Open and Shut

Typical lab applications involving shutters include laser pulse picking and modulation, x-ray switching, video exposure control, microscopy, and more. In choosing a shutter for microscopy applications such as epi-fluorescence excitation, fluorescence imaging, and bright-field microscopy, you should consider several parameters.

Fluorescence microscopy often involves controlling the high-intensity UV exposure of the sample under test to avoid overexposure. For the most part, the mercury lamps generally used for such experiments cannot be directly controlled to provide this light burst since turning the lamp on and off will cause it to destabilize. Many industry microscopes either do not have internal shutters or, if they do, they often lack the switching speed required for some applications. External shutters provide an option for exposure control.

An external shutter must be able to completely transmit or attenuate the illumination beam without damage to the switching mechanism (the shutter blades) and be easily installed between the lamp house and the microscope. These are some basic requirements. Other parameters include switching speed, aperture size, coating, and damping. If space constraints are a concern, you may choose to use an iris-type shutter, which will allow you to place an equivalent-size aperture in a smaller area. The tradeoff here is generally activation speed. All else being equal, if speed is of concern in a particular application and size is not an issue, choose a dual-bladed device.

To specify a shutter, first determine the aperture size and the minimum switching speed required. For an external 25-mm shutter, the typical transfer time (minimum time required to transfer from close to open) is 5 ms; for a 35-mm device, the transfer time rises to 13 ms. You may use a smaller aperture to gain speed, or look for a crystal type or another type of device that can provide the speed you require at a loss of transmittance.

Next, choose a coating for the shutter's blades. Most mercury-lamp fluorescence applications require a broad-spectrum reflective surface that offers a higher reflectivity in the UV region of the spectrum, such as an aluminum-magnesium-fluoride coating. This surface reflects source light off of the blade surface and keeps the absorbed light to a minimum. For visible-light applications using halogen or xenon lamps, an aluminum-silicon-oxide coating is also available. For IR applications, gold-magnesium fluoride is sometimes used, as well as silver-magnesium fluoride.

Many applications require an upgrade to the shutter's damping mechanism to prevent the heat produced by the lamp from causing the shutter to close prematurely (losing capture). A high-temperature damping system modification allows these devices to operate in the temperature range produced by some lamps. An air gap allows airflow around the shutter's reflective blades and also reduces heating of the shutter body by reducing conduction through the mounting system. To finish specifying, choose the correct mounting system for the specific type of microscope.

A shutter controller provides the electrical interface necessary to operate the shutter and provide a signal interface to the user's computer or control system. Signals from the computer or control system can be applied via transistor-transistor logic, low-voltage differential signaling, or serial communications. In most microscopy applications, the computer system controls the timing, so built-in timing may not be necessary. For applications requiring testing or manual exposure control operations (as well as computer control), the best choices are devices that contain built-in selectable timing.

One desire by most users of shutter equipment that require devices of this nature is to keep the integration and learning curve short. Keeping the system not only easy to integrate but also easy to use is a paramount design goal. oe