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I think we'd need to be told a whole lot more - 
what the dye was, what the bandpass was, etc.
A mercury lamp has very powerful spectral lines,
so (especially in the green) the measured power
is not spread across the spectral range but 
concentrated at one wavelength.  If this happens 
to hit a particular transition the effect on the 
fluorochrome could be different from excitation
by a broad band.

                                       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)
Electron Microscope Unit, Madsen Building F09,
University of Sydney, NSW 2006
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-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Mark Cannell
Sent: Wednesday, 12 March 2008 8:02 AM
To: [log in to unmask]
Subject: Re: Fluorophore bleaching by excitation light sources

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I don't understand this. The only explanation I can think of is that that the excitation wavelengths are _not_ the same in the two cases.(if they were and the power is the same the photon flux is the same).  Any other comments/views?

Regards.

Gerard Whoriskey wrote:
> Commercial interest.
>
> We have recently had preliminary feedback from a number of independent 
> sources that show much reduced bleaching when a sample is excited 
> using an LED source rather than a Hg bulb. These tests, carried out 
> with identical powers in the excitation bandpass region, showed that 
> imaging could be carried out for up to three times longer.
> On live tests cells were seen to be still living happily after being 
> exposed for twice the time it took to kill the cells under Hg excitation.
> We are still gathering information on this and intend to publish in 
> due course.
>   

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