Kipp & Zonen Brewer Mk III Operator's Manual page 22

Photomultiplier Detector (PMT)
Light passing through the exit slits is collected on the cathode of a low-noise PMT detector [PM 1].
The photon pulses are amplified, discriminated, and divided by 4, before being transmitted to a
counter. The resulting photon count is registered in one of six wavelength channels.
Radiation through the exit slits is focused onto the cathode of the PMT by a 38.1 mm focal-length
quartz Fabry lens [LE 6].
The cathode of the PMT is located at the ultraviolet focus of the Fabry lens. The spacing between
the lens and the PMT takes into account the apparent change in focal length due to the presence
of the filters. The PMT is enclosed in a magnetic shield which is maintained at cathode potential
(approximately -1300 V) to minimize dark noise. The PMT and its shield are spring-loaded against
a PTFE ring which locates the cathode at the center-of-focus of the Fabry lens and also insulates
the magnetic shield from the PMT housing.
The housing has been designed to allow access to the photon amplifying and discriminating
circuitry without disturbing the optics.
Electronics
The Brewer Spectrophotometer contains its own microprocessor board, an 8051 based
microprocessor, which controls all internal instrument operations. Specifically, the microprocessor
is responsible for:
• positioning the diffraction gratings - the microprocessor controls stepper motors which
drive micrometers connected by lever arms to rotating gratings.
• positioning the slit mask to allow light to pass through one of six exit-plane slits to the
photomultiplier: this mask is driven by a second microprocessor-controlled stepper
motor.
• synchronizing the photon-count circuitry with the movement of the slit mask.
• turning the halogen and mercury calibration lamps on and off.
• reading optical position sensors.
• reading analog monitor voltages.
• moving motors to track the sun.
• moving neutral density, diffusing, and polarizing filters into the optical path.
• opening and closing a field-of-view defining iris.
• provides an RS-422C communications link to an external computer.
The microprocessor is programmed to accept commands from the external Computer, execute the
commands, and return results to the Computer. An IBM compatible computer is used as the control
console to facilitate programmed command sequencing as well as automatic data logging and
processing. Raw data is recorded on hard disk drive, and real-time results may be printed on hard
copy or printed to disk for later printing.
The major electronic subsystems of the instrument are:
• Main power supply.
• Main Electronics board - carries control program Flash EPROMs, and a serial
communications interface which runs at 1200 baud (bits per second) and provides the
following functions:
• Input/Output Interface - on/off control of the calibration lamps, drives the wavelengh-
micrometer stepper motor and slit mask stepper motor.
• Photon Counter - accumulates the amplified and scaled photon counts from the Pulse
Amplifier, and transfers these counts to the microprocessor.
• Clock-Calendar - a real-time clock / calendar which, with the RAM, has battery
protection.
• Analog-to-Digital (A/D) conversion - 24 single-ended, 10-bit A/D channels for
monitoring instrument voltages, currents, temperatures, and moisture.
MKIII OPERATOR'S MANUAL
2. SYSTEM DESCRIPTION
19