In the attached Proteus circuit I used optocouplers NSL-32SR3. Its dependence on the LED current is depicted in the attached graph.
Unfortunately for some reason the ohhmeters show always 100MOhms, and it doesn't matter on the applied to LED current.

 

Do you have any series Collector or Emitter Resistance ? Does your Vs/Vcc/Vdd has any internal resistance ? (←at least one of these is required)

 

Typically an ohmmeter does not work well to measure the resistance of a n active device.
Why do you want to measure it?

 

How about describing in detail your measurement process and exactly what you used for the measurement process?
How to test an optocoupler
Even that method is not exactly reliable.

Ron

 

Do you have any series Collector or Emitter Resistance ? Does your Vs/Vcc/Vdd has any internal resistance ? (←at least one of these is required)

An ohm meter in a simulator will not be an accurate device, as you have discovered. And a photo transistor needs a bias source and your circuit does not have one. Make it a series loop with a 10K ohm resistor and a 10 volt battery and then read the voltage across the photo-transistor. And add a series current meter and you may be able to calculate the resistance.

 

as any transistor the coupler also has the V.collector dependency usually the lower V.c --
-- (the higher the in series collector resistance) the lower the R.CE.ON

 

An ohm meter in a simulator will not be an accurate device, as you have discovered. And a photo transistor needs a bias source and your circuit does not have one. Make it a series loop with a 10K ohm resistor and a 10 volt battery and then read the voltage across the photo-transistor. And add a series current meter and you may be able to calculate the resistance.

Is it the same procedure if we use photoresistor instead of phototransistor?

 

Is it the same procedure if we use photoresistor instead of phototransistor?

Difference between photoresistor and phototransistor | photoresistor vs phototransistor
Read and learn. The answer to your question is no. Photo transistors have a fast response, even faster than a speeding bullet. Photo transistors also tend more to be On/Off devices and used in applications where their use is practical. You choose a component based on its intended application. Before asking more questions how about answering some which have been asked of you?

Ron