In short: FOM is resolution multiplied by signal-to-noise ratio (SNR). It’s a useful indicative number, but the image also depends on background (EBI), halo, blemishes and the quality of the optics. A datasheet only counts if its serial number matches the serial number on the tube.
Decoder: enter the values from your datasheet
Tube datasheet decoder
FOM (resolution × SNR):
This tool doesn't rate anything or compare manufacturers. Check the values on the original datasheet whose serial number matches the tube.
Key parameters
| Parameter | Unit | What it means | Direction |
|---|---|---|---|
| Limiting resolution | lp/mm | How many line pairs per millimetre the tube can resolve. | higher = finer detail |
| SNR (signal-to-noise ratio) | – | At a given light level, how much the signal is corrupted by the tube’s noise. | higher = less scintillation |
| FOM (figure of merit) | – | Resolution × SNR. | indicative |
| EBI (equivalent background illumination) | µlx | Tube background brightness with no light at the input. | lower = darker background |
| Halo | mm | Size of the bright ring around a point light source (measured with a standardised input spot). | smaller = better |
| Gain | cd/m²/lx | How much the tube amplifies light. | within the maker’s range |
| Autogating | yes/no | The power supply automatically regulates voltage as light changes, protecting the tube. | yes = faster response to light |
| Phosphor | P43 / P45 | Image colour: green (P43) or white (P45). | personal preference |
Why a higher FOM doesn’t always mean a better image
FOM combines only two numbers. Two tubes with the same FOM can look different in practice – they can differ in background (EBI), halo size, blemishes or the quality of the optics and housing. Treat FOM as a first comparison, not the only criterion.
How to check that a datasheet is genuine
- The serial number on the datasheet must match the serial number on the tube.
- The datasheet should show the tube manufacturer and the test date.
- A datasheet without a serial number, or one downloaded from the internet, tells you nothing about a specific tube.
- For a used device, ask for the original or a legible photo of the whole datasheet.
Sources
- Photonis (Exosens) – Night vision at a glance, 2025 – definition of FOM, resolution and SNR (checked 9 Nov 2026)
- Photonis 4G – technical leaflet – EBI in µlx, halo with 0.35 mm input spot, gain, autogating (checked 9 Nov 2026)