Photons per pixel = (Sample brightness × NA² × Transmittance × Pixel area at sample) / Magnification². Larger pixels at lower magnification collect more photons per pixel despite potentially lower NA. The photon budget determines ultimate SNR: even 100% QE can't create signal from photons that weren't collected.
Sensitivity Metric
Photons per Pixel
The fundamental signal budget
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Definition
The number of photons collected by each pixel during an exposure, before conversion to electrons. This is the starting point for all sensitivity calculations and depends on sample brightness, collection efficiency, and pixel area.
Technical Details
Simplified
Each pixel is like a bucket catching raindrops of light. Photons per pixel tells you how full each bucket gets. Bigger buckets (larger effective pixel area) catch more drops, but you need enough buckets (pixels) to see the detail.
Why It Matters
Understanding photons per pixel reveals why the 25× E9 system can match the 60× Kinetix for sensitivity despite lower NA—the larger sample area per pixel collects proportionally more photons.
Practical Example
For equal sample brightness: 25× with 1.1 NA collects from 148nm × 148nm = 0.022 μm² per pixel. 60× with 1.2 NA collects from 108nm × 108nm = 0.012 μm². The 25× system collects 1.8× more photons per pixel despite slightly lower NA.