An optical comparator — also called a profile projector — projects a magnified silhouette of a workpiece through a lens onto a ground-glass screen, where the operator overlays a chart or reads a digital encoder to compare the part against a drawing [S8]. On a 2026 shop floor the equipment is no longer a one-size-fits-all box: floor-standing horizontal units like the Starrett HF600 carry 24 in screens and 12 in X / 8 in Y travel [S3], while the larger HF750 steps to a 30 in screen for stampings that exceed the smaller field of view [S5]. Selection is driven by the workpiece geometry, the ambient conditions around the cell, and how much of the measurement loop will be CNC-controlled versus operator-judged.
Buyers who treat an optical comparator as a single SKU typically end up re-fixturing parts to fit the machine, rather than the other way around. The reliable path is to start from the part population — flat stampings, turned shafts, threaded fasteners, precision industrial valve bodies — and let the orientation, screen, travel, and software fall out of that matrix [S4][S9].
Optical path orientation: vertical for flat parts, horizontal for turned parts
Vertical comparators illuminate the part from below, lay it on a glass stage, and read its silhouette against an overhead screen — the natural fit for flat, stable workpieces such as gaskets, gears, and stamped sheet metal that need no fixturing beyond a flat rest [S4]. A QVP400-class vertical unit is the workhorse configuration for this case because the operator does not have to clamp a rolling cylinder on an inclined surface.
Horizontal comparators send light through the workpiece from one side and project the silhouette onto a vertical screen, which lets the operator mount a shaft, screw, or heavy cylindrical part between centers or in a V-block — geometries that are unsafe to balance on a vertical stage [S4]. Starrett's HF600 and HF750 are both horizontal floor-standing units, with the 30 in HF750 reserved for parts too long for the 24 in HF600 to project in a single field [S3][S5]. A turned servo motor shaft, for example, is mounted between centers on the horizontal unit and indexed with the Q-axis digital protractor to 1 arc-minute resolution [S3].
Screen diameter, lens magnification, and the visible-field math
Optical comparators ship with screen diameters from 12 in to 32 in, and the visible field at any given lens follows a strict ratio: visible field = screen diameter ÷ lens magnification, so a 16 in screen with a 10X lens shows 1.6 in of the workpiece [S9]. Standard lens sets typically run 5X, 10X, 20X, 25X, 50X, and 100X, and a four-position lens turret (as fitted on the HF600) lets the operator step magnification without re-collimating [S3].
The practical rule of thumb is to keep the projected image within roughly 1 in of the screen edge so the overlay chart or digital reticle does not clip the silhouette at the bezel [S9]. If the part is longer than that envelope at the lowest useful magnification, the buyer should step up to the next screen size rather than pay for a longer-travel stage they will not use.
Stage travel, fixturing, and workpiece weight
Stage travel on the HF600 and HF750 horizontal units is 12 in X by 8 in Y as standard, with X optionally extended to 16 in for long shafts and stampings [S3][S5]. Motorized Y-axis travel is standard on the HF600, and CNC control is offered as an option for cells that want scripted sweeps across multiple datums rather than hand-cranked jogging [S3]. Stage load rating is the next gate: heavy castings and forged pressure transmitter bodies can exceed the weight capacity of a bench-top unit and force the buyer to a floor-standing configuration with a heavier cast-iron base.
Fixturing determines the rest of the budget. Horizontal cells typically add a V-block set, a center adapter, and a rotary chuck for shaft work; vertical cells add overlay chart holders and vacuum chucks for thin stampings. Skip the fixture line item at quote time and the comparator will be parked beside the cell waiting for a custom soft jaw.
Operator skill, vibration, and other shop-floor failure modes
Optical comparators are operator-sensitive by design: focus position drifts with the individual, and measurement values vary with the skill of the person at the screen, not just the part [S2]. Ambient vibration from nearby milling, grinding, or stamping presses can disrupt the projected edge and add measurement noise, and a heavily shared comparator across a three-shift cell compounds the repeatability problem [S2].
Temperature drift, oil mist, and dust are equally real failure modes in a machine-shop cell rather than a metrology lab [S10]. Buyers in a CNC-heavy cell should specify a canopy and curtain set (standard on the HF600) and locate the unit away from chip-producing stations. Where the cell cannot guarantee a clean, low-vibration envelope, the right answer may be to skip the optical comparator entirely and move to a video or image-dimension system with place-and-press operation (the KEYENCE IM-X family is the cited example), which removes operator focus drift from the loop [S7].
Readout, software, and the CNC upgrade path
Digital readout has three practical tiers: a Q-axis digital protractor to 1 arc-minute for one-off angle checks (standard on the HF600) [S3]; a 2-axis digital readout (X/Y) for manual sweep measurements; and a full CNC software stack — either Quadra-Chek or MetLogix on the Starrett platform — that drives motorized stages and runs programmed measurement routines [S3]. The CNC path matters once a cell exceeds the throughput that manual logging can sustain, because manual logging becomes the bottleneck at high part counts.
Software also governs data export to the cell's PLC supervisory layer or to a customer-facing inspection report. A buyer who plans to feed inspection results back to a flow-line controller should validate the readout's CSV / QDAS export before signing the PO, not after [S10].
Selection matrix at a glance
For flat gaskets, stamped sheet, and thin gears, pick a vertical unit (QVP400-class) on a glass stage with 5X–20X lens coverage and a 14 in–20 in screen [S4][S9]. For turned shafts, threaded fasteners, and cylindrical bodies between centers, pick a horizontal unit — HF600 (24 in screen, 12 × 8 travel) for the mid range, HF750 (30 in screen) for long parts [S3][S5]. For heavy or tall parts, confirm stage load rating before screen diameter. For cells with high operator turnover or heavy shop-floor vibration, plan to evaluate a video-based image dimension system alongside the optical comparator rather than treating the comparator as the only option [S2][S7].
Sourcing, calibration, and what to verify before signing
Optical comparators are sold through specialized metrology distributors — Fox Valley Metrology and Willrich Precision Instruments are the cited channels for the Starrett HF line — and through factory-direct OEM sales for the KEYENCE IM-X line [S3][S5][S7]. The IQS Directory catalog lists the broader vendor set, but selection should be narrowed to vendors who can deliver on-site calibration with a traceable artifact, not just a sales quote [S1].
Before signing, confirm the lens set, the readout / software tier, the CNC option (if any), the fixture kit, the canopy / curtain set, and the on-site calibration schedule in writing. A comparator that arrives without the fixture kit is the single most common cause of a metrology cell standing idle for the first 30 days [S1][S10].
Two signals worth tracking over the next two quarters: (1) Starrett's CNC option take-rate on the HF600 / HF750, which sets the used-market price floor on manual units and is a leading indicator of how fast 2-D vision is displacing silhouette projection; (2) the IQS Directory vendor count, which has compressed in 2024–2026 as vision-based systems have absorbed the lower end of the comparator market [S1][S7].