Microscope Stages

A digital microscope uses a digital camera rather than a traditional eyepiece. An image of the sample is observed and analyzed directly on an electronic monitor display in real time.

Modern biomedical, life science and diagnostic instruments rely on automated digital microscope technologies. DNA sequencers, cell imaging instruments, and digital pathology scanners are just a few of the many applications for digital microscopy solutions. When designing these complex systems, there are a lot of options to consider which involve tradeoffs in performance, throughput, and cost.

The functions of a digital microscope are to control the digital camera, illumination and filter selection. In addition, microscopes require two critical positioning functions. The first is to precisely position the sample on the microscope slide in the XY plane. This can either be done with fast moves between adjacent fields (field sequential imaging), or stable constant velocity motion (scanning imaging). The second key positioning task for a microscope stage in an automated microscope is the focus axis. This moves the objective relative to the sample to maintain critical objective focus at all times.

Digital microscopes are designed to include stages which move a sample or objective in x, y, or z motion. We’ve created a unique microscope stage architecture by embedding high performance precision electronics required to control motion within the microscope stage without increasing its size.

Microscope stages move a sample or objective in x, y, or z motion and are typically available as either single axis, an XY stage, or XYZ stages. Microscope stages are also commonly referred to as XY tables, linear actuators, motorized stages, or linear translation stages.

XY motion is performed by an XY microscope stage, which is typically a thin, low-profile stage with a central opening for the transmissive imaging. XY microscope stages are typically actuated with stepper motors and lead screws, while cutting-edge designs employ a linear motor and a linear encoder on each axis and close a servo feedback loop.

Automated microscope focus axes typically use piezo or direct drive technology. In a microscope stage, direct drive linear motors offer extended range, allowing the objective to be retracted during sample load and unload, together with very high resolution and repeatability. The focus axis position is typically commanded by an autofocus sensor, which derives a focus error signal via a laser diode passing through the objective or by a software autofocus algorithm.

Once the optical imaging elements are selected, the mechanical microscope stage architecture can be finalized. A typical field of view is much smaller than the sample being imaged. Thus, in order to image an entire sample, either the sample or the camera/objective will need to move along two perpendicular axes (XY). In addition, in order to properly resolve the image, the distance between the magnification objective and the camera (or image sensor) needs to be precisely adjusted. This is referred to as the Z axis. The Z axis is typically vertical and motion along it occurs perpendicular to the XY plane to move the sample into the imaging field of view.

The microscope field of view calculation relates the sensor size to the magnification and will determine how many XY movements will be required to image the area of interest on the sample.

There are three common configurations of the mechanical microscope stage motion hardware. Selecting the best one depends upon the complexity of the particular application.

1. The XY microscope stage moves the sample below a Z stage that is moving the objective or camera. This is the most common configuration. The benefit of this approach is that the image becomes stable after motion more quickly because the microscope slide is only moving on one motorized axis. This means it can be mounted to a sturdy structure instead of a stack of three moving axes whose resonances need to damp. During sample loading, the objective can be moved vertically away from the sample mounting area, which makes changing samples easier. Also, Abbe errors are reduced because the overall stack is shorter. To understand the impacts of Abbe errors on stage positioning, see our Abbe error white paper. This method works for regular or inverted microscope configurations.
   
2. Three motion axes move the object being imaged in X, Y and Z directions while the camera and objective remain stationary. In this situation, the camera is very stable because it is mounted to a rigid surface. The drawback is that the travel required for the Z axis may be longer because the objective-to-imaging sensor distance is fixed. Typically, it requires less travel to move the objective with respect to the imaging sensor or camera.
3. The objective or camera is moved in X, Y and Z directions, while the sample remains fixed. This approach works best when imaging small parts such as a microscope slide. As part size increases, the complexity of this approach also increases. Larger travels tend to require two axes to support a beam that has the vertical focus axis with both the imaging sensor and objective mounted to it.