The term “aperture” when used in reference to an X/Y scan head, is defined by the diameter of the laser beam, which the scan head can handle over its total specified scan angle. The aperture of a scan head relates to the focused spot size that the scan head can generate for a given objective (or f-theta) lens. Larger apertures can produce smaller focused spot sizes. Smaller apertures produce larger focused spot sizes. The negative consequence of a larger aperture is a slower scan speed as a result of having to use larger mirrors.

And conversely, smaller aperture scan heads can scan faster. A rule of thumb is to double the scan speed for an aperture, which is one half its original size.

The aperture or beam diameter is a measure of the total diameter of the beam including virtually all of the beam’s energy. This is typically the 1/e2 of a Gaussian beam times the square root of 2 (or 1.41). For a 1/e2 beam of 7 millimeters, the full beam diameter is ~9.9 millimeters. A 10mm aperture scan head would be a suitable choice for such a beam.

Therefore, when describing the aperture for a set of X/Y mirrors or a scan head, we refer to the beam diameter. A 10 millimeter mirror set or scan head can handle a 10 millimeter diameter beam through its specified scan angle. Most scan heads are specified to handle a +/- 10 degrees mechanical (+/- 20 degree optical) scan angle. Although some scan heads may have different scan angles.

In the case of a 10mm scan head, specified to scan a +/- 20 degree optical scan angle, the mirrors are designed to hold the 10mm beam on the mirrors as they scan through their total maximum angle. The mirrors in a 10 millimeter scan head are larger than 10 millimeters.

The first mirror that the laser beam strikes in an X/Y scan head is the “X” mirror. The X mirror is designed to hold the full diameter of the laser beam at an incident angle of 45 degrees. The beam forms an ellipse as it strikes the X mirror due to this incident angle. As the mirror scans back and forth through its total scan angle, the ellipse can elongate or become shorter as seen in the flash diagram. The X mirror is usually a little higher than the beam aperture, to accommodate slight misalignment. And the mirror is much wider to accommodate the elongation of the elliptical shape the beam forms on the mirror.

The second mirror encountered by the laser beam is called the “Y” mirror. The Y mirror must accommodate the ellipse formed by the incident angle of the beam coming off the X mirror as it strikes the Y mirror, along with the elongation of that ellipse as the Y mirror scans back and forth. This accounts for the width of the Y mirror. Furthermore, the Y mirror must accommodate the fact that the X mirror “scans” the beam up and down the Y mirror, which accounts for the height of the Y mirror. This can be seen in the flash diagram. Y mirrors are usually larger than the X mirror and typically limit the speed of an X/Y scan head.

In both the X and Y mirrors, the corners are cut off to reduce the total inertia of the mirrors. Any area on the mirror, which does not reflect the laser beam is a candidate for removal. If one studies the areas which the beam imparts on the mirrors, one will recognize that the Y mirror can be cut asymmetrically to further reduce inertia. That is why NTI uses asymmetrical Y mirrors in its larger aperture designs.

For more detailed information about Scan Lenses, click on this link: Scan Lens Theory