TZM-N: Manual telecentric zoom
Why do we need telecentric optics at all? Or rather, when should telecentric optics be used? Basically, it is used to measure different heights within an image, to avoid distortions, i.e. incorrect measurements in the peripheral area of an optic, and to measure grinding wheels or parts with similar geometry.
As with the standard ratchet zoom systems, the telecentric 12x zoom has 14 ratchet steps that can be calibrated in the Metric measuring software. The working distance (lens end to object) of approx. 180 mm does not have to be changed. The zoom range covers a field of view from 4 mm to 50 mm when combined with a 1 1/8" camera chip. The object depth of field varies in this range from 1.3 mm to 38.8 mm. The manual or motorised systems are often used for upgrading grinding machines or dressing machines.
Of course, these high-quality optical systems are not only intended for installation in machines. We can supply the manual and motorised telecentric zoom systems with tripods on aluminium or granite bases, as well as in combination with positioning or cross measuring stages. These in turn are available as manual or motorised versions. So there is a wide range of possibilities.
The scope of delivery includes the manual telecentric zoom optics, Metric MT measuring software, calibration disc with DAkkS protocol, a USB 2.0 camera with 1280 x 1024 pixels as well as a 5 metre USB cable and protective housing for the USB camera. The optics have an additional, replaceable protective glass for the optics. To prevent abrasive dust from entering the inside of the optics, a cover sleeve is mounted over the zoom adjustment (movable). On request, we can provide stepfiles with the external dimensions if you wish to manufacture a fixture to hold the system.
Zeichnung - Drawing - TZM-N
Are you still interested in a little theory? Then we have some basic information on telecentricity for you here:
Why do we need telecentric lenses in optical measuring instruments? Imagine a plate with a number of holes where all twelve holes are visible within the field of view of an objective lens. With a standard optical system (entocentric optical system) we would experience perspective distortion as shown in the illustration at the left (1).
Since with telecentric optical systems the opening angle is zero degrees within a certain range, chief rays are absolutely parallel in the ideal case and the image is depicted without any perspective error i.e. the chief rays are shown parallel to one another. Expressed in another manner: The telecentric optical system reconstructs the image perpendicularly at the edge of the image as well as in the middle, or: The optical system “looks” into the holes at the edge absolutely straight instead of at an angle.
A further, very important reason is that telecentric lenses offer the same reproduction scale even in the Z direction (axial) within a defined range. Imagine a plate equipped with objects of different heights to be measured within one field of vision. The left the illustration below (2) shows that the three objects with different heights exhibit different sizes with a standard objective lens. The model at the right shows the result with a telecentric lens. Expressed in simple terms, this means, that lenses which are sometimes closer and sometimes further away do not indicate any difference in the distance measured. However this is true only as long as the areas are in the telecentric range, which, for its part, is located within the depth-of-field range, however is not identical to it. Telecentric lenses are also interesting when parts are fed to the optical system on belts and the positioning is not precisely reproducible. Here the differences in height are again compensated in the telecentric range. Telecentric optics are also essential for measuring holes (top/bottom).
Object space telecentricity
Object space telecentricity is used to depict objects without perspective distortion. The entrance pupil is located at infinity, so that the chief rays in the object space are parallel to the optical axis. For this reason the front lens must be at least as large as the object to be represented. A further property of this this optical beam is that the image scale does not change when the object is shifted axially. The image always appears the same size regardless of the distance to the object. However it does go out of focus when the object is located outside the ideal object plane. This characteristic is used in measurement lenses to allow a certain tolerance range for the specimen. The tolerable distance range is determined by the depth of field and is specified in the data sheets. With microscopes the constant image scale allows easy focus. On the object side the telecentric optical beam can be realized most simply by a simple convex lens with aperture diaphragm in the focal plane in the image space.
Image space telecentricity
Image space telecentricity services primarily to ensure that the chief rays are parallel to one another. It is used, among other applications, for digital camera lenses to prevent pixel vignetting. The exit pupil is located at infinity, so that the chief rays all strike the image plate perpendicularly. The simplest design consists of a single convex lens with aperture diaphragm in the focal plane in the object space.
Double telecentricity
Double telecentricity is a combination of object-space and image-space telecentricity. Such lenses are used particularly for measuring technology, however, they are also used in photo-lithographic production processes. The entrance and exit pupils are located at infinity, so that the system is afocal. In contrast to purely object-space telecentricity, the tolerable object position is not limited here by the depth of field. It is possible to refocus the image plate without changing the size of the image. The simplest design for this consists of two convex lenses with an aperture diaphragm located in between. The distance between a lens and the aperture diaphragm must be equal to the specific focal length. Theoretically a double telecentric lens has no aberration such as distortion. (Source: Gottfried Schröder: Technische Optik [Technical Optics], Vogel-Verlag Würzburg 1977, ISBN 3-8023-0067-X)
Telecentric lighting
Telecentric lighting is a special form of focussed lighting with strong directional properties. This application is accomplished almost exclusively with transmitted light. A light source (usually LED) of known, small illumination aperture is positioned in the focal plane of the light’s optical system. The result: Parallel chief rays. Telecentric lighting is not parallel lighting (defined aperture). This makes it considerably less sensitive to vibration or maladjustment.
Telecentric lights supply a very homogeneous, high contrast illumination of the field of vision. It is always necessary to use it in combination with telecentric lenses, because with an entocentric lens, the light source appears to be located at infinity due to the parallel chief rays. Blue is used primarily as the light source wave length (maximum accuracy) due to the minimal diffraction. The highly directional properties of telecentric lighting allow it to suppress extraneous light well.
Application
In combination with telecentric lenses wherever bright, high contrast illumination is required and it where it is necessary to precisely recognize or measure objects, which are difficult to handle optically. The distinguished preferred direction of the light rays requires exact focussing. For this reason telecentric lights require a solid, adjustable mount. Calibration of the light aperture and image aperture is the primary factor determining the position of the edge location when using telecentric components. The products below are all object-space telecentric optical elements.
