In this article we assume that both detectors are of the same kind (both Excelitas SPADs, both MPD PDMs, or both PMA Hybrid detectors).
If you are using either a proper (see below) 50/50 beamsplitter to split the signal between two detectors or a 100% mirror to direct the signal to one detector and then the other, but still observe very different count rates on the two detectors, this discrepancy could be due to the following reasons:
1. Misalignment of the Lens in Front of the Detector
The lens alignment in front of the detector may be incorrect. Typically, the detector mounted in reflection requires adjustment because beamsplitters might not all have exactly the same angle.
Solution: Follow the "Everyday Alignment" Procedure from the MicroTime 200 manual to correctly align the lens. It might be that "Fundamental Alignment" is also necessary.
2. Beamsplitter Variability
PicoQuant offers different types of beamsplitters:
- Simple 50/50 splitters (delivered with each beamsplitter tower) → split rather 30/70, depending on wavelength and polarization
- 50/50 cube (nonpolarizing) → should split 50/50
- 50/50 cube (polarization splitting) → splits 50/50 only for randomly polarized signals
- 50/50 plate (labeled "50/50, 400–1000 nm") → should split 50/50
Solution: To determine whether misalignment or the splitter itself is causing the discrepancy, direct all light to Detector 1 and record the count rate (adjust laser power so the count rate is between 100 kcps and 700 kcps). Then use a 100% mirror to redirect all light to Detector 2 and record the count rate. If the difference is more than 10%, follow the "Everyday Alignment" procedure for the detector with the lower count rate. Then introduce the beamsplitter and verify the combined count rate from both detectors matches the single-detector measurement (allow for ~10% error).
3. Detector Efficiency Differences
Even two detectors of the same model can have somewhat different detection efficiencies due to normal unit-to-unit variation from manufacturing. This is expected and does not indicate a fault. Manufacturers specify PDE as a minimum or typical value rather than an exact figure, and each unit ships with its own individually measured performance data — confirming that individual units of the same model will differ to some degree.
SPADs (Excelitas SPCMs, MPD PDMs):
- For Excelitas SPCMs, peak PDE is typically above 70% near 650 nm. Individual units of the same model can show some typical variation around this figure.
- MPD PDMs show a similar picture, with a typical peak PDE of around 49% at 550 nm. Some variation between units is normal.
PMA Hybrid detectors:
- PMA Hybrids use Hamamatsu hybrid PMT tubes. GaAsP photocathode versions peak at around 40–45% quantum efficiency near 580 nm; multialkali versions peak lower at around 20–25%.
- Some unit-to-unit variation is expected here as well, similar in nature to SPADs.
- PMA Hybrids are additionally sensitive to how uniformly the photocathode area is illuminated, so coupling or focusing differences can compound efficiency-related count rate differences.
General notes:
- PDE is strongly wavelength-dependent for all detector types, so differences can be more pronounced at wavelengths away from the peak, especially if the two detectors are from different production batches.
- Ageing also affects detection efficiency over time — an older detector in a two-detector setup may show noticeably lower efficiency than a newer one.
A count rate difference of up to ~10% between two nominally identical detectors is generally within the normal range, even with perfect alignment and a perfect 50/50 beamsplitter. Differences beyond ~15–20% are worth investigating further (alignment, detector health, or beamsplitter type).