From: michael jacobsen <jacobsen@biosci.uq.edu.au>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Mon, 24 Jul 1995 19:09:37 +1000

burnt rings, stray light, radial calibration errors, and light-source potentiometer adjustments

Jack Correia wrote some time ago..."We ended up reducing the light intensity
and getting back to 15% variability.  Note this means the LSU, 60000 lamp is
probably not good, too much light, and should be "tuned" down to a more
reasonable intensity level."

There has been much talk of lamp cleaning of late and associated problems
with intensity variations across the cell.

I wish to share one such experience that occurred well over a year ago now
and a remedy that no one has mentioned (at least not directly) nor even may
be aware of.

A routine scan after cleaning of the lamp resulted in a "radial calibration
error" message that persisted for some time such little confidence, if any,
was placed in results during that period even though subsequent absorption
scans appeared quite normal. In attempts to diagnose the problem (a learning
curve for both myself and the local service engineer, Jim Russell) the lamp
incident intensity was checked and found to be in the vicinity of 100,000.
This intensity decreased markedly with usage but was initially thought to be
par for the course. The diagnosis then was that the lamp needed to be
cleaned on a regular basis of the order of once every couple of weeks so as
to maintain a resemblance of constant incident intensity as well as avoiding
errors in radial distance calibration. The naive impression then was that
other users must be experiencing the same "problem".

It is my belief now that incident intensity within this range is "too much"
for the lamp "face" (which demanded, in retrospect, more regular lamp
cleaning steps than usual). Unfortunately with this regime (again in
retrospect) the radial calibration procedure regularly failed to detect the
correct radial reference position in both attempts (no doubt now due to
stray light). In short when the "burnt" ring reappeared the calibration
error disappeared!

Jim Russell (our local Beckman Service Engineer) found and suggested that a
potentiometer be adjusted within the light source supply manufactured for
Beckman as a separate unit ("black box") I believe. This adjustment can be
made using a small screw-driver through an opening in the light-source
supply box (a procedure not recommended to be undertaken by non Beckman
personnel).

Because of this experience I strongly suspect that not all light-source
supplies are factory adjusted to suit anciliary light source components of
the XLA to within the same tolerance when assembled (if only) - an
additional variable effecting light-source incident intensities perhaps not
recognise nor accepted widely?

Perhaps someone else has found the light-source potentiometer adjustment a
means of compomise between frequent lamp cleaning, incident intensity
variations and repeated radial calibration error messages?