How does Depth of Field affect Telecentric Lenses?

It is a general misunderstanding that the telecentric lenses innately enjoy a preponderant depth of field than the usual conventional lenses. While depth of field is still conclusively governed by the wavelength and f/# of the lens, it is true that the telecentric lenses can have a greater adaptable depth of field than the conventional lenses due to the symmetrical glaring on either side of best focus. As the part under examination shifts toward or away from the lens, it will follow the angular field of view (or the main ray) that is correlated with it. In a non-telecentric lens, when an object is shifted in and out of focus, the part blurs asymmetrically due to the prolix and the magnification change that is correlated with its angular field of view. However, telecentric Lenses, blur symmetrically since there is no angular component to the field of view. In practice, this means that features such as edges possess their center of mass location; a judicious measurement can still be made when the object is beyond best focus as long as the variation remains high enough for the contrivance being used by the machine vision system to work properly.

While it may seem counterintuitive, blur can be used advantageously in certain applications with Telecentric Lenses. For example, if a machine vision system needs to find the center location of a pin, as shown in Figure 3a, the transition from white to black is quite sharp when the lens is in focus. In Figure 3b, the same pin is shown slightly defocused.

The Same Pin Imaged both In and Out of Focus

Figure 3: The Same Pin Imaged both In and Out of Focus. Note that the transition from white to black covers many more pixels when the lens is slightly out of focus (b). This can be advantageous!

Looking at a plot of the image grey levels from a line profile taken across the edge of the part, as in Figure 4, the slope of the line is much shallower for the slightly defocused image, as the pin edge is spread over more pixels. Due to the symmetric blurring of the Telecentric Lens, this blur is still usable as the centroid has not moved and the amount of sub-pixel interpolation needed is decreased. This reduces sensitivity to grey level fluctuations caused by sensor noise and allows the pin center location to be found more reliably and with higher repeatability.

Plot showing the difference in Slope between a Focused and Defocused Edge

Figure 4: Plot showing the difference in Slope between a Focused and Defocused Edge where the defocused edge takes up more pixels; searching the edge becomes easier without relying on sub-pixel interpolation.

 Article is posted by Opticsforhire.com –  An Optical Design Consultant.

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