an IR filter built in to stop that happening.
I´m not sure, but I think that that type of photodiode hasn´t any filter. Is a simple photodiode, not a phototransistor like V1838. (if it is a photodiode, of course). So natural light might affect it because natural light has IR waves too.
MarkT:
What forward voltage do you get at room temperature with 1 -- 10mA or so forward current?
5V with 1kOhm resistor gives us 5mA. So, second case:
- Polarizing at 5V with 1kOhm resistor. Multimeter at 20V range:
+ Directly polarized: Didn´t light on. Multimeter measured 0.28
+ Inverse polarized: Didn´t light on. Multimeter measured 0.34
Covering from light or exposing to it DIDN´T make difference.
MarkT:
Silicon photodetectors (0.7V) will be a lot less than a IR LED (1.1V), IIRC.
Photo emitters are not silicon, but a direct-bandgap semiconductor (normally). Of course
a photodiode doesn't have to be silicon, but many are.
I don´t understand what do you mean