Optical Comparator

What is an Optical Comparator

An optical comparator applies the principles of optics to the inspection of manufactured parts. A collimated light source illuminates a workpiece held on a stage, the part casts a shadow, and a projection lens magnifies that shadow and throws it onto a large ground-glass screen. On the screen the operator reads the part outline against crosshairs, a graduated protractor scale, or a transparent overlay chart that carries the nominal profile and its tolerance band. Because the part is measured as a backlit silhouette, the instrument is also called a shadowgraph, and because the magnified profile is projected for inspection, European and Asian metrology literature calls it a profile projector. All three names describe the same machine.

Structurally a comparator has five subsystems: (1) the illumination unit, a halogen or LED lamp with a condenser that produces either transmitted (contour) or reflected (surface) light; (2) the workstage, a glass or open fixture that holds the part and traverses in X and Y on precision ways carrying glass scales; (3) the telecentric projection lens, interchangeable to set magnification; (4) the mirror train and ground-glass screen, often rotatable through a full circle for angle measurement; and (5) the readout, ranging from a simple chart to a digital readout (DRO) with edge detection. The combination of a non-contact optical front end and a coordinate-measuring stage is what separates a comparator from a simple magnifier.

The instrument has a clear inventor and date. James Hartness, an optical engineer and telescope builder who chaired the United States National Screw-Thread Commission, worked with Russell W. Porter to combine astronomical optics with the practical problem of inspecting screw threads. The resulting Hartness Screw-Thread Comparator, filed in 1925 and granted in 1929, became a profitable product for the Jones and Lamson Machine Company. The comparator solved a problem that micrometers and ring gauges could not: it let an inspector see and measure the full form of a thread, the flank angle and root radius, not just a diameter at one point.

The comparator filled a unique niche between hand gauges and the later coordinate measuring machine. A caliper or micrometer reads one dimension at one location with physical contact. A comparator displays an entire two-dimensional profile at once, without contact, at magnifications from 10X to 100X, so a thread crest, root, and flank are all visible and measurable in a single view. This full-field, non-contact, high-magnification capability is why comparators remain in toolrooms, edged-tool factories, gear shops, and high-volume stamping lines nearly a century after Hartness, even as video measuring machines take over documented variable-data work.

Four engineering metrics determine comparator quality: magnification accuracy, glass-scale resolution, screen size, and stage load capacity. Magnification accuracy governs how faithfully the screen image represents the part. Scale resolution sets the smallest reading. Screen size sets how much of a part is visible at once, which drives inspection speed for overlay-chart work. Stage capacity decides whether a heavy shaft can even be loaded. These four, far more than headline magnification numbers, separate a production-grade comparator from a benchtop demonstrator.

Types: Horizontal vs Vertical

Optical comparators are classified first by the orientation of the optical axis, which dictates how the part is held and what it weighs. The two families are horizontal-beam and vertical-beam, and a third distinction (benchtop versus floor-standing) follows from screen size. Choosing the wrong orientation is the most common selection error, because a part that is natural to support on one layout is awkward or impossible on the other. The table below contrasts the two principal layouts.

Horizontal comparators beam light along a horizontal optical axis and present the screen vertically in front of the operator. Because the part axis lies horizontal, gravity loads the workstage straight down, which suits shafts, fasteners, turned components, and other heavy or long parts that can rest on the stage or be held between centres. The Starrett HB400 horizontal benchtop carries a 50 kg (110 lb) workstage load, and the Mitutoyo PH-3515 carries 45 kg (about 100 lb), figures that a flat vertical stage cannot match. Horizontal machines are the default in screw, gear, and edged-tool factories.

Vertical projectors point the optical axis at a horizontal glass stage, so the part lies flat and the feature of interest is in the top plane. This layout is natural for stampings, gaskets, printed circuit boards, sheet contours, and small flat machined parts where the silhouette to inspect is the outline as seen from above. Vertical models such as the Mitutoyo PJ-A3000, a midsize family with a 300 mm (11.8 in) screen, tend to be more compact and lower cost than horizontal machines of similar screen size, because they do not need the heavy stage and centres a shaft demands.

A secondary split is image orientation. A simple comparator projects an inverted, laterally reversed image, the natural result of the lens and mirror train, which trained operators read without difficulty. Plus-image or erect-image systems add optics to present a correctly oriented image at higher cost and, historically, slightly reduced accuracy. The Mitutoyo PH-3515 is notable for delivering an erect image on a high-brightness screen, which speeds operator orientation on the shop floor. For overlay-chart work the orientation must match the chart, so erect-image machines and erect charts are paired deliberately.

The final practical split is benchtop versus floor-standing, which tracks screen diameter. Benchtop machines with 300 to 400 mm screens serve most toolrooms and quality labs. Floor-standing machines with 500 mm and larger screens, such as the Mitutoyo PV-5110 with its 508 mm forward-tilted screen, exist to show an entire large part, a complete gear or a long stamped contour, in one glance, which is decisive for fast overlay-chart go-no-go inspection in mass production.

Optics and Illumination

The optical heart of a comparator is the telecentric projection lens and the illumination scheme that backs or fronts the part. These two choices set the achievable magnification accuracy and decide which features can be measured at all. The table below compares the mainstream projection lenses and the two illumination modes, with the magnification-accuracy figures published by Mitutoyo for its PH, PJ, and PV projectors.

Telecentric optics are the defining requirement. A telecentric projection lens accepts only rays parallel to the optical axis, so the projected size of a feature stays constant regardless of how far the part sits from the lens within the depth of field. With an ordinary lens, nudging a shaft a few millimetres toward the lens would enlarge its shadow and falsify the diameter reading. Telecentricity is what makes a stated magnification accuracy meaningful, and it is why every reputable comparator and profile projector, vertical or horizontal, uses a telecentric system. Magnification is fixed per lens; operators change magnification by swapping bayonet-mounted lenses, not by zooming.

Magnification accuracy is the deviation between the nominal lens ratio and the actual ratio on the screen, verified with a certified stage micrometer. Mitutoyo specifies plus-or-minus 0.1 percent or better for contour (transmitted) illumination and plus-or-minus 0.15 percent or better for surface (reflected) illumination across its PH, PJ, and PV projectors. On a 10X lens projecting a 1 mm feature, 0.1 percent is a 1 micrometre image error before any operator or scale error is added. The surface figure is looser because a reflected image has a softer edge than a sharply backlit silhouette, so reflected-light readings are inherently less repeatable.

Contour or transmitted illumination, the classic diascopic scheme, lights the part from behind so it appears as a crisp black silhouette against a bright screen. This is the most accurate mode and the reason comparators excel at profile work: the edge between part and background is sharp, so the operator or the edge detector can locate it precisely. Transmitted light only works for the outline of an opaque part or the full body of a translucent one. Surface or reflected illumination, the episcopic scheme, lights the part from the front to reveal top-face features, bore edges, printed marks, and details that have no silhouette. It is essential for many real parts but always less precise than contour light.

The light source has shifted from halogen to LED. Legacy and many current Mitutoyo projectors use a halogen lamp, for example a 24 V, 150 W bulb on the PV-5110, which produces a bright even field but runs hot and has a finite bulb life. Modern machines such as the Starrett HB400 and HE400 use LED profile and surface illumination, which runs cool, lasts far longer, holds a stable colour temperature, and reaches full brightness instantly. LED is the clear trend for new purchases and lowers the consumable and maintenance burden over the machine life.

Screens, Overlay Charts and Standards

The screen is where measurement actually happens, and its size, rotation, and graticule decide how a comparator is used. A larger screen shows more of a part in one view, which is decisive for overlay-chart inspection speed. The screen typically rotates through a full circle and carries a fine protractor scale so the operator can measure angles directly off the projected image. The table below lists screen specifications across representative production comparators.

The overlay chart is the comparator inspection method that has no equivalent on other instruments. Also called a Mylar chart or comparator chart, it is a transparent film printed at the projection magnification with the nominal part profile surrounded by upper and lower tolerance lines. The inspector clips the chart to the screen, projects the part shadow over it, and accepts the part if the entire silhouette falls inside the tolerance band. This gives a full-form go-no-go verdict in seconds with no number ever recorded, which is why charts dominate high-volume inspection of threads, gear teeth, and stamped contours. A chart must match both the magnification and the image orientation of the machine.

The digital readout (DRO) is the variable-data alternative to the chart. Glass linear scales on the X and Y stage ways feed a counter so the operator measures point to point by aligning the screen crosshair to each edge. Resolution is the smallest scale increment: the Starrett HB400 uses glass scales with 0.5 micrometre (20 microinch) resolution on both axes, and the Mitutoyo PV-5110 reads to 0.001 mm. Angle is read either from the rotating screen protractor or a digital Q-axis; Starrett and Mitutoyo both reach 1 arc-minute resolution. The DRO turns the comparator into a true two-axis coordinate measuring device for the projected plane.

Optical edge detection is the most consequential accuracy upgrade. A sensor mounted at the screen detects the light-to-dark transition of a projected edge and triggers the DRO automatically, removing the operator's subjective judgement of where a fuzzy edge sits. This both improves repeatability and lets less-trained operators produce consistent results. Mitutoyo offers it as the OptoEye accessory on the PH, PJ, and PV families, and Starrett offers automatic optical edge detection on the HB400. For any shop collecting variable data for SPC, edge detection is worth specifying from the outset.

Standards govern how a comparator is evaluated and accepted. Within the ASME B89 dimensional-metrology framework, ASME B89.4.18 specifically addresses performance evaluation of video systems and comparators, defining the tests that reveal a machine's sensitivity to environmental and other effects. The broader ISO 10360 series and ASME B89 acceptance principles inform calibration intervals and uncertainty budgets. In practice, a comparator should be verified with certified stage micrometers and gauge blocks traceable to a national standard, on a schedule set by the quality system, typically annually for production machines.

Key Specification Parameters

Reading a comparator datasheet means separating the headline numbers from the figures that actually govern fitness for purpose. A profile-projector spec sheet lists a dozen or more entries, but seven parameters drive nearly every selection decision: magnification accuracy, available lens set, screen diameter and rotation, X-Y stage travel and load, scale resolution, angle resolution, and illumination type. Each is explained below, with verified values from Mitutoyo and Starrett machines.

Magnification accuracy is the primary metrology spec, expressed as a percentage deviation of the screen image from the true part size. Quality machines specify plus-or-minus 0.1 percent or better for contour illumination and plus-or-minus 0.15 percent or better for surface illumination, the figures published across the Mitutoyo PH, PJ, and PV ranges. Treat any comparator that does not state a magnification-accuracy figure, separately for contour and surface light, as unqualified for serious dimensional work.

Available lens set determines the feature sizes you can resolve. A 10X lens is the standard accessory and gives the full-profile overview; optional 5X, 20X, 50X, and 100X lenses extend the range from whole-part viewing down to thread roots and micro-fractures. Because lenses are interchangeable bayonet units, you can buy magnification as your inspection needs grow, but confirm the machine accepts the magnification you will need before purchase.

Screen diameter and rotation set inspection speed and angle capability. Diameters run from 300 mm on a midsize vertical projector to 508 mm on a large floor-standing machine. A bigger screen shows more of the part at once, the single biggest factor in overlay-chart throughput. Full-circle or plus-or-minus 360-degree screen rotation with a fine protractor is what lets the operator read angles directly from the image without any extra fixture.

Stage travel and load decide what parts physically fit. Representative X-Y travels are 250 by 150 mm (10 by 6 in) on the Mitutoyo PH-3515 and 200 by 100 mm on the PV-5110, with the Starrett HB400 offering 400 mm (16 in) X travel and 150 mm (6 in) Y. Load capacity matters most on horizontal machines: 45 kg on the PH-3515 and 50 kg on the HB400. A part heavier than the rated stage load cannot be measured, full stop, so verify mass before screen size.

Scale resolution and angle resolution set the smallest readings. Production comparators use glass linear scales reading to 0.5 micrometre (Starrett HB400) or 0.001 mm (Mitutoyo PV-5110), and digital protractors or Q-axes reading to 1 arc-minute or 0.01 degree. Remember that resolution is not accuracy: the scale resolution is finer than the achievable measurement uncertainty, which is dominated by magnification accuracy and operator or edge-detector edge-finding.

Illumination type closes the spec list. Confirm both contour (transmitted) and surface (reflected) illumination are present if the part has top-face features as well as a profile, and confirm whether the source is halogen, which is bright but hot and consumable, or LED, which is cool, long-lived, and the modern default on machines like the Starrett HB400 and HE400. A few machines add coaxial surface light for measuring inside bores.

Selection Decision Factors

To convert the preceding five chapters into a specific model, follow the decision sequence below. Most selection mistakes come not from one wrong number but from settling secondary details before the part geometry and weight have driven the layout choice. These steps double as an RFQ template.

1. Part geometry and weight first: Decide horizontal versus vertical from the dominant part. Shafts, turned parts, and heavy work demand a horizontal machine with a 45 to 50 kg stage. Flat stampings, gaskets, and boards suit a vertical projector. Confirm the heaviest part is within stage load before anything else.
2. Screen diameter for inspection speed: If the workflow is overlay-chart go-no-go on production volume, size the screen so the whole part fits in one view, which favours 400 to 508 mm screens. If the work is occasional variable-data point measurement, a 300 to 353 mm screen is adequate.
3. Magnification range: Standard 10X covers most profiles. Add 50X or 100X if you must inspect thread roots, small fillets, or micro-fractures; add 5X if parts are large. Verify the body accepts the lenses you will need.
4. Magnification accuracy class: Require plus-or-minus 0.1 percent contour and plus-or-minus 0.15 percent surface as a baseline. Anything looser, or unstated, is unsuitable for documented dimensional inspection.
5. Readout method: Choose overlay charts for fast go-no-go, a glass-scale DRO for variable data, and add optical edge detection (OptoEye, Starrett auto edge) wherever operator-independent repeatability or SPC data is required. Edge detection is the highest-value accuracy upgrade.
6. Illumination and light source: Confirm both contour and surface illumination if the part needs them, and prefer LED over halogen for cool running, long life, and stable output on new purchases.
7. Stage travel and fixturing: Match X-Y travel to the largest feature span, and budget for centres, V-blocks, or custom fixtures, especially on horizontal machines holding shafts between centres.
8. Standards and traceability: Require evaluation consistent with ASME B89.4.18 and a calibration certificate traceable to a national standard, with certified stage micrometers and gauge blocks for in-house verification on the quality-system schedule.

One last commonly overlooked dimension is manufacturer serviceability: availability of overlay charts cut to your part profiles, replacement lamps or LED modules, glass-scale and DRO support, edge-detector calibration, and local field-calibration service. A comparator is a 15 to 25 year asset, so spare-part continuity and chart-making support matter more than headline price. Mitutoyo and Starrett both maintain calibration laboratories and accessory supply networks across major manufacturing regions, which makes them safe choices for production-line installations; video measuring machines from the same vendors are the natural upgrade path when documented variable data eventually dominates the workload.