Keeping Optical Tables Steady Tech
lenge for precision experiments and manufacturing processes in optoelectronics. Optical tables and breadboards in various shapes and forms are the de facto standard of vibration control. They rest on soft isola-tors that effectively filter out vibration from the floor. However, other sources of disturbance – acoustic noise, atmospheric turbulence and onboard sources – still exist and may become prevalent vibration sources at higher frequencies. These disturbances can have a significant negative impact on the optomechanical performance if their spectral contents are close to the resonance frequencies of the platform. For this reason, internal damping, or dissipation of mechanical energy, that mitigates the effect of resonance is very important for high-quality vibration-isolated platforms. Active vibration damping that involves electromechanical sensors, actua-tors and control systems has proved most effective for suppressing resonance vibrations. Recent extension of this technology by Newport with add-on SmartTable ADD active dampers expands the limits of applications beyond standard optical tables to virtually any vibration-isolated platform that can experience unwanted resonance vibrations. The theory of active vibration control predicts that a small number of active dampers placed near antinodes of the main vibration modes suffice to suppress resonance vibration over the whole structure. In the case of a typical rectangular platform, two dampers placed at adjacent corners do the job. This has been confirmed by theoretical analysis and by multiple measurements of the dynamic reaction at remote locations on the table by highly sensitive seismic ac-celerometers. 3 To illustrate this in a more vivid demonstration, researchers at New-port Corporation turned to the powerful experimental means provided by the scanning laser Doppler vibrometer from Polytec Inc. The experiment also promised valuable information about the proper tuning of the active control feedback loops. A custom all-steel optical breadboard was chosen for testing the effect of add-on active dampers (Figure 1). The breadboard, whose size was 4 ft × 5 ft × 4.5 in., was supported by four spring-based vibration isolators, so that the " rigid body " natural frequency of the suspension equaled 5.25 Hz. Two SmartTable ADD dampers were bolted onto the two front corners of the breadboard. To obtain a consistent stationary vibration signal, the breadboard was excited by a stationary random force. A miniature electromagnetic shaker was used for this purpose; it is seen at the far left corner of the breadboard in Figure 1. The resulting vibration of the bread-board did not exceed the levels routinely …
Optical table with embedded active vibration dampers (smart table)
This paper describes the actively damped optical table developed and introduced as a standard product, ST series SmartTable(TM), by Newport Corporation. The active damping system is self-adjusting and robust with respect to changes in payload and vibration environment. It outperforms not only the broadband damped optical tables, but also the top-of-the-line tables equipped with tuned passive vibration absorbers. The maximum resonance vibration amplitudes are reduced about ten times. Additionally, the user has the benefit of being able to monitor and analyze vibration of the table by the conditioned low-noise signals from the embedded vibration sensors. Theoretical background, analysis, design rationale and experimental verification of the system are presented, with emphasis on sensor-actuator pairs architecture, signal processing and adaptive controls.
UV-B radiometry and dosimetry for solar measurements.
Biologically effective irradiance or dose of solar UV radiation was determined using a spectroradiometer, two broadband radiometers and two types of passive UV-dosimeters. The absolute erythema irradiance and the actinic irradiance were calculated from the solar spectrum measured with the spectroradiometer. It was demonstrated that the erythema irradiance is proportional to the actinic irradiance of solar radiation. The erythema irradiance was also determined using the two broadband radiometers which utilize a filter transmitting erythema spectra. Personal UV-dosimeters such as polysulphone and CR-39 were used to determine the erythema dose for a selected period of time. These results were used to estimate the accuracy of the broadband radiometers and UV-dosimeters. It was found that the results obtained from the broadband radiometers deviate from the absolute erythema irradiance by less than 20% during clear days between the hours of 11:00 and 13:00 Eastern Standard Time (EST) in Australia. The assessment of the erythema dose using passive dosimeters such as polysulphone and CR-39 could introduce an error as high as 40% if the calibration was not performed before undertaking experimental measurements.