When selecting a camera for machine vision applications, factors beyond resolution and frame speed are critical. Among these are full well capacity, signal-to-noise ratio (SNR), and dynamic range (DR). However, interpreting these specifications can be tricky because the definitions used by sensor manufacturers often differ from those in camera datasheets. Let’s clarify these concepts using the example of the OnSemi Vita25k sensor found in Adimec cameras like the S-25A70/CXP and S-25A30/CL.

Full Well Capacity: What It Means

The full well capacity refers to the maximum number of electrons a pixel can hold. Sensor manufacturers typically list this as 22,000 electrons (e). However, this value represents the theoretical limit—the sensor’s behavior becomes nonlinear beyond that point. For machine vision applications like automated inspection, we focus on the linear range where measurements remain accurate. Adimec measures each sensor to determine its usable linear capacity: 13,000 e in this case.

Signal-to-Noise Ratio (SNR)

The SNR is often calculated as (20 \cdot \log_{10}(\sqrt{\text{Full Well}})). While the manufacturer’s data sheet might list a value like 43.4 dB based on their full well capacity, this includes nonlinear operation and doesn’t reflect real-world usability. Adimec recalibrates sensors to ensure SNR calculations align with the linear operating range.

Dynamic Range (DR)

Dynamic range is defined as (20 \cdot \log_{10}(\text{Full Well / Dark Noise})), where “Full Well” refers to the usable capacity in the linear regime. This specification indicates how well a camera can capture both bright and dark details in an image.

Example Comparison

The table below compares specifications based on different definitions of full well capacity:

Parameter Sensor Manufacturer (22,000e) Adimec Linear Capacity (13,000e)
Dark Noise 34 e 34 e
Full Well Capacity 22,000 e 13,000 e
Dynamic Range (DR) 56.2 dB 51.6 dB
Signal-to-Noise Ratio (SNR) 43.4 dB 41.1 dB

Adimec’s measurements show that the usable full well capacity is 13,000 e, with corresponding DR and SNR values calculated accordingly.

Why Definitions Matter

Camera manufacturers often set their analog-to-digital converters (ADCs) to handle a larger well size than necessary for linear operation. This prevents clipping but can mislead users if specifications aren’t aligned with the usable limits. Adimec provides practical specs based on actual testing data, as shown in our internal test results from 45 camera tests:

Parameter Average DR/SNR (from 45 Tests
Dark Noise 34 e
Full Well Capacity 16,119 e
Dynamic Range 53.7 dB

Final Thoughts

The way you define full well capacity directly impacts the reported DR and SNR values. The highest achievable specifications (from Adimec’s testing) appear in the final table below:

Parameter Full Well Capacity
Dark Noise 34 e
Dynamic Range (DR) 53.7 dB
Signal-to-Noise Ratio (SNR) 42.0 dB

Conclusion

Always verify how full well capacity, DR, and SNR are defined when evaluating camera datasheets. If you need precise results for machine vision tasks, ensure the specifications align with your application’s linear measurement requirements.

For further reading, explore our related articles on dynamic range, SNR, and other technical topics relevant to machine vision cameras.

Last Updated: 2025-09-04 21:00:28