Why Are Low-Pressure Sodium Lamps Used to Measure Mechanical Seal Flatness?

When you think about low-pressure sodium lamps, street lighting is probably the first application that comes to mind.

Low-pressure sodium (LPS) lamps are famous for their distinctive yellow-orange light and extremely narrow spectral output around the sodium D-lines near 589 nm.

But there is another, much less obvious application: precision flatness measurement.

A recent customer application provided an interesting real-world example. A mechanical seal manufacturer in Chile uses a SPEEDFAM CHECKLITE system for precision flatness inspection of mechanical seal components. According to the customer, the device uses a low-pressure sodium lamp as its monochromatic light source.

This application illustrates one of the most interesting characteristics of LPS technology:

Sometimes the value of a light source is not how much light it produces, but how spectrally specific that light is.

What Is a SPEEDFAM CHECKLITE?

The SPEEDFAM CHECKLITE is a specialized instrument designed to measure the flatness of finished, polished components.

Speedfam describes its Checklite as a monochromatic flatness measurement system capable of measuring flatness down to approximately one light band, or 0.3 micron, when used with a Speedfam optical flat.

The system is not simply shining a bright lamp onto a component. Instead, it uses a combination of:

  • A monochromatic light source
  • An optical flat
  • A highly reflective finished surface
  • Optical interference
  • The resulting light bands

to reveal extremely small deviations from flatness. That is where low-pressure sodium light becomes interesting.

Why Does Flatness Measurement Need Monochromatic Light?

The basic principle is optical interference.

An optical flat is placed gently against the polished surface being tested. There is an extremely small gap between the two surfaces. When monochromatic light is introduced, light reflected from the different surfaces interacts with itself. The result is a pattern of bright and dark interference fringes, often referred to as light bands.

The geometry of those bands provides information about the surface. Speedfam explains that straight, parallel bands indicate a flat surface, while curved or distorted patterns can reveal different forms of deviation, including concavity, convexity, grooves, and edge rounding.

This is a remarkably elegant measurement technique. No complex camera system is necessarily required. The surface itself effectively creates an optical pattern that reveals its own deviation from flatness.

Why Is a Narrow 589 nm Light Source Useful?

This is where low-pressure sodium lamps have a unique advantage.

A low-pressure sodium lamp produces light concentrated around the sodium D-lines at approximately 589 nm. Instead of illuminating the test surface with a broad mixture of wavelengths, LPS provides a highly concentrated spectral source.

For an interference-based measurement, this is extremely useful. If many different wavelengths are present simultaneously, interference patterns can overlap and become more complicated to interpret. With a highly monochromatic source, the resulting interference fringes are much easier to observe and read.

That is one reason monochromatic sodium illumination has remained relevant in precision optical measurement even though LED technology has replaced LPS in many general-lighting applications. The requirement is not simply illumination — it is controlled optical wavelength.

From Light Bands to Microns

The concept is surprisingly simple.

Imagine placing an optical flat on a polished mechanical seal face. If the surface were perfectly flat, the interference bands would appear straight and parallel. If the surface deviates from flatness, the bands bend or change shape.

By examining these patterns, the operator can determine the direction and approximate magnitude of the deviation. Speedfam provides examples showing how different light-band patterns correspond to different surface conditions — allowing an optical phenomenon to become a practical manufacturing measurement.

And the scale involved is tiny. Speedfam specifies its Checklite system for measurements around 0.3 micron per light band. For comparison, 0.3 micron is only 0.0003 mm.

Why Does Flatness Matter for Mechanical Seals?

Mechanical seals depend on extremely close contact between sealing faces. The two primary seal faces must maintain the appropriate geometry and surface condition to prevent unwanted leakage while allowing the seal to operate reliably. That makes flatness a critical manufacturing parameter.

This is not merely theoretical. Sealmatic Chile, a manufacturer of mechanical seals, states that its quality-control process includes precise inspection of seal components and that its primary seal face flatness is maintained within 1–2 helium light bands.

This provides a useful real-world example of why extremely precise flatness measurement matters in mechanical seal manufacturing. The connection is therefore:

Precision machining → polished seal face → optical flat → monochromatic illumination → interference bands → flatness measurement

And somewhere in that chain, a low-pressure sodium lamp can play a surprisingly important role.

Why Not Use a White LED?

Modern LEDs are excellent light sources. They are efficient, compact, instantly controllable, and available in almost any color. But a white LED is fundamentally different from a low-pressure sodium lamp.

A typical white LED system produces a relatively broad spectrum. A low-pressure sodium lamp produces an exceptionally narrow spectral output centered around the sodium D-lines. For ordinary lighting, the broad spectrum of an LED is usually an advantage. For an interference-based optical measurement, however, spectral simplicity can be an advantage.

The question is therefore not: Is LPS better than LED?

It is: Does the measurement require a highly monochromatic light source?

If the answer is yes, an LPS lamp can remain relevant even in a modern precision-manufacturing environment.

A Technology That Survived for a Reason

Low-pressure sodium technology is often considered an old lighting technology. That is certainly true in the context of general illumination — LEDs have replaced LPS in most applications where efficiency, color rendering, instant startup, controllability, and compact size are important.

But specialized measurement systems are different. A technology does not have to be new to be useful. It only needs to have the right physical properties for the application.

In the case of low-pressure sodium, one of those properties is its exceptionally narrow spectral output around 589 nm. That makes LPS interesting not only for specialized illumination, but also for certain optical measurement and inspection systems.

From Street Lighting to Precision Manufacturing

This is one of the more unusual stories in lighting technology. The same basic light source historically used to illuminate roads and highways can also be found in equipment designed to reveal microscopic deviations in the flatness of precision-manufactured components.

The applications could hardly be more different. One is about illuminating a large outdoor area. The other is about producing a controlled optical phenomenon that can reveal deviations measured in fractions of a micron.

Yet both applications take advantage of the same fundamental property: low-pressure sodium produces a highly concentrated, highly predictable wavelength of light. That is why LPS remains relevant in specialized optical applications long after it has disappeared from many mainstream lighting applications.

A Real-World Example: SPEEDFAM CHECKLITE

The SPEEDFAM CHECKLITE is manufactured by Speedfam (India) Pvt. Ltd. and is designed to measure the flatness of polished components using an optical flat and interference light bands. In this particular customer application, the Checklite is used for mechanical seal inspection, with a low-pressure sodium lamp providing the monochromatic illumination.

It is a good example of something that is easy to overlook:

A low-pressure sodium lamp does not always exist to illuminate a space. Sometimes it exists to make a measurement possible.

Frequently Asked Questions

Why are low-pressure sodium lamps used to measure mechanical seal flatness?

Mechanical seal flatness is measured using optical interference — a technique that requires a highly monochromatic light source. Low-pressure sodium lamps produce light concentrated near 589 nm with an extremely narrow spectral output, which creates sharp, easily readable interference fringe patterns when used with an optical flat. Broadband white light sources produce overlapping fringe patterns at multiple wavelengths that are much harder to interpret.

What is a light band in flatness measurement?

A light band is a unit of measurement used in optical flatness inspection. One light band corresponds to half the wavelength of the illuminating light source. For a 589 nm sodium lamp, one light band equals approximately 0.3 micron (0.0003 mm). A flatness specification of 1–2 light bands means the surface deviation is within 0.3–0.6 micron — an extremely tight tolerance used in precision seal manufacturing.

What is the SPEEDFAM CHECKLITE used for?

The SPEEDFAM CHECKLITE is a monochromatic flatness measurement instrument used to inspect the surface flatness of polished precision components. It uses a sodium lamp as its light source and a Speedfam optical flat to generate interference fringe patterns. These patterns are used to assess flatness, identify concavity or convexity, and detect surface irregularities such as grooves or edge rounding.

What is an optical flat and how does it work?

An optical flat is a highly polished, extremely flat reference surface made from glass or fused silica. When placed against a polished test surface and illuminated with monochromatic light, the tiny air gap between the two surfaces produces an optical interference pattern. The shape and spacing of the resulting light bands reveal how closely the test surface matches the reference flatness of the optical flat.

Can a white LED replace a sodium lamp in a SPEEDFAM CHECKLITE or similar system?

A standard white LED cannot directly replace a low-pressure sodium lamp in an interference-based flatness measurement system, because a white LED produces a broad spectrum that creates overlapping fringe patterns at multiple wavelengths. A narrowband 589 nm LED source with an appropriate optical filter could approximate the monochromatic output of an LPS lamp, but the spectral purity and output characteristics would differ. In systems specifically designed around LPS illumination, a direct lamp replacement is usually the most straightforward approach.

What wavelength does a low-pressure sodium lamp produce?

A low-pressure sodium lamp produces light concentrated almost entirely around the sodium D-lines at approximately 589.0 and 589.6 nm. This extremely narrow spectral output — sometimes described as near-monochromatic — is what makes LPS lamps useful for interference-based optical measurement, where spectral purity directly affects the clarity and interpretability of fringe patterns.

How accurate is optical flatness measurement using sodium light?

When used with a precision optical flat, sodium light flatness measurement systems such as the SPEEDFAM CHECKLITE are specified for measurement resolution down to approximately one light band, or 0.3 micron. This makes the technique suitable for verifying surface flatness on precision-machined components such as mechanical seal faces, optical elements, and lapped or polished industrial components.

What types of surface defects can optical flatness inspection reveal?

Optical interference flatness inspection can reveal a range of surface conditions, including overall concavity or convexity, surface grooves or tool marks, edge rounding or rolloff, and localized irregularities in an otherwise flat surface. The specific fringe pattern produced by the interference interaction between the optical flat and the test surface provides visual information about the direction and approximate magnitude of any deviation from flatness.

Where can I source a replacement LPS lamp for a SPEEDFAM CHECKLITE or similar flatness inspection system?

Glow Object® supplies low-pressure sodium lamps — including 35W SOX lamps — and compatible electronic ballasts for optical inspection and flatness measurement applications. If you are maintaining or replacing a monochromatic flatness measurement system that uses an LPS lamp, Glow Object can help identify a suitable replacement lamp and ballast.

Low-Pressure Sodium Lamps for Optical Measurement

Glow Object® supplies low-pressure sodium lamps and ballasts for specialized applications where the unique spectral characteristics of LPS remain useful. Applications include:

  • Optical inspection and precision flatness measurement
  • Interference-based measurement systems
  • Scientific instruments and laboratory use
  • Specialized industrial inspection
  • Semiconductor and precision manufacturing
  • Darkroom and photolithography applications

If you are maintaining or replacing a monochromatic optical inspection system that uses a low-pressure sodium lamp, Glow Object® can help identify a suitable LPS lamp and compatible ballast.

Explore LPS Products →

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Last Updated: September 2026

This article is part of the Low Pressure Sodium (LPS) Lighting Knowledge Hub, a technical resource covering the physics, engineering, and specialized applications of spectral design and lighting systems.