Exposure time of area scan cameras
The calculation of the correct exposure time in case of moving parts prevents blurred images. They are produced when the pixels shift in the range of the camera while being exposed. Depending on the application, different tolerances are possible. A smear effect of 0.5 to 2 pixels is not visible to the eye most of the time, however, it still affects precision measurement results. Yet this value is acceptable for many standard applications. Setting shorter and shorter shutter times is nevertheless not unrestricted: at 30 - 50 µs the image becomes "grainy" in many CCD sensors. The minimum shutter time strongly depends on the sensor chip used. The image can also be impaired in case of excessive exposure times (500 ms and more), the image gets noisy. In practice, typical values often range between 0.1 and 20 ms.Tip:
An online wizard for the calculation of the exposure time for moving objects can be found in the "Service" area.
Exemplary calculation for an area scan camera:
A conveyor speed of 6 metres per minute is supposed to be inspected using a camera image field of 128 mm. The maximum permissible smear effect should not be more than half a pixel. The camera has a resolution of 1280 x 1024 pixels.
- Camera resolution 1280 x 1024 pixels
- Part size 128 mm
- Resolution corresponds to 1280 pixels on 128 mm, i.e. 10 pixels / mm
- Conveyor speed 6 m per minute
- This corresponds to 6,000 mm per 60 seconds
- i.e. the conveyor moves 100 mm per second in the camera image field
- 100 mm (camera resolution 10 pixels/mm) correspond to 1000 pixels by which the conveyor moves during inspection.
Exposure time calculation:
- 1000 pixels correspond to 1 second of conveyor carriage
- 1 pixel corresponds to 1/1000 second
- 1/2 pixel error corresponds to 1/2000 second.
In our example, an exposure time of 500 microseconds would lead to a smear effect of 0.5 pixels.