Telecentric Scanning Lenses
Telecentric scanning lenses are a special configuration in which the arrangement of optics are designed to focus
down the beam such that it is always perpendicular to the flat field (Figure 10.41). This is accomplished by
ensuring that the system ‘stop’ is located at the front focal point of the lens system. The ‘stop’ is located at the
position where the beam is deflected from the axis.
In a single-axis scanning system, this location is at the scanning mirror. For two-axis scanning, the stop
is mid-way between the mirrors. In the latter case, since the beam is actually deflected at the mirrors,
which are either side of the stop, the focused beam is not quite perpendicular to the flat field. Typically it
is a few degrees to the perpendicular.
The advantages of using a telecentric scanning lens are they avoid beam ovality that could affect very
critical marking applications and drilling holes through a material will not produce an inclined hole. The
typical practical example of the latter is drilling via holes in printed circuit boards.
Telecentric scanning lenses are always multi-element designs supplied in a housing. It should be obvious
that at least one lens element will be larger than the field size to be scanned. In practice, this means that
for reasons of manufacture and cost, only small field sizes are possible. This in turn implies short focal
lengths. Most telecentric lenses will also tend to be customer specific designs.
General technical specifications
Construction: All lens elements are made from laser grade ZnSe.
Housings are made of black-anodised aluminium.
Coatings: The standard coating is a low absorption anti-reflection (AR) for 10.6mm,
applied to all lens surfaces. Reflectance is less than 0.25% per surface. Other
wavelengths are possible on request.
Mounting: Usually this will be a thread, the size of which will be partly determined by the
Window: Since the lens elements are large and expensive, and working distances will be
similar to the (short) focal length, the last lens surface can be prone to coating
with process debris. As a result, many customers opt for a protection window
as the last element. Please note, however, that this should not been seen as
an alternative to debris extraction.
Housings can be designed to take a window even if the window itself is not
fitted. The difference in performance with and without the window is not usually
significant, but a shift in focus does occur.
The following details are necessary to attempt a telecentric design (see figure10.42):
1.) Scanned field size and whether square or circular (2Y).
2.) Focused spot diameter (d).
3.) Incident beam 1/e2 diameter (D).
4.) Maximum optical field angle, equal to double the mirror tilt movement (q).
5.) Mirror spacing.
6.) Clearance (minimum) spacing between the Y mirror and the lens housing (to avoid
7.) Wavelength (l).
8.) Laser beam M2 factor.
9.) Laser power.
Note that not all the above are independent but are related to the focal length, F, in a diffraction limited
F = Y/q = pdD/4M2l
The first part of the equation above implies the ‘F-theta’ characteristic whereby the position of the
focused spot, Y, is given by the Fq rather than the distortion free relation of F.tanq. Whenever possible,
the F-Theta characteristic will also be a target in designs, but sometimes it can lead to sacrificing
focused spot diameter.