Optical Table Guide

Optical table tops and supports can be selected and ordered separately as stand-alone items, or they can be ordered as frame based combined systems.

An optical table is a stiff platform supporting vibration sensitive equipment. The most critical vibration characteristic of an optical table is its resonances. This is because the table is designed to be as stiff as possible and acts as a rigid body when its resonances are not excited. In other words, when the table is a rigid body there is no dynamic deflection, and the beam alignment on top of the table would not be disturbed. However, a typical optical table has one or sometimes two major resonances and several minor ones. Therefore, damping is needed to reduce the resonance amplitudes and minimize the system deflection when the resonances are excited by ambient vibrations. Three types of damping methodologies are available: Broadband, Tuned Mass, and Active/Hybrid.

If the optical table will be in an environment subject to seismic floor vibrations – for example, from foot and vehicle traffic and building vibrations – then the type of optical table supports should be selected depending on how much vibration isolation is needed by your application. Newport offers pneumatic vibration isolators, passive air mounts and rigid legs to support optical tables.

Table systems were born due to the increasing demand of easier set up, mobility and accessories capabilities. They usually feature a robust frame system with either rigid support or integrated isolators and could be combined with various accessories such as casters, overhead shelves, hip guards, side shelves, laser safety curtains, etc.

Optical table systems can be characterized by compliance curves for the optical table top and transmissibility curves for the isolation provided by the table supports. Taken together, these two plots show the primary performance capabilities of the optical table system. Knowing how to interpret the two graphs is vital when choosing the most appropriate table system for your application.

If our standard optical tables and isolation systems do not quite meet the requirements for your application or environment, please contact us to discuss a custom solution. For over 50 years our extensive design and manufacturing capabilities have delivered customized, damped platforms to numerous academic and industrial customers. Our capabilities include custom shaped granite, honeycomb or Invar structures, non-magnetic honeycomb tables, cleanroom and vacuum compatible structures with either broadband or tuned dampers for optimal stability and vibration reduction. For more information, please see Custom Vibration Isolation Solutions.

Q: What is the typical thermal bending of optical tables and what type of table should I choose if thermal stability is needed for my application?

A: Under localized heat sources (laser, lamp, etc.), all standard optical tables in the market are very likely to bend a little bit (µrads) because the thermal gradient exists between top and bottom skin, which are thermally isolated by air – no matter whether it is an “all-steel symmetrical isotropic” design. If minimum thermal expansion is absolutely needed, the best way is to isolate your thermal source from tables, or choose Super Invar™ as your table material.

Q: What is the difference between vibration damping and isolation? Why do I need isolators when I already have an optical table top?

A: Vibrations transmitted from the floor to the tabletop is handled by isolators. Those floating legs provide isolation to your experiment and eliminate surrounding floor vibrations like building sway, street traffic or even people walking nearby. Damping, on the other hand, targets tabletop and minimizes its resonances caused by the rest of your experiments or environment. This part is handled mainly by tabletop. Combined together, they kill vibrations and make your optical table the cornerstone of your experiment.

Q: Is optical table the heavier the better? Why not use granite as optical table top?

A: Surprisingly, we don’t want our tables to be too heavy. What matters is the stiffness to mass ratio, and we want this ratio to be as large as possible so that the table has a higher stiffness and expanded rigid body zone. Granite is very flat but it is also very heavy. The mass in granite does not contribute to its structural stiffness; therefore granite is not the ideal candidate for optical table top. Instead, honeycomb structure provides less mass and better stiffness and delivers the best vibration control performance.

Q: Why use composite wood for side panels? Isn’t wood more vulnerable compared to steel in terms of environmental instability?

A: Side panels and edge finish for optical tables do more than just covers. They should contribute to the overall damping performance of the table. The advantage of steel is that it is stiff and provides good environmental stability. The drawbacks are: Like many other hard metals with high elasticity and high density, steel tends to allow vibration or ringing and would resonate with very little natural damping. The wood, on the other hand, is very good at natural damping and eliminate vibrations. This is why ringing bells are always made out of steel/metal and high end hi-fi speakers that require acoustic damping are almost all enclosed by composite wood. But wood is not without any drawbacks – it tends to be more vulnerable than steel under difficult environmental factors. Considering the pros and cons from each side and the fact that optical tables are generally used in house and under controlled environment, Newport decides to use composite wood with moisture protective paint layers which provides superior damping and prevent the side panels from introducing vibrations to the tabletop.

Q: What is the proper way to compare the performances of different optical tables?

A: Optical tables are not complex structures. Modes of rectangular plate-like structures like optical tables had been discussed and depicted in numerous handbooks and textbooks. The most direct way is to compare the compliance curves. Compliance test is the vibration control industry standard to test the vibration damping characteristics of a structure; it is well documented and has been used for decades. The test is done on all corners of the table as those tend to have the highest level of vibration.

Other tests such as non-contact laser scanning vibrometry (LSV) is also sometimes performed. However, LSV test could lead to erroneous conclusions if the test is not staged properly. It requires the laser head to be calibrated and mounted on a tall structure in order to capture the entire table surface. Moreover, it requires a calibrated reference signal for excitation in order to get meaningful data. It is used widely for testing complicated miniature structures, instead of a well-defined large structure such as optical table.