ScanImage 2018 : Slow (Y) Scan Dimension Basics


ScanImage uses a galvanometer in conjunction with a resonant scanner to form a rectangular image. After capturing the last line of the frame, the galvanometer must return to capture the first line of the next frame. This movement is called Flyback, and the flyback duration is a function of (among other parameters) the angular velocity of the galvanometer and the total angular displacement of the flyback. ScanImage accounts for the time necessary for this flyback to occur, so that artifacts due to flyback do not appear in the frame.


Unlike the X command signal (in most cases), the flyback of the Y command signal is instantaneous (in all cases).


 Future versions of ScanImage may provide an option to have a Y command signal with non-instantaneous flyback


If the physical scanner lacks slew rate limiting (see Physical Scanner Limitations), then the instantaneous Y command signal generated by ScanImage may cause harm to the scanner. Fortunately, commercial scanners typically do have slew rate limiting.


In reality, however, the flyback of the physical scanner will not follow this command. As a result, the physically scanned trajectory during frame scans must either coincide with first or final line of the Frame:


Laser scan trajectory during frame scans including Slow (Y) dimension flyback


Flyback on Final Line OFF

When Flyback on Final Line is disabled (default), the flyback of prior Frame is coincident with first line of succeeding Frame. In this mode, the beginning portion of the first line of eachFrame (starting with the second) contains the artifact of scanning diagonally across the field-of-view.


This mode is provided to mostly match the behavior of previous versions of ScanImage


This artifact will appear both in the image display figures and in the saved image file

Flyback on Final Line ON

When Flyback on Final Line is enabled, the final line of each Frame is effectively skipped, and the (instantaneous) Y command signal flyback occurs immediately following the end of the second-to-last line in the Frame. In other words, the flyback is considered to comprise the entire Line Period of the final line.

Within the image display figures and in the saved image file, this simply moves the Y flyback artifact from the first line of image to the final line. Importantly, however, if Pockels modulation is enabled and the Blank Flyback option is enabled, then the Beam(s) will be 'blanked' during this entire Line Period.

In addition, when Flyback on Final Line is enabled, the option to Discard Flyback Line becomes available, which allows the final line to be discarded during processing, so that it does not appear in either the image display figures or in the saved image file. In other words, the displayed/stored lines per Frame is one less than configured Lines/Frame.


The mode Flyback on Final Line is recommended in particular for cases where the number of Lines/Frame is low, with either the Discard Flyback Line option or the use of a Pockels modulator with Blank Flyback enabled


When using Discard Flyback Line, it may be desired to set Lines/Frame to an odd value (e.g. 257) in order that displayed/stored images are a more common even value (e.g. 256)