[Imgcif-l] Polarization flashbacks

[Graeme.Winter at Diamond.ac.uk]

Aha,

It seems that the 2000-vintage conversation which moved on to fine points of English spelling was not completely futile, as I now see

_diffrn_radiation.polarizn_source_norm

Name:
'_diffrn_radiation.polarizn_source_norm'
Definition:

        The angle in degrees, as viewed from the specimen, between
               the normal to the polarization plane and the laboratory Y
               axis as defined in the AXIS category.

               Note that this is the angle of polarization of the source
               photons, either directly from a synchrotron beamline or
               from a monochromater.

               This differs from the value of
               _diffrn_radiation.polarisn_norm
               in that _diffrn_radiation.polarisn_norm refers to
               polarization relative to the diffraction plane rather than
               to the laboratory axis system.

               In the case of an unpolarized beam, or a beam with true
               circular polarization, in which no single plane of
               polarization can be determined, the plane should be taken
               as the XZ plane and the angle as 0.

               See _diffrn_radiation.polarizn_source_ratio.

In the updated dictionary. 

Sorted, I believe the expression is.

Cheerio,

Graeme



-----Original Message-----
From: imgcif-l-bounces at iucr.org [mailto:imgcif-l-bounces at iucr.org] On Behalf Of Graeme.Winter at Diamond.ac.uk
Sent: 18 July 2011 11:23
To: imgcif-l at iucr.org
Subject: [Imgcif-l] Polarization flashbacks

Hi Folks,

As a part of my ongoing struggle to make sense of imgCIF I have found my way to the definitions of polarization. Looking at some decade-old imgCIF-l discussions it seems that the definitions were chewed over in the last millennium and have not really changed a lot since.

However as was discussed before the definitions are defined in terms of the incoming and diffracted rays, which is poorly defined if we have several diffracted rays.

Not wishing to get an argument going about the right  / wrong way to express the fraction, the polarization plane does need to be defined in a useful way which depends on the agreed reference frame i.e. the principle scan axis and beam - in the case of a well aligned synchrotron I guess the plane normal would be approximately coincident with Y however there is no way to write this down.

At least, none that I could see in the big book.

Is there one?

Many thanks,

Graeme

Dr. Graeme Winter
Senior Software Scientist
Diamond Light Source

+44 1235 778091 (work)
+44 7786 662784 (work mobile)





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Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message.
Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom