转载至Exposure time
6.1.3. Exposure time
What does the camera sensor actually detect? It detects photon counts; the more photons that hit the sensor elements, the more those elements increment their counters. As our camera has no physical shutter (unlike a DSLR) we can’t prevent light falling on the elements and incrementing the counts. In fact we can only perform two operations on the sensor: reset a row of elements, or read a row of elements.
To understand a typical frame capture, let’s walk through the capture of a couple of frames of data with a hypothetical camera sensor, with only 8x8 pixels and no Bayer filter. The sensor is sat in bright light, but as it’s just been initialized, all the elements start off with a count of 0. The sensor’s elements are shown on the left, and the frame buffer, that we’ll read values into, is on the right:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||
The first line of data is reset (in this case that doesn’t change the state of any of the sensor elements). Whilst resetting that line, light is still falling on all the other elements so they increment by 1:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
The second line of data is reset (this time some sensor element states change). All other elements increment by 1. We’ve not read anything yet, because we want to leave a delay for the first row to “see” enough light before we read it:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
The third line of data is reset. Again, all other elements increment by 1:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |||||||||
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |||||||||
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |||||||||
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |||||||||
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |||||||||
Now the camera starts reading and resetting. The first line is read and the fourth line is reset:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | |||||||||
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | |||||||||
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | |||||||||
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | |||||||||
The second line is read whilst the fifth line is reset:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | |||||||||
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | |||||||||
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | |||||||||
At this point it should be fairly clear what’s going on, so let’s fast-forward to the point where the final line is reset:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
At this point, the camera can start resetting the first line again while continuing to read the remaining lines from the sensor:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
Let’s fast-forward to the state where the last row has been read. Our first frame is now complete:
| Sensor elements | –> | Frame 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | |
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
At this stage, Frame 1 would be sent off for post-processing and Frame 2 would be read into a new buffer:
| Sensor elements | –> | Frame 2 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | –> | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||||||
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | Rst | ||||||||
| 7 | 7 | 7 | 7 | 7 | 7 | 7 | 7 | |||||||||
| 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | |||||||||
| 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | |||||||||
| 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | |||||||||
From the example above it should be clear that we can control the exposure time of a frame by varying the delay between resetting a line and reading it (reset and read don’t really happen simultaneously, but they are synchronized which is all that matters for this process).
Rst与第一次readout的时间即为每一行的曝光时间。
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