Fastec IL3, Руководство оператора

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IL3 User’s Guide
page 68
11S-3002B
Application Note 4: Frame Rate, Resolution, and Exposure
Scale and Resolution/Frame Rate
Selecting the proper resolution and frame rate for a given high-speed event is important. It is based
on the Field of View (FOV) required to get a good image of your object of interest and the speed at
which the object will move through that FOV.
For example, if you wish to image an automobile traveling at 50 mph across an intersection, full
resolution and a relatively slow frame rate will work because your field of view (FOV) will be large and
the car will not be moving through it very quickly.
Imaging a bird traveling at the same speed will require a much smaller FOV as the bird is 1/20th the
size of the car. If you wish to use the same scale (object size/FOV), the FOV becomes 1/20th the size
and the bird moves through it 20x as fast.
If you got acceptable imaging of an auto at 60FPS, it may take 1250FPS to get similarly acceptable
imaging of a bird at moving the same speed.
Aliasing and Frame Rate
If you are imaging a motion that is cyclical in nature like a wheel spinning or a lever moving up and
down, it is important to use a high enough frame rate to avoid motion aliasing. If you know the
speed of the object, use a frame rate at least a few times as fast as the repetition rate to get a valid
characterization of the motion. If you don’t know the speed, use as high a frame rate as possible
to start with and adjust from there. (Be sure to analyze the movement one frame at a time as the
playback speed may cause aliasing as well.)
Generally you will choose to use the largest resolution possible for the frame rate required. This will
give you the best definition of your object of interest. Smaller resolutions may be desired in order
to increase the record time. Choosing the right shutter speed is dependent not only on the speed at
which an object is traveling through the FOV, it is also dependent on how the imagery is going to be
used.
For motion analysis it is best to get as short an exposure as possible in order to limit motion blur.
(Motion blur can be defined as the number of pixels traversed by an edge of an object during an
exposure.)
For smooth video, on the other hand, long exposures are best. These make for more attractive
movies, but blurry stills.