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From: Arthur Rowe <arthur.rowe@nottingham.ac.uk>
To : lkott@lanet.lv
Date: Fri, 20 Oct 2000 10:33:48 +0000
Re: 3mm centerpieces
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Laila -
Jim Cole is absolutely right to offer cautionary comment about the need to
have the optics focussed exactly on the 2/3rds plane is you want to avoid
Wiener skewing problems when using short optical pathlengths and hence high
refraction gradients.
What is often overlooked however is that the focussing precision required is
fixed not in absolute space along the optical axis, but in relative
'pathlength in solution space'. That means that if you use a centrepiece
which is 10 times thinner, you need to achieve 10 times greater precision in
focussing this optics to end up with a tolerable level of error. Clearly,
this puts impossible demands upon your ability to focus the optics when the
column is really short - with a 'squashed gasket' centrepiece, even getting
the plane of focus inside the solution column at all would be miraculous !
What is even more often overlooked is that with interference optics
(although not with differential refractometric - or schlieren - optics)
there is an 'aperture' effect, which can only be avoided by focussing
precisely on the 50% plane ! Older literature - about half a century back -
does imply that this effect causes fringe distortion/blurring rather than
displacement, so may cause noise rather than systematic error, but it would
be nice to have some certain knowledge that contemporary approaches to
fringe displacement evaluation(1) are unaffected by this problem.
To summarise: if you use short optical pathlength cells to examine
concentrated solutions with their inevitably steep refraction gradients(2),
then you have to accept some significant degree of loss of precision as
compared to working under more usual conditions. And worryingly, this loss
of precision will in general be both systematic and r-dependent. But of
course the problems which you are tackling may be adequately error-robust.
Best wishes
Arthur
.
notes:
(1) there are people out there who will note my restraint in not enlarging
on this issue !
(2) errors are refraction gradient-, not absolute refraction dependent.
*****************************************************
Arthur J Rowe
Professor of Biomolecular Technology
University of Nottingham
School of Biological Sciences
Sutton Bonington
Leicestershire LE12 5RD UK
Phone/voicemail +44 (0)115 951 6156
Phone/fax +44 (0)115 951 6156/7
email arthur.rowe@nottingham.ac.uk
arthur.rowe@connectfree.co.uk
Web http://www.nottingham.ac.uk/ncmh/business
*****************************************************
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<HEAD>
<TITLE>Re: 3mm centerpieces</TITLE>
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<FONT SIZE=3D"2">Laila -<BR>
<BR>
Jim Cole is absolutely right to offer cautionary comment about the need to =
have the optics focussed exactly on the 2/3rds plane is you want to avoid Wi=
ener skewing problems when using short optical pathlengths and hence high re=
fraction gradients. <BR>
What is often overlooked however is that the focussing precision required i=
s fixed </FONT><U>not</U><FONT SIZE=3D"2"> in absolute space along the optical=
axis, but in </FONT><U>relative 'pathlength in solution space</U><FONT SIZE=
=3D"2">'. That means that if you use a centrepiece which is 10 times thinner,=
you need to achieve 10 times greater precision in focussing this optics to =
end up with a tolerable level of error. Clearly, this puts impossible deman=
ds upon your ability to focus the optics when the column is really short - w=
ith a 'squashed gasket' centrepiece, even getting the plane of focus inside =
the solution column at all would be miraculous !<BR>
What is even more often overlooked is that with interference optics (althou=
gh not with differential refractometric - or schlieren - optics) there is an=
'aperture' effect, which can only be avoided by focussing precisely on the =
50% plane ! Older literature - about half a century back - does imply that =
this effect causes fringe distortion/blurring rather than displacement, so m=
ay cause noise rather than systematic error, but it would be nice to have so=
me certain knowledge that contemporary approaches to fringe displacement eva=
luation(1) are unaffected by this problem.<BR>
To summarise: if you use short optical pathlength cells to examine concentr=
ated solutions with their inevitably steep refraction gradients(2), then you=
have to accept some significant degree of loss of precision as compared to =
working under more usual conditions. And worryingly, this loss of precision =
will in general be both systematic and r-dependent. But of course the proble=
ms which you are tackling may be adequately error-robust. <BR>
<BR>
Best wishes<BR>
Arthur<BR>
.<BR>
notes:<BR>
(1) there are people out there who will note my restraint in not enlarging =
on this issue !<BR>
(2) errors are refraction </FONT><U>gradient-</U><FONT SIZE=3D"2">, not absol=
ute refraction dependent.<BR>
<BR>
*****************************************************<BR>
Arthur J Rowe<BR>
Professor of Biomolecular Technology<BR>
University of Nottingham<BR>
School of Biological Sciences<BR>
Sutton Bonington<BR>
Leicestershire LE12 5RD UK<BR>
<BR>
Phone/voicemail +44 (0)115 951 6156<BR>
Phone/fax +44 (0)115 951 6156/7<BR>
email arthur.rowe@nottingham.ac.uk<BR>
arthur.rowe@connectfree.co.uk<BR>
Web http://www.nottingham.ac.uk/ncmh/business<BR>
*****************************************************<BR>
<BR>
<BR>
<BR>
</FONT>
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