Detection Questions & Answers

Which detector gives me the best signal-to-noise ratio (SNR)?

Which photodetector has the best SNR varies greatly with light level, speed, and support electronics. It’s usually best to determine the light level and which noise source limits you: your signal shot noise, your electronics/amplifier noise, or your detector’s dark noise.

For high light levels where amplifier noise is negligible and you are signal shot noise limited, detectors with the highest quantum efficiency (QE) will have the best SNR.

For lower light levels where the readout or amplifier noise dominates, detectors with high internal gain are required for good SNR. The detector’s internal gain reduces the importance of the amplifier noise, so you are then limited by dark or signal shot noise.

For the lowest light levels, detectors with the lowest dark noise will have the best SNR.

If you’re not sure, we can help! Please contact us to get an SNR calculation for your unique situation.

For more info about choosing a detector, read our Guide to detector selection article.

What is photon counting?

Photon counting is being able to measure and observe a single incident photon. It is typically used with a photon counting specific circuit that discriminates electronics noise out from the observation and removes the excess noise factor, which is noise from the detector’s intrinsic gain mechanism. Using this circuit and because the light levels are so low, we are typically limited by the dark noise of the detector.

What detector options are available for photon counting?

For photon counting, detectors with intrinsic gain are necessary. A commonly used detector is a photomultiplier tube, which uses dynodes to provide intrinsic gain to the detector. Photodiodes do not have intrinsic gain which means 1 incident photon will, at most, allow the flow of 1 electron, which is too small of a signal to overcome the noise. Avalanche photodiodes (APD) have intrinsic gain, but the gain is too small to overcome noise in most cases, unless the APD is run in Geiger mode. Single-photon avalanche diode (SPAD) and Multi-Pixel Photon Counters (MPPC) are based on APDs in Geiger mode, so these two types of detectors have enough intrinsic gain to show single photon pulses.

How can I improve my signal-to-noise ratio (SNR) if I am barely able to differentiate my signal from noise in an MPPC?

If you are limited by the dark noise, it benefits you to try and decrease the dark noise as much as you can to see if you can detect your single photons. One improvement can be using a cooled detector. Examples of a cooled MPPC are the S13362-3050DG and S13362-1350DG. Cooling will help reduce the number of dark counts and dark noise at the cost of needing more power to drive the detector with cooling. If a module is preferred, we do have a variety of cooled modules including the new C14455-GA and C14456-GA series, which have peak sensitivities at 600 nm. If cooling is not preferable for the MPPC and other aspects of the signal/noise cannot be changed, switching the detector family to PMTs may be preferred since PMTs have lower dark counts per unit active area. Please contact Hamamatsu’s Technical Support team if you would like additional information about the tradeoffs and options.

How else can I improve my SNR when I’m photon counting?

Increasing the measurement time can improve your SNR by a factor of the square root of your integration time. Meaning if your current measurements is only 1 second and now you measure the number of signal counts and dark counts for 4 seconds, you will approximately double your SNR!

Here’s the equation for photon counting SNR:

SNR=Detected counts - Dark counts * Measurement TimeDetected counts+2 *Dark Counts

For more information, view a presentation about low light detection.

If you’ve got a technical question you’d like to see answered on this page, email us.

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Meet the engineers

Eric Mesa is an Applications Engineer out of NJ, who understands the intricacies of signal-to-noise comparisons. He likes baseball and most other sports, and when it comes to detector selection, he always hits a home run!

Neil Patel enjoys the majesty of narwhals and photons. He glides through technical issues just as the unicorn of the sea glides through the water. Fun fact: The narwhal’s tusk is actually a protruding tooth.

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