Optical comparators
Profile-projector illumination, screen and stage measurement, calibration, and application limits.
Tags: inspection, dimensional-metrology, optical-measurement
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Measurement principle and scope
A profile projector magnifies a visible contour. Stage coordinates and screen measurements use different measurement chains. KEYENCE: optical-comparator fundamentals
Illustrative screen calculation: assume an image measures 100.0 mm and the applicable magnification is exactly 20×. The corresponding object length is 5.000 mm. A 0.1 mm image-reading change corresponds to 0.005 mm at the object. These assumed numbers demonstrate scaling; they omit magnification error, image quality, reference-scale uncertainty, and operator effects. They do not imply that the instrument can measure to 0.005 mm uncertainty.
Define the required result before choosing the method: an external width in a specified section, an angle between fitted lines, or agreement with an overlay contour. A projected outline alone does not establish the dimensions of hidden features or a complete three-dimensional surface. Record the viewing direction and datum alignment so another operator can reproduce the inspection.
Illumination and the visible feature
| Illumination | Useful observation | Qualification question |
|---|---|---|
| Transmitted or contour | Silhouette against the background | Does the visible boundary represent the drawing feature rather than a burr, chamfer, or overlapping edge? |
| Coaxial surface | Reflected image of a surface feature | Is the required boundary visible with adequate contrast at the selected focus? |
| Oblique surface | Enhanced surface contrast and relief | Does changing the illumination direction move the apparent boundary? |
Mitutoyo's guide distinguishes transmitted contour lighting, coaxial surface lighting through the projection optics, and oblique surface lighting. It specifically cautions that oblique illumination is prone to dimensional measurement error. A visually useful surface image therefore needs its own validated edge-selection method. Mitutoyo: Quick Guide, profile-projector section
Specify illumination mode and settings as part of the inspection method. If different operators choose different visible edges, resolve the feature definition before averaging their results. An automated edge detector also needs a defined applicable contrast, illumination, and feature type; automation does not establish which physical boundary the drawing requires.
Screen magnification and stage accuracy
Screen measurement converts an image length back to object length using magnification. Stage measurement instead observes successive edges against a reference and reads the displacement. Mitutoyo's 2017 bulletin EDU-15002A distinguishes magnification accuracy from X/Y length accuracy and identifies stage squareness as an additional influence when both axes move. Its specifications derived from JIS B 7184:1999 are not universal specifications for every machine. Mitutoyo: Accuracy of Profile Projectors
Higher magnification trades field of view for a larger displayed feature. Mitutoyo gives field diameter as screen diameter divided by lens magnification. Its guide also distinguishes magnification accuracy from measurement accuracy and explains that telecentric imaging can retain image size during axial defocus while the image becomes blurred. Telecentric optics therefore do not remove the need to focus or verify imaging performance. Mitutoyo: Quick Guide
Bounded named example
Mitutoyo's E14005-US catalog, printed August 2025, lists the PJ-H30A2010B, code 303-717-13A, with 200 × 100 mm X/Y travel. The PJ-H30 specifications give axis measuring accuracy as ±(3 + 0.02L) μm, where L is measured length in millimetres, using the JIS B 7184 method. For L = 100 mm, the formula evaluates to ±5 μm; this is an axis specification, not the uncertainty of a finished-part measurement. Mitutoyo: Profile Projector catalog, pages 10 and 16
The same catalog separately lists contour magnification accuracy of ±0.1% or less of nominal magnification, surface magnification accuracy of ±0.15% or less, and standard X/Y counter resolution of 0.001 mm. The PJ-H30 lens table gives a 30.6 mm field with its 10× lens and 3.06 mm with its 100× lens. Preserve those distinctions; finer counter increments or a larger image do not substitute for a measurement uncertainty evaluation. Check the actual model, lens, configuration, and current supplied documentation. Mitutoyo: catalog, page 12 and associated footnote
Calibration and local verification
Calibration must cover the measurement functions actually used. The 2017 Mitutoyo bulletin describes checking X and Y displacement using a suitable glass scale, checking magnification with object-side and screen-side standards at multiple screen locations, and checking squareness with a calibrated square. A center-field magnification check alone does not demonstrate performance across the field. The bulletin also identifies possible checks of screen eccentricity, cross-hairs, and rotating-screen accuracy. Mitutoyo: calibration methods
For local qualification, select representative feature sizes, stage positions, lenses, and illumination modes. Investigate position-dependent discrepancies as possible imaging or stage effects rather than assuming a single scale correction resolves them. Include distortion or field variation in the checks relevant to screen comparison.
Starrett describes calibration after maintenance or adjustment using traceable chrome-on-glass standards and gauge blocks, with laser interferometry available for larger systems. Its guidance relates calibration interval to usage, loading, tolerance demands, and customer requirements. Use that application-dependent approach rather than treating a calendar interval as proof that the machine remains suitable. Starrett: calibration and preventive maintenance
Focus, fixture, and uncertainty
KEYENCE identifies operator-dependent focus as a source of measurement variation and notes that stepped features require focusing at different heights. For a reproducible method, define the surface or edge to focus on, datum construction, sampled locations, and point-selection or fitting strategy. Repeated measurements should include unloading and repositioning when those operations occur in production. KEYENCE: measurement stability and operation
Non-contact observation removes probe contact force, but the fixture can still influence a flexible component. Support it in the specified condition, avoid obscuring the measurement boundary, and evaluate clamping effects. Assess temperature differences and expansion where they matter to the tolerance; a stable readout does not establish thermal equilibrium.
Build an uncertainty model for the chosen method. Relevant contributions can include calibrated stage or magnification performance, reference artifacts, edge localization, focus, repositioning, datum alignment, temperature, and fitting. NIST TN 1297 explains combining standard uncertainty components with covariances where appropriate; expanded uncertainty requires a stated coverage factor. A manufacturer error limit is not automatically a standard uncertainty to insert unchanged into that calculation. NIST: combined standard uncertainty, NIST: uncertainty guidance
Report the measured feature, method, instrument/lens, illumination, fixture and datum setup, relevant calibration status, result, and required uncertainty and acceptance rule. This makes the result usable independently of the projected image.