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Just to add to Craig's post, AOTF-equipped confocals do blank the beam
on the return trace, which significantly reduces bleaching.  Someone in
the early days - I think it was Watt Webb's lab - added a high-speed
shutter to a regular confocal to do the same thing, but I don't think
any microscope manufacturer took it up.  How fast your flyback can be
depends, in principle, on your scan speed but I think a lot of systems
actually scan both directions at the same speed, and of course resonant
scanners can only do that, so the bleaching on flyback can be
significant.  And many years ago I did a bleach test and the vertical
line at the 'turn-round' was quite visible - but I think that's
atypical.  

                                     Guy

Optical Imaging Techniques in Cell Biology
by Guy Cox    CRC Press / Taylor & Francis
     http://www.guycox.com/optical.htm
______________________________________________
Associate Professor Guy Cox, MA, DPhil(Oxon) 
Australian Centre for Microscopy & Microanalysis, 
Madsen Building F09, University of Sydney, NSW 2006 

Phone +61 2 9351 3176     Fax +61 2 9351 7682
             Mobile 0413 281 861
______________________________________________
      http://www.guycox.net
 


-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]]
On Behalf Of Craig Brideau
Sent: Friday, 1 July 2011 4:49 AM
To: [log in to unmask]
Subject: Re: confocal scanning

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For mono-directional scanning, the return to the start of line is called
the
'flyback' time.  Usually it is quite short as the galvo typically moves
at
top speed to 'snap' back to the start of the next line.  Unless your
laser
powers are extremely high, the flyback exposure the sample receives is
probably negligible.  You do get a bit of lag time at the start of the
line
though.  What happens is the galvo snaps to the position, but then has
to
reverse direction to begin sweeping out the line.  One way to deal with
this
is to have an edge clipping the beam off just before the start of a
line.
 That way when the galvo is reversing direction and the laser is
dwelling
for a little bit, the beam just strikes the blocking edge before it
starts
sweeping out the line.  Some scan heads are designed this way, others do
it
through clipping off the inherent apertures in the system rather than
having
a dedicated edge.

Craig



On Thu, Jun 30, 2011 at 7:47 AM, Sandrine
<[log in to unmask]>wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> *****
>
> Hello
> One quick question about bi and mono directional scanning in confocal
> microscopy. In mono directional scanning, what happen to the laser
light
> when
> the galvanometer mirror brings the beam back to the beginning of the
line?
> My
> understanding is that it does not reach the sample if the system is
> equipped
> with an AOM or EOM, but I am not absolutely sure about why. And what
> happens during a dual color bidirectional scan?
> Thanks a lot!
>