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What is a Resonant Scanner?

Technology of SP8 confocal microscope

A resonant scanner is a type of galvanometric mirror scanner that allows fast image acquisition with single-point scanning microscopes (true confocal and multiphoton laser scanning). High acquisition speed is required to track fast processes, especially in living samples. It has the additional benefit of providing a better fluorescence signal and reduced photobleaching (1).

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Resonant scan speed

As a resonant scanner will oscillate only at a defined frequency, it consequently lacks position and speed control but offers the highest image acquisition speed. Scanners with frequencies up to 12,000 Hz are currently used in Leica scanning microscopes (Jan. 2019). In bidirectional scanning, this translates to 24,000 lines per second. A frame with 512 lines will need only about 20 milliseconds for recording.

Pixel clock control

In resonance, a scanner can only operate sinusoidally, like a pendulum clock or a child’s swing. The pixel clock must, therefore, have a non-linear synchronization with the scanner’s movement. A smart control (Fig 1) adapts to the scanner’s amplitude and phase. That adaptation allows the full field of view [FOV] (2) to be scanned with a freely tunable scanning resolution; i.e., the pixels per line can be set to any integer number up to 2496.

X2Y scanner design

The Leica resonant scanner employs a three-mirror concept that offers superior homogeneity in illumination compared to classical two-mirror scanners. The tree-mirror concept is known as the “x2y scanner”. This approach solves the general problem of scanner systems with two mirrors which cannot be placed in the correct position (the back focal plane) at the same time without additional relay optics. Usually, deviations are accepted by compromising the homogeneity of illumination (with negative effects on the uniformity of brightness and resolution). A single mirror solution (gimbal/cardanic mount) offers homogeneous illumination, but is restricted to very low scanning speeds.

The x2y scanner illuminates the full FOV homogeneously and allows high scanning speed in both the y- and x-directions.

Fig 1: Animated schematic that illustrates the principle of the x2y scanner concept. The x2y design ensures always correct filling of the entrance pupil of the objective lens and still allows high scan speeds.

The resonant scanner device is composed of the two y-scanning mirrors (standard non-resonance) in combination with a resonant galvanometric scanner for the x-direction. Such a combination allows scans at very high line frequencies, up to 24,000 lines per second (in bidirectional mode) without compromising homogeneity. The y-movement by the two low-inertia galvanometric mirrors is fast enough to keep pace with the high line frequencies and that finally results in high frame rates. High frame rates are required to study time-resolved fast processes in living biological samples.