Photodiode measurement method? - Page 1
Forum section: electronics / projects

author: Tandy
author_rank: Frequent Contributor
timestamp: June 13, 2015, 04:55:45 pm
content: I am planning on measuring light levels using a photodiode and considering the best way to do it. The ultimate aim is to have reasonably accurate measurement of light level such as would be needed to for example measure the brightness of an LED.The traditional way to do it is with a transimpedance amplifier to obtain a linear voltage scale. However I have also seen people measuring the capacitance of the photodiode by timing the discharge. Most of the circuits using this method are simply using it to detect the presence of light, i.e is it daylight or night. Anyone had experience of using the capacitance method for measuring light levels, is it accurate?The capacitive method does seem attractive due to the minimal component count but I don't see any examples of it being used for accurate measurements.I'm hoping someone might be familiar with both methods and have some idea of the merits of each.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 13, 2015, 07:25:37 pm
content: The method measuring the time to charge / discharge a capacitance is also measuring current and not the capacitance.A method in between uses a charge amplifier to convert current to frequency with a separate amplifier, rather similar to the TIA.The difficulty is often the large range - here measuring times may have an advantage.

author: Tandy
author_rank: Frequent Contributor
timestamp: June 14, 2015, 08:47:34 am
content: Thanks, it would seem that the discharge timing route would require an op-amp as a charge amplifier anyway in order to get a measurable range, so either way the component count is similar.The intention is to use photovoltaic mode as lower dark current and noise is more desirable than fast response time, as I am trying to measure a static light source rather than a transient signal. So for now I think I will stick with the TIA as that is the method I am familiar with.Now just got to figure out how best to deal with the horrible temperature coefficient of photodiodes. It seems that there are two issues, that the spectral absorption range widens with increases in temperature resulting in higher currents being produced as well as an apparent reduction in shunt resistance. I was thinking that maybe I could pass a current through the photodiode to warm it when the temperature is below 20 degC so that the measurement is never taken with a junction temperature below 20 C but not really got a solution to dealing with higher temperatures.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2015, 09:38:16 am
content: Are you sure you can't afford an instrument instead? A proper one will have calibrated scales, and some understanding and compensation of its nuances (like tempco and such). I should think they're available in the $100s range, depending on spec.If your intent was maybe more to design a kit to do it, you'll still need the calibrated sensor to begin with, which is probably almost as much trouble..Tim

author: TMM
author_rank: Frequent Contributor
timestamp: June 14, 2015, 09:59:00 am
content: The only feasible reason i would see in using the capacitance method is if you want to save the cost of using an ADC - capacitance method can be impelemented with a digital I/O pin instead.

author: Kalvin
author_rank: Super Contributor
timestamp: June 14, 2015, 10:11:44 am
content: Here is a patent using CTMU, A/D-converter and its internal sampling capacitor for measuring sensor current:http://www.google.com/patents/WO2013096371A1?cl=en

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2015, 11:33:35 am
content: Silicon photodiodes are not very temperature sensitive. The only thing that changes a lot with temperature is the dark current, but this is often very small unless you have really high temperature (e.g. more than 50 C). Temperature coefficient is a little larger at the long wavelength limit.If really needed one could measure the temperature of the diode, but usually this not even needed.Temperature dependence is large only if you use photovoltaic mode and direct voltage measurement to get a logarithmic scale. This definitely needs compensation or temperature regulation. So it's usually better to use an extra diode / transistor to do the log conversion if needed.The more important part is the Temperature of the LEDs.For low frequency data, it's often best to use a TIA and operate the detector at nominally zero voltage.One can still use the timing method without an external amplifier: with a bias, the photo-current can be used to charge a capacitor up to a predefined value, checked by the comparator that many µCs include. So the only external part needed would be a capacitor in the nF range.

author: tggzzz
author_rank: Super Contributor
timestamp: June 14, 2015, 11:52:33 am
content: How many watts of illumination, P, will be incident on your photodiode?What will be the photodiode's dark current, Id. That Idwill be equivalent to a noise-equivalent-power NEP = 2*Idwatts of illumination.If NEP ~= P, then you have an noticable inaccuracy.The simplest solution is to chop the illumination P at a frequency f, either mechanically with a shutter or electrically by switching the source on and off. Then use a bandpass filter to only amplify the AC component of the photodiode's output.Even 35 years ago I was able to make asimplefront end with NEP = 1pW which had alinearresponse up to 1mW. That's an optical dynamic range of 90dB. Since 1pW optical -> 1pA current, that corresponds to an electronic dynamic range of 180dB.

author: Tandy
author_rank: Frequent Contributor
timestamp: June 14, 2015, 01:11:38 pm
content: I am at the early prototype stage having simply created a TIA circuit on a breadboard with a common TL081 op-amp to do some testing. I have found that if I refrigerate the photodiode to 5 C and put it under a bright halogen light so that the temperature increases due to the heat from the bulb the voltage reading climbs quite a lot from 1.85V to 2V over a few minutes. Initially I thought if I put a second diode (in the dark) attached to the opposite input on the opamp that it would offset the changes but that didn't work. Further reading suggests that the sensitivity spectrum widens with temperature, so the diode is producing more current due to the wider range. So unless I used a narrow spectrum light source such as a coloured LED there will be a significant variation in readings dependent on temperature.On a broad spectrum light source the compensation is easy as I can simply apply compensation according to a reading from a temperature sensor. But in a narrow band source the problem becomes more difficult. If I shine a yellow LED at the sensor that is in the middle of the spectrum so the widening of the spectrum does not cause any change. However if I shine a blue LED that is on the edge of the sensor spectrum that the wider spectrum will change the reading.As I don't have control over the light source I have to do my best to mitigate problems at the receiver end. I can't see a solution to the widening spectrum sensitivity issue other than controlling the light with an optical filter. Accurate filters get expensive and degrade over time.My ultimate goal is to do automated relatively precise automatic measurements for example place a sensor in a greenhouse and measure the amount of light received by plants or photodiodes with narrower spectrum sensitivity to measure climate conditions such as measuring water vapour in the atmosphere by comparing visible light with near infrared. In these kinds of conditions the temperature swing can be quite large.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2015, 05:10:29 pm
content: The temperature sensitivity is mainly near the high wavelength limit. The light bulb has its maximum intensity in this range. Natural light normally contains rather little intensity in this range as it is absorbed by water vapor. So I won't worry so much about this.I don't remember much trouble at the short wavelength end.With LEDs, the temperature sensitivity of the LEDs it often 10 times higher than that of the detector.

author: Tandy
author_rank: Frequent Contributor
timestamp: June 14, 2015, 06:23:24 pm
content: Thanks for your input, I will do a bit more experimenting, could do with a monochromator really to do wavelength specific tests.I am doing it as much for the experience as anything. I picked up a Sun and Sky monitoring station originally designed by Forrest Mims III for RadioShack. The idea of measuring absorption of different wavelengths to determine the water vapour in the atmosphere seemed interesting. The RadioShack kit has no form of temperature compensation so the station has to be kept indoors and taken out every time to make a measurement. If I can figure out a reliable way compensate for the temperature then I can build something I can leave in place to take its own measurements. The only ongoing maintenance being occasionally cleaning the sensor lens.