Measuring Microscopes and Projectors

The measuring microscope is considered to be the “father” of all coordinate measuring machines. Here, a microscope and the human eye are used as a “sensor” (Fig. 3a). The operator uses a reticle integrated in the beam path to aim at the points of the object being measured. In the simplest case, all one has to do is read off the coordinates indicated on the scales of the measuring table. In modern measuring microscopes, the coordinates are captured and evaluated directly by the connected analysis computers. Today, instruments of this type are used only for simpler, two-dimensional measuring tasks. One of the chief disadvantages of this measuring technique is the fact that the operator directly influences the measured result by visually approaching the measured points (subjective measurement). In addition, the manual execution of complex measurement programs is very costly and quite involved.

Fig. 3: Basic design of the measuring microscope and measuring projector with the Werth Tastauge: a) Measuring microscope; b) Measuring projector; c) Werth Tastauge and projector beam path.

In the case of the profile projector, the instrument’s optics project a correctly scaled image of the workpiece onto a ground glass screen (Fig. 3b). Using an equally scaled drawing, a direct comparison can be made between the imaged workpiece area and the dimensions of the drawing. This procedure might be regarded as the original form of measurement “in the image”. The advantages it offers include fast checking of a wide variety of features and extremely simple operation. Generally speaking, however, only “go/no-go” information can be obtained in this manner. And once again, human judgement influences the measured result. Moreover, the lens system must have an extremely low distortion factor in order to achieve acceptable measuring uncertainties.

Fig. 4: Modern measuring projector (Werth OPTIMUS® 300) with integrated optoelectronic sensor (Werth Tastauge).

The measuring projector (Fig. 4) combines the advantages of the measuring microscope with those of the profile projector. On its ground glass screen, the image can be compared with a drawing just as quickly as with a profile projector. Since it is equipped with scales along its coordinate axes, individual points can be probed with the reticle in the same manner as with the measuring microscope. In the 1970s, Dr. Siegfried Werth developed the “Tastauge” optical edge sensor – the first optoelectronic sensor for measuring projectors designed to enable automatic probing of object points. The Tastauge performed the same function normally performed by the human eye, but only for highly contrasted objects. Introduced in 1980 and combined with a CNC control, this new sensor technology made it possible to automate optical coordinate measuring machines for the first time. Interestingly, the measuring uncertainty of the machines built at that time was in the range of only a few micrometers.