Staging and Measuring Capacity

For the selection of the proper size of optical projector, the screen diameter may be regarded as the most characteristic single dimension. The envelope dimensions of the optical system, and thereby the minimum outside dimensions of the entire instrument, are a function of the screen diameter.

From an application point of view, it is that characteristic dimension, generally the diameter of the round screen that determines the area of the object that can be contained in the screen image at any specific magnification. Although it is quite feasible to move the object and thereby to bring its various sections into the field of view of the system, for many applications it is desirable to have the entire object, or a whole detail contained in the screen image. This is particularly the case for comparison inspection by means of contour charts.

The size of the largest object that can be mounted on the projector stage may also need to be investigated. This is the case when the object has at different locations, details that must be explored by optical projection, or even measured in controlled dimensional relation to each other. Factors that need to be checked in such cases are the following:

a. The working distance of the optical system — the distance of the focal plane from the relay lens. This is the lens behind the object stage, which picks up the image of the part and transmits it to the magnifying lens system. It is also called focal clearance.

b. The throat clearance —the free distance between the light source condenser and the relay lens. For all practical purposes, this is the sum of the condenser clearance, which is the distance between condensing lens and focal plane, and of the working distance.

c. The vertical clearance— the maximum distance between the table top and the lens centerline.

d. The range of coordinate movements is still another significant aspect for evaluating the capacity of optical projectors. It is also called the measuring range, and it is not necessarily identical with the total table travel that, in some models, may exceed the distances of the measuring movements.

The purpose of the coordinate movements is to bring, by means of controlled displacement, distant details of the object into the focal field, and also to establish coincidence between pertinent image elements and screen index lines or chart outlines.

The following are typical capacity data for different sizes of standard models of a well-known make of optical projectors: