Double Telecentric

Image: Double-Sided Telecentric The single telecentric lens is becoming increasingly sought after, both in the world of metrology, and other automated applications where pattern recognition relies on a representative image. Because every point in the field is looked at with the same perspective, there is no "perspective distortion". With proper collimated illumination, slight variations (within the depth of field) of focus will not produce a shift in magnification. The image may get a little "fuzzy", but the magnification won't change. This is most useful where the subjects are varying slightly in distance. The Navitar lenses are supplied with a "fixed" stop, optimized to balance resolution and depth of field. Custom lenses may be ordered with smaller stops to increase the depth of field.

With the single telecentric lens, the rays at the image plane are still coming in at strong angles. If we "double up" on the lenses, we now have telecentric performance front and back. The resulting system is less prone to optical aberrations and geometrical distortions. Image sizes will not vary across the field due to positioning of the sensor.

Modern sensor pixels are both getting smaller, and more cluttered up with electronic circuitry. As light collection is normally a function of active area, manufacturers are adding micro lenses over each pixel to aid in the collection of light. These micro lenses are most effective when the incoming rays are 5 degrees, or less, off normal. The telecentric cone of light is a major benefit in these situations.

The penalty, as with all Telecentrics, is that to keep the telecentric performance, the speed of the lens is relatively slow, therefore a strong, collimated, illumination source is required. The slow lens, coupled with relatively low magnification, limits the final resolution. Also, there is no provision for adjusting magnification, and the working distance must be held to achieve the final numbers.