Teledyne UA-6, Installation And Operation Manual

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UA-6 Detector
Section 4 Theory of Operation
4-8
4.3 Type 11 Optical Unit
Theory of Operation
This section explains the optical unit theory of operation. To
provide a clear, basic understanding of the optical unit circuitry,
equivalent circuits and mechanical functional diagrams are used
in the following paragraphs. An equivalent circuit is a simple
electronic symbol which is used in place of the detailed circuitry,
but still maintains identical terminal characteristics of the
detailed circuitry. The theory of operation is presented at two
levels in the following paragraphs. The first level provides
general data on the overall optical unit system, and the second
level provides information on specific individual system circuits.
4.3.1 General
The optical unit (Figure 4-3) is an electro-optical transducer that
develops an electrical signal (varying resistance) in response to
changes in sensed light levels. The optical unit consists basically
of a UV light source, flow cells, two reflectors, a reference and
measuring photocell, two filter assemblies, a regulating photocell
assembly, source screen, and a baseline adjusting aperture plate
assembly.
The UV light source is at one focus of an oblate spheroid (ellipse
of revolution) formed by the two reflectors and the source screen
is at the other focus, resulting in almost total capture of radiation
in the system. When the same wavelength is used in both the
MEASURING CELL A and REFERENCE CELL B light paths,
only one source screen is used.
UV light is reflected from the two reflectors and excites both
sides of a light source, which is part of the source screen,
resulting in a light emission spectrum at a certain wavelength
region. Depending upon the source screen used, the emitted light
is either diffused light from the UV lamp, or is fluorescent light
excited by the UV lamp. This light emission spectrum is radiated
from the source screens to the flow cells, through an aperture in
each reflector. Transverse openings in the flow cells provide a
path for the light through an absorbing medium by intersecting
longitudinal openings in the flow cell which provide the path for
the absorbing medium.
The spatial integrating action of the elliptical reflectors and the
translucent diffusing action of the source screen light source area
produce extremely good optical balance between MEASURING
CELL A and REFERENCE CELL B light paths. This results in a
very low recorder noise and drift regardless of the spatial or
angular fluctuations in the light emitted from the UV lamp. This
important feature (patent no. U.S. 3,822,097; 3,859,539; other
patents pending) contributes to the Type 11 optical unit’s high
performance.
Light radiating through the reflector aperture to the reference
flow cell is interrupted by the aperture plate assembly once it has
passed through the flow cell. The aperture plate assembly con-
tains a teardrop-shaped aperture that controls the amount of
light passed on to the filter assembly and photodetector. This pro-
vides a coarse baseline adjustment during calibration proce-
dures.