# Simplest possible light sensor relative calibration

1. Feb 22, 2015

### john824

I am looking for way how to do relative calibration of light sensor (PMT). I would appreciate any suggestions or links to sources with description how to do it in very simple (understand also cheap if possible) way.

For example. Let say that I have integration sphere with two outpust. I have two optical cables connecting to sphere. Lets assume that those two cables are identical. Optical cable enlight PMT sensor from some distance in black box. I can measure light characterstics by PMT (not calibrated). With distribution maximum at point I_max_one_source.

If I connect two cables from integration sphere to black box, playing role of two very close sources, should I see 2 times more pulses on PMT? So will I see distribution with maximum at point

I_max_two_sources = 2 x I_max_one_source?

Any other way or suggestions how to do simple calbration are appreciated.

Thanks for answers to naive questions from beginner in the field.

2. Feb 22, 2015

### Staff: Mentor

If the sources are close enough to give two times the light intensity at the position of the PMT, then you can get approximately twice the pulse rate. Not exactly due to the dead time of the PMT.

3. Feb 22, 2015

### john824

If laser light (green one for example) will be used as input to integration sphere (as light source), what hapend to output (from integration sphere) light wavelenght? Output wavelength(s) will be simmilar or same or different (how much?) to laser wavelength?

4. Feb 22, 2015

### Staff: Mentor

Is there something that would change the frequency of the light? I don't know the setup.

5. Feb 22, 2015

### john824

I am not sure, because that, let me ask in diffrent way - my questions then are:

Can integrating sphere change wavelength? I suppose no, but because I am beginner, I would like to ask it.
Can optical cable change wavelength?

There is nothing more, just air in black box and integrating sphere. So in simple setup without anything else I am not expecting change of wavelength. Could you confirm it?

6. Feb 22, 2015

### Staff: Mentor

(Significant) frequency shifts come from materials only - and you need special materials for that. Light passing through vacuum or air will simply keep its frequency.
The wavelength in a medium depends on the local speed of light within, but once the light leaves the material it goes back to its initial value if no frequency shift occured.

7. Feb 22, 2015

### john824

Thank you.

Another question. Output from integrating sphere will be carried in-by optical fiber cable. Is there reduction of light intensity in cable? So if cable will be let say 1, 10 cm and 100 cm long should be any reduction expected?

8. Feb 22, 2015

### Staff: Mentor

No cable is completely loss-free. The loss (as function of the light frequency) should be given in the datasheet of the cable.