Naked Science Forum

Non Life Sciences => Technology => Topic started by: Atomic-S on 06/02/2016 05:36:10

Title: How does illumination affect phototransistor noise?
Post by: Atomic-S on 06/02/2016 05:36:10

What is the relationship between the amount of noise created in a phototransistor that is illuminated by a steady omnidirectional source at a certain distance away, compared to the noise created when the transistor is illuminated from the same source N times further away? (No other source of illumination being present.)

Title: Re: How does illumination affect phototransistor noise?
Post by: evan_au on 06/02/2016 08:20:42

Quote from: Atomic-S

a phototransistor that is illuminated by a steady omnidirectional source at a certain distance away

I am imagining a phototransistor mounted at the center of a uniformly illuminated sphere (say, 0.1m radius).

Quote

when the transistor is illuminated from the same source N times further away?

I am imagining the same phototransistor, but this time mounted at the center of a larger, uniformly illuminated sphere (say, 1m radius, N=10).
- The illumination intensity of the sphere's inner surface has not changed (in terms of lumens per cm²).
- But the surface is now 10 times further away
- By the inverse square law, the intensity should decrease by a factor of 100 from a given cm² of the sphere
- But by geometry, the second sphere has 100 times the area
- So this suggests that the illumination is the same at the center of both spheres

I suggest that the noise should be the same in both cases.

Title: Re: How does illumination affect phototransistor noise?
Post by: alancalverd on 06/02/2016 08:44:28

"Omnidirectional" implies a point source in my understanding, so the illumination  will follow an inverse square law.

NB We generally distinguish between signal, the photocurrent associated with the light source, and noise, due to thermal effects within the transistor.

Title: Re: How does illumination affect phototransistor noise?
Post by: Colin2B on 06/02/2016 10:03:43

The light source will contain noise, mainly thermal noise, but this will be a constant proportion with distance as said in previous replies.

All transistors generate noise, mainly thermal and shot noise, and there will be variations depending on bias current, frequency and impedance matching which will be on the detailed data sheets for specific devices. In terms of light levels you will want to look at fluctuations in the generation of free carriers, diffusion and recombination fluctuations, and 1/f mechanisms. I would contact the manufacturer of some devices, you'll find them very helpful as there has been a lot of research in this area, particularly for low light levels.