Sensitivity and ISO 12232
Sensitivity, pixel resolution, and throughput are the three major characteristics to consider when choosing a high-speed camera. While many other features enhance the camera system and workflow, these three are crucial in differentiating one camera from another.
‘Sensitivity’ is a critical parameter specified by most camera manufacturers using the ISO 12232 methodology. This saturation-based method indicates that a higher ISO value allows the image to reach saturation with less light. However, it does not precisely define the sensor’s sensitivity to light (i.e., how many electrons are generated per incident photon) relative to the sensor noise.
ISO 12232 was originally established to compare digital sensors to film speed, but it does not provide a full range of sensor performance. High-speed cameras are often required to perform well in low-light environments, which cannot be accurately determined by the ISO specification. Achieving a higher ISO rating can be manipulated by adjusting display settings, such as reducing bit depth or full-well capacity. However, these changes might make the image appear brighter but adversely affect image quality and overall performance.
When using ISO 12232 to compare high-speed cameras, consider the following limitations:
- ISO 12232 does not account for noise performance, which is essential for discerning low-light camera performance. Key attributes like Temporal Dark Noise (Read Noise) and Absolute Sensitivity Threshold, indicating how well the sensor can distinguish detail from noise, should be considered instead.
- ISO can be easily manipulated with added gain, which lowers the Signal-to-Noise Ratio (SNR) and Dynamic Range (DR). Unfortunately, SNR and DR are not always reported, especially not in relation to ISO, making this trade-off less apparent.
- ISO values are often rounded up to the closest defined ISO value above the actual measurement. This rounding factor can inflate the ISO value by up to 1/3 F-stop, making it unsuitable for accurate photon level measurements and sensitivity comparisons.
- The light source used for measuring ISO can be Tungsten or Daylight, leading to significantly higher specs for monochrome cameras without an IR filter when using a Tungsten source. However, many applications do not involve scattered or reflected light matching the spectrum of these sources. Best practice involves using Spectral Responsivity plots (or QE curves) to determine the number of electrons generated relative to incident photons across the visible and NIR spectrum. Spectral response curves are available for all Phantom cameras.
Due to these uncertainties and limitations, relying solely on ISO 12232 to compare sensitivity or image quality aspects is not advisable. Vision Research has shifted away from ISO as the standard for specifying sensitivity in favor of the EMVA 1288 standard. EMVA 1288 provides customers with a comprehensive set of sensor parameters to accurately evaluate a camera’s imaging performance.
To learn more about EMVA, you can visit the page on the Vision Research website by following this link.
For a deeper dive into ISO, including the mathematical aspects, refer to our application note that you can find here.
