Biologists that use, or are interested in using, microscope cameras and don’t speak engineering
Clarity on the relevance of camera specs to biological experimentation
|Synonyms used by other vendors
|Full well capacity
|Pixel well depth
|Full well capacity EM-CCD readout
|Shift register well depth, Gain register pixel well depth
|A/D conversion factor, Digitization, Digital output format
As with many scientific questions, the answer to the seemingly simple question, “Will I be able to differentiate 1500 molecules from 1475,” is not straightforward. It depends on the photon emission properties of the fluorophore, the number of fluorophores per molecule, and the camera. The heart of this question is understanding the detectable signal change that your camera is able to detect for your specific experiment. And to understand detectable signal change, we need to look at full well capacity.
Detectable signal change (DSC) describes sensitivity to change. For the case of a microscope camera, DSC describes the measureable precision of a change in the signal, making total signal acquired the limiting factor. Full well capacity—which defines the maximum amount of signal the camera can detect—is the most important feature and camera noise is of minimal importance.
Back to the question of detecting 1500 molecules versus 1475, the question becomes, “can I detect a 2 % difference?” Using the following equations for DSC and % DSC:
and solving for S:
Our camera would need to be able to have enough full well capacity to capture 2500 electrons before saturating.
Of course, our sample would need to produce enough photons to create that many electrons in the detector in the time allotted by the exposure and the quantum efficiency of the system.
But what if you wanted to detect this 2 % difference while also keeping other features in the image visible (above noise) and non-saturating. For example, if you were trying to image neurons, where the cell bodies are bright, but the dendrites are much dimmer. Now you need to consider the dynamic range (DR) of the camera.
The total DR of the camera is also a function of the full well capacity, but here camera noise is important to consider. Note that DR does not consider the sample, it is only a property of the camera.
DR is defined as the maximum achievable signal divided by the camera noise.
And is sometimes presented in decibels instead of as a ratio:
Using the ORCA-Flash4.0 as an example, we divide the full well capacity of 30,000 electrons by the read noise at slow scan, which is 0.9 electrons, to get a dynamic range of 33,000: 1, or 90.4 Db.
Note that for EM-CCDs, when using the EM register dynamic range can be reduced. Once the full well capacity of the EM gain register exceeds the amplified full well capacity of the individual pixel, dynamic range decreases. The net result is a tradeoff between high EM gain and wide dynamic range.
It’s important to recognize that bit depth and dynamic range are not synonymous. Full well capacity divided by bit depth establishes the limit of precision for each gray level—it gives the value for how many electrons per gray level (digitizer bit).
For quantitative image analysis and feature classification, high bit depth is essential.
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