The rapid development of digital camera technology extends the range of applications for camera-based mesurements continuously.
The camera hardware designed for scientific and industrial applications has several special features compared to the consumer cameras:
Exposure time and triggering
Accurate timing and control of the exposure time is crucial. Exposure times in the range of 1-100 µs are typically used and the image acquisition is synchronized with external signals,
such as pulsed laser illumination. Thus, very low jitter in triggering and exposure timing is essential. These aspects set high requirements for the camera electronics.
Image quality
Image quality is a key factor in producing reliable measurement data. Important parameters include image resolution, contrast and sharpness. Emerging trend in the digital camera
technology is towards higher resolution, i.e. increasing the number of pixels while decreasing the pixel size. However, due to light diffraction-limited spot size, the modulation
transfer function (MTF) of the camera sensor is at its best around 3.5 µm pixel size – smaller pixels do not improve camera’s information transfer (i.e. resolution). There is also
a trade-off in decreasing the pixel size. Smaller pixels mean less light sensitive area in the sensor decreasing the low-light performance of the camera. The camera sensor has to
produce a linear response to incident light with high dynamic range and low noise level. Scientific cameras typically utilize 12/16bit dynamic range.
Data transfer
Since a large number of images is typically needed for statistical analysis, efficient data transfer from the camera to the computer / storage space is needed. Cameras may provide
imaging rates of 10-50 images/sec. There are several camera interfaces to connect the camera to the computer. Fastest data transfer is provided by cameraLink, Gigabit Ethernet (GigE) and
FireWire (IEEE1394b). High-speed cameras utilize a large internal memory to store the images. Special memory is used to allow writing speeds of more than 8 Gb/sec. The image data
is downloaded from the high-speed camera only after the recording.
Other Special features
In order to measure the displacement or velocity of a fast-moving object, the camera needs either a high frame rate or a short interframing time. Short interframing time allows the recording
of two images with short time delay. Modern CCD sensors can achieve an interframing time of 500 ns. This feature is necessary for example in Particle Image Velocimetry (PIV) and
Particle Tracking Velocimetry (PTV) techniques.
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