← AskMachinist · Guides · Calculator · 🔎 Search
Optical Comparator Basics
Guide · Updated October 27, 2026
An optical comparator (profile projector) shines light past a part and projects its magnified silhouette on a screen. It is quick for profiles, radii, threads and small features that are hard to reach with contact tools.
Need an exact reference size?
Build any gage block stack with the free Gage Block Calculator. Free, no login.
Step by step
- Pick the magnification: Choose the lens magnification (10×, 20×, 50×) to suit the feature size, and fit the matching overlay chart.
- Fixture the part: Set the part on the stage or in centers, square to the light.
- Focus: Focus until the edges are sharp, with no halo.
- Measure: Use the stage micrometers or DRO to move the edge from one crosshair point to another, or compare the profile to an overlay chart.
- Verify the stage: Measure a gage block to check stage travel.
Tips
- Overlay chart values are only correct at the lens magnification printed on the chart.
- Clean the lens and screen.
- For threads, tilt the light to the helix angle.
Related: checking radius gages, checking chamfers.
Worked example: checking a radius at 20X
At 20X magnification, a 0.050 in radius appears as 1.000 in on the screen. Put a 20X radius chart on the screen, focus on the part edge and line it up with the 0.050 in line. If the edge falls between the 0.048 and 0.052 in lines, the radius is within ±0.002 in. For linear dimensions, use the stage micrometers or DRO, not the screen scale, and touch off on each edge with the crosshair.
Check magnification by measuring a gage block or pin of known size: a 0.2500 in pin should measure 0.2500 in on the DRO when you cross both edges. Use profile (back) lighting for outlines and surface lighting for features on the face. Thread forms are checked with thread overlay charts at the same magnification; make sure the helix angle is set so both flanks are in focus.
Keep the screen and lenses clean and the room lights low enough to see a sharp edge. Fixture parts so the feature is square to the light; a tilted part makes edges look soft and shifts readings.
Quick shop check
Example: at 10× magnification, a 0.010 in deviation on the part appears as 0.100 in on the screen. Use the correct chart for the magnification; using a 20× chart on 10× doubles the error.
FAQ
What is an optical comparator used for?
Checking profiles, radii, angles and thread forms by projecting a magnified silhouette.
Comparator vs microscope?
A comparator projects onto a large screen for quick comparison. A measuring microscope is for very small features.
How do I check accuracy?
Measure gage blocks on the stage. Build sizes free at gageblockcalc.com.
Can it check radius gages?
Yes. Compare the projected profile to a radius overlay.
Related guides and calculators
- How to build a gage block stack
- How to wring gage blocks
- How to choose a gage block set
- Gage block care and wear blocks
- Carbide vs steel vs ceramic gage blocks
- How to check a height gage with gage blocks
- Machinist Christmas gift guide
- Surface finish basics
- All free machinist calculators
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your gage manufacturer's instructions and your quality system.