What is a Brewer
The Brewer MkIII Spectrophotometer is a sophisticated optical instrument. It provides near-simultaneous observations of the Total Ozone Column (TOC) and Sulphur Dioxide (SO2) between the instrument and the sun. It can also make high resolution spectral measurements of the ultraviolet (UV) radiation in the direct sun beam or from the whole sky.
The Brewer makes measurements by a combination of firmware running autonomously within the spectrophotometer and software running on a computer that must be connected to the Brewer. When the Brewer has been installed and levelled; the latitude, longitude, altitude, date and time are entered via the software. An algorithm calculates the position of the sun and the Brewer moves to point in that direction. After alignment is optimized the Brewer will automatically point at the sun throughout the day when it is above the horizon.
The spectrophotometer is mounted onto the azimuth tracker, which has a stepping motor and drive system controlled by the firmware. The tracker has a very rugged and stable tripod stand with adjustable feet for levelling.
The direct sun is viewed through a flat quartz window. A stepping motor controlled by the firmware moves the zenith prism so that the direct radiation from the sun passes through the fore-optics to the spectrometers.
The spectrophotometer contains two modified Ebert f/6 spectrometers in series. This reduces the unwanted ‘stray’ light by a factor of 1000 compared to a single spectrometer. Each utilises a 3600 line/mm holographic diffraction grating operated in the first order.
The zenith prism
The zenith prism can also be rotated towards a prism to view the global UV radiation through a diffuser and quartz dome.
The third function of the zenith prism is to look at the Tungsten Halogen ‘standard’ lamp to check the stability of the measurement chain, or the Mercury lamp to check the wavelength settings of the two spectrometers.
Along with the zenith prism and lamps, the fore-optics contain lenses, a viewing port to check the alignment on the sun, a number of filters that ensure that the detector is operating in its optimum intensity range, and a motor-driven adjustable iris. A window in the instrument cover allows the filter and iris positions and the sun position to be seen.
Light from the sun, sky or lamps is directed by the fore-optics into the upper spectrometer through a slit. The upper spectrometer disperses the incoming light into a ‘fan’ of ultraviolet radiation of wavelengths from 286.5 nm to 363 nm (nanometers, 10-9 m). The main components of the spectrometer are a spherical mirror and a holographic grating, the angle of which is accurately controlled using a high precision micrometer driven by a stepping motor.
Changing the angle of the grating selects a narrow band of the ‘fan’ of UV radiation and this is directed to the lower spectrometer. The grating of the lower spectrometer is moved by a micrometer and motor in synchronism with the upper unit, so that only the selected waveband of radiation is re-combined and directed, via an exit slit, to the specially made UV-enhanced photomultiplier tube (PMT) detector. The PMT and its associated electronics count the photons of UV radiation.
It is this synchronised dual-spectrometer system that reduces the unwanted ‘stray’ light by three orders of magnitude compared to a single spectrometer of similar type, enabling much improved performance at low solar elevations (high zenith angles), which is very important in polar regions, and also at short wavelengths in the UVB region of the spectrum.
The correct wavelength setting of the two gratings is checked using the Mercury lamp. The overall response of the complete optical system and the PMT is checked using the Tungsten Halogen standard lamp.
Brewers have a unique design of spectrometer that is self-compensating for the expansion and contraction of components caused by changes in temperature. This means that it can be used around the world outdoors without the need for complex temperature stabilisation.
It is critical that the spectrophotometer stays dry and that no condensation forms on the optical surfaces. To ensure this, the Brewer has number of humidity monitors and desiccant containers.
In the UV scanning mode the two gratings, GR1 and GR2, are moved simultaneously to change the wavelength of radiation reaching the detector. When measuring Ozone, Sulphur Dioxide and Aerosol Optical Depth the gratings are fixed and six wavelengths are selected using a motor controlled slit mask, SL1.
For measurement of Ozone, Sulphur Dioxide and Aerosol Optical Depth the absolute value of the irradiance is not important, if there is sufficient for an acceptable signal-to-noise ratio. It is the ratio of the signals at each of the 6 slit mask wavelengths that is used to calculate the values.
From the spectral scan information UVA, UVB, UVE (Erythemally weighted) irradiances and UVI (Index) can be calculated. For UV measurements the absolute sensitivity is critical and this must be periodically calibrated using a traceable UV radiation source and can be regularly checked using the extra cost accessory UV Stability Kit.
The electronics assembly carries a large main board that runs the firmware and controls the background Brewer functions. Apart from the internal response and wavelength stability checks there are a large number of diagnostic and performance tests that can be run to ensure optimum operation of the Brewer. It also responds to and carries out commands from the operating software running on a connected computer, and sends measurement and diagnostic data to the computer for processing and storage.
The Spectrophotometer has an AC-to-DC main power supply that provides various voltages to the electronics, an internal fan to ensure even temperature distribution and a heater that switches on at low temperatures to avoid excessive load on the motors.
At the rear of the electronics assembly is a board with high stability constant current supplies for the Tungsten Halogen standard lamp and the Mercury lamp, and a module with the amplifier and high stability, high voltage power supply for the PMT detector.