Lesson 4 – Height Gages and Sag Measurement

What You Will Learn

• Explain how height gages are used for thickness, height, depth, and sag measurements in precision optics manufacturing.
• Set, verify, and maintain an accurate zero reference before taking measurements.
• Measure center thickness and sag as controlled optical dimensions.
• Identify common setup and operator errors that reduce measurement repeatability and accuracy.
• Apply good shop-floor practices to improve consistency and measurement reliability.

Lesson Video

Video Placeholder: Insert the narrated lesson video for this metrology topic here.

Height Gage Purpose

A precision height gage is a versatile dimensional measurement instrument used throughout optics manufacturing and inspection. Depending on the probe style and setup, a height gage can measure height, thickness, depth, step height, and surface sag. Height gages are commonly found in both manufacturing and quality laboratories because they provide excellent resolution over a relatively large measurement range.

In optics manufacturing, height gages are frequently used to measure center thickness, edge thickness, and sag values on lenses, mirrors, windows, and other precision components. These measurements help verify that the optic meets both optical and mechanical drawing requirements.

Although modern digital height gages may display extremely fine resolution, accurate measurements depend on far more than the instrument itself. Reliable measurements require a stable surface plate, a clean contact point, proper fixturing, and consistent operator technique. A height gage measurement should always be viewed as a complete measurement system consisting of the instrument, fixture, reference surface, optical component, and technician.

Establishing a Zero Reference

Before any measurement can be made, the height gage must be referenced or zeroed. The zero reference establishes the baseline from which all measurements are taken. Zeroing is typically performed by touching the probe to a certified reference surface, gage block stack, fixture datum, or directly to the surface plate. Because all subsequent measurements depend on this reference, technicians must ensure that the surface plate is clean, the probe is free of damage, and the fixture is properly seated. Even small amounts of debris can alter the zero position and introduce significant error.

Zero should be checked periodically throughout the inspection process, particularly when measuring tight-tolerance optical components. Many technicians verify zero before and after a series of measurements. If the instrument does not return to zero, previous measurements may need to be repeated.

Measuring Center Thickness

Center thickness is one of the most commonly controlled dimensions on optical drawings because it directly influences optical performance, element spacing, and mechanical fit. An incorrect center thickness can change focal length, interfere with assembly, or prevent an optical system from meeting design requirements. When measuring center thickness with a height gage, the optic must be properly supported so it cannot rock, tilt, or shift during measurement. The probe must contact the specified location identified on the drawing. Since optical surfaces are often curved, small changes in contact location can produce different results. A typical measurement sequence begins by cleaning the optic and fixture, verifying the zero reference, positioning the optic securely, and taking multiple measurements to confirm repeatability. Consistent readings indicate that the setup is stable and that the measurement is reliable.

Sag Measurement

Sag, or sagitta, is the height difference between a curved optical surface and a reference plane. Sag is a critical optical dimension because it directly relates to surface curvature and radius. Throughout the manufacturing process, sag measurements are frequently used to monitor grinding and polishing progress, verify generated geometry, and support process adjustments. Sag measurements are often very small and tightly controlled. Because of this, cleanliness becomes extremely important. Dust, polishing residue, fingerprints, or debris trapped beneath the optic can significantly affect the measured value. Likewise, a worn probe tip or damaged fixture can introduce measurement errors.

Technicians should carefully inspect and clean all contact surfaces before making sag measurements. Repeating the measurement several times helps confirm that the optic is properly seated and that the result is repeatable.

Sources of Measurement Error

Height gage measurements can appear highly repeatable while still being inaccurate if systematic errors exist within the setup. Common sources of error include improper zeroing, dirty fixtures, incorrect probe placement, unstable part support, excessive measuring force, or failure to follow drawing requirements. Temperature can also influence measurements. Optical components, fixtures, and measuring equipment expand and contract with temperature changes. For critical measurements, the optic and measurement equipment should be allowed to stabilize in the inspection environment before measurements are taken. Whenever measurements differ significantly from expected values, technicians should stop and investigate the setup rather than simply averaging inconsistent readings.

Shop Floor Practice

Successful optical metrology requires selecting the correct measurement method for the specified tolerance and documenting results in a manner that another technician can reproduce. Before beginning any inspection, technicians should review the engineering drawing, identify the feature being measured, verify the required tolerance, and confirm that the selected instrument is capable of meeting the measurement requirement. Throughout the measurement process, technicians should evaluate whether cleanliness, fixturing, operator technique, environmental conditions, or instrument condition could influence the result. Well-documented and repeatable measurement practices help ensure consistency across shifts, operators, and facilities.

Knowledge Check

1. What dimensions can a height gage measure?
2. Why must the zero position be checked repeatedly during inspection?
3. How does sag relate to optical geometry?
4. Name two setup issues that can affect height gage readings.
5. Why is repeatability important when measuring center thickness?
6. How can contamination affect sag measurements?