On conventional horizontal optical comparators, a projected shadow of a component is projected onto a frosted screen and is there compared against a Mylar overlay chart, or using stage movements physical dimensions are measured using cross hairs on the projected image to align the features.
This method with the use of a mechanical overlay on the projection screen works effectively and lateral measurements are made using a digital read-out (DRO) providing a resolution of up to 0.5µm, however comparisons are subjective and can lead to inaccuracies due to the subjective nature of the comparison and change of operator.
A more accurate comparison can be made using a digitised instrument where overlays are replaced by imported data files providing improved accuracy. These instruments can also double up as a vision system making a cost-effective acquisition and rapid return on investment.
An example of this type of system is the HDV Horizontal Digital Video comparator from Starrett, available in the UK from Market Harborough-based
Optimax Imaging and Inspection Ltd, which combines the best features of a horizontal optical comparator and a vision metrology system. It is available with three-stage travel options of 300, 400 or 500mm in the X axis x 150mm in the Y axis and is built on a rigid steel frame design and configured like a traditional horizontal comparator.
However, the system also has a mounted 5MP digital video camera, which replaces the frosted screen. This is coupled with a choice of quick-change telecentric lenses or 6.5:1 zoom for micron-level resolution.
The HDV system includes a powerful 64-bit PC and touchscreen monitor, which runs MetLogix M3 Metrology software. With this software, DXF CAD files can be imported and 2D Go-No-Go gauges created, this is like using a Mylar overlay chart on a comparator screen with greater accuracy. Video edge detection (VED) allows real-time interaction of the imported file with the video image of the part being inspected. As a result, productivity, speed and accuracy are all improved.
The HDV featurs a steel construction with nickel plated stage tooling plate, 540 x 130mm workstage top plate, CNC controls, 150kg maximum load capacity, 75mm of focus travel, Helix angle adjustment with ±15° Vernier scale, X-Y and focus positioning via joystick and trackball positioning, Heidenhain glass scales for 0.5μm X and Y resolution, LED illumination for surface and profile lighting, and a 5 mega-pixel black and white digital video camera (2,448 x 20,58 pixels).
The instrument is available either as a bench top or as a floor-standing version, the bench top model is more compact, making it a better fit for small workshops, laboratories, or environments where space is limited.