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I am currently trying to amplify a tiny amount of light so that a photodiode will recognize it.

In order to amplify the light I thought about using a magnifying glass since I got to know that such convex glasses might help to spread light. I was positively surprised as I saw the result as the light really got amplified using a magnifying glass (see drawing). However the issue is that the magnifying glass requires a distance of 9-10 cm between the light source and the glass in order to spread the light in the required strong way.

I was wondering whether there is any way to get the same light amount spread without having a gap between the lens / glass and the light source (or having a small gap)?

enter image description here

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    $\begingroup$ You can use a large area sensor, even though that leads to other problems (high frequency noise and response time). A solar cell might do just fine. Please not that you are not "amplifying" light, you are only collecting more of it. What a lens does is to trade "area against angle", and in this case it only works because the light that you want to detect is nearly parallel. Now, you can use a mirror instead of lens, or a combination of mirrors or multiple lenses, which will greatly reduce the thickness of the setup, but that's ultimately an engineering optimization. $\endgroup$
    – CuriousOne
    Commented May 25, 2015 at 22:28
  • $\begingroup$ Thx! Do I understand correctly that u mean that a solar cell will sprad the light as the magnifying glass does? $\endgroup$
    – Sathees
    Commented May 25, 2015 at 22:52
  • $\begingroup$ A solar cell is basically a large area photo diode, except that it doesn't cost nearly as much. The signal to noise ratio that you will get out of a solar cell is, of course, horrible, since the capacitance is so large. On the other hand, since you didn't tell us anything about the signal that you want to measure, that may or may not be a problem. $\endgroup$
    – CuriousOne
    Commented May 26, 2015 at 3:20
  • $\begingroup$ Basically due to the given circumstances an led light becomes weak however visible. Unfortunately the photodiode ( which will be connected to a microcontroller) is not able to recognize the light in order to take further action. Using a magnifying glass the photodiode recognizes the light. The gap of arround 9-10 cm distrurbs me using the glass.... $\endgroup$
    – Sathees
    Commented May 26, 2015 at 7:06
  • $\begingroup$ There are a couple of things you have to do. 1) use a small lens in front of the LED to create a narrow directional beam 2) use a small lens in front of the photodiode 3) modulate the signal from the LED at e.g. 1 kHz and use a synchronous detector on the highly amplified ac signal from the photodiode. These three measures will buy you three or four orders of magnitude of sensitivity while making the setup insensitive to stray light. $\endgroup$
    – CuriousOne
    Commented May 26, 2015 at 7:13

2 Answers 2

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A lens with more curvature will have a focus point closer to the lens. Unfortunately, they are more difficult to make. Since these lenses need good precision, they are made with glass and poor quality lenses are usually discarded.

On the up side, LEDs and sensors are designed to work as a pair and may be used up to (about) one meter, so lenses are not needed.

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  • $\begingroup$ Thanks! Well unfortunately that LED light power gets weakened and the significantly weakened light should be recognized by the photodiode. Since the light is too weak I had to make use of a magnifying glass.. $\endgroup$
    – Sathees
    Commented May 25, 2015 at 22:51
  • $\begingroup$ There might be a solution, did you get the LED and photodiode together? Does the wavelength of maximum output of the LED match the wavelength of maximum sensitivity of the photodiode? $\endgroup$
    – LDC3
    Commented May 25, 2015 at 22:53
  • $\begingroup$ I did my homework on selecting the right photodiode....which has its peak sensitivity at 580nm $\endgroup$
    – Sathees
    Commented May 25, 2015 at 22:58
  • $\begingroup$ How close together can the LED and photodiode be? less than 1 cm? What do you want to do with the beam? $\endgroup$
    – LDC3
    Commented May 25, 2015 at 23:40
  • $\begingroup$ Pls check the updated drawing above! The issue is the very low level light that I wanna have detected by the photodiode $\endgroup$
    – Sathees
    Commented May 25, 2015 at 23:50
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What you want is a lens with a shorter focal length. As a rough number, you can work with a LED to lens distance of twice the focal length of the lens. Note that that is what you are doing with your magnifying glass. It's possible to get lenses with focal lengths less than 10 mm, so you can operate at less than 2 cm separation. Since I assume you don't want to spend much money on this, I recommend an optical surplus site. Surplusshed.com is a good place to start, and they have some very low-cost items.

And you seem to misunderstand how a lens works in this case. What you are trying to do is focus the LED on the photodiode. Depending on the size of the LED vs the diode, for best results you may have to get a photodiode with a bigger active area.

And finally, the simplest way to do what you seem to want to do is simply butt the LED against the front of the photodiode.

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  • $\begingroup$ Thx! Well basically I want to reduce the gap between the lens and light source whilst the light gets powered/amplified somehow so that it is signficant enough for the photodiode. I did consider on selecting a photodiode with a bigge sensitivity area. $\endgroup$
    – Sathees
    Commented May 25, 2015 at 23:35

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