Introduction

Why can’t humans see gas, but OGI can visualize it?

Methane and most volatile organic compounds (VOCs) are invisible to the human eye. When gas leaks occur in industrial facilities, operators usually rely on handheld detectors for point-by-point inspection or fixed monitoring systems for continuous observation.

While these methods remain essential for industrial safety management, they may become less efficient when inspecting large-scale assets such as storage tanks, processing facilities, elevated equipment, or long-distance pipelines.

With increasing attention on methane emissions reduction, LDAR (Leak Detection and Repair) programs, and industrial emission management, technologies that enable remote, non-contact, and visual gas leak detectionhave become increasingly important.

One of these technologies is:

OGI — Optical Gas Imaging.

But how can OGI visualize something that is invisible to the human eye?

The answer lies in the interaction between gas molecules and infrared radiation.

1. What Is Optical Gas Imaging (OGI)?

Many people seeing an OGI camera for the first time assume that it directly “captures gas.”

However, OGI does not photograph gas molecules themselves.

Instead, it detects the changes in infrared radiation caused by gas absorption and converts these invisible differences into visible images.

In simple terms:

OGI does not see the gas itself.
It visualizes the infrared radiation changes caused by gas molecules.

The basic imaging process can be summarized as follows:

① Infrared radiation from the background reaches the camera

Industrial equipment and surrounding objects continuously emit infrared radiation.

② Gas molecules absorb specific infrared wavelengths

When gas passes through the optical path, molecules absorb infrared radiation at characteristic wavelengths.

③ Infrared differences are detected by the camera

The OGI system identifies changes in infrared intensity within the target spectral range.

④ Image processing enhances gas visualization

The detected infrared variations are processed and displayed as a visible gas plume.

As a result, invisible gas leakage can be visually located in real time.

2. Why Can OGI Detect Methane and VOCs?

The key is:

Infrared absorption characteristics

Different gas molecules interact with infrared radiation in different ways. Each gas has unique absorption characteristics, often described as its infrared fingerprint.

Methane (CH₄) and many hydrocarbon-based VOCs have strong absorption features in the mid-wave infrared (MWIR) region, particularly around 3.2–3.4 μm.

This absorption behavior is mainly associated with:

C–H bond stretching vibrations

OGI systems designed around these characteristic absorption bands can detect infrared radiation differences caused by methane and hydrocarbon gases, allowing the formation of visible gas plumes.

However, gas visualization depends on multiple factors, including:

• Gas absorption characteristics

• Gas concentration

• Distance between the camera and leak source

• Background infrared contrast

• Environmental conditions

A gas must have sufficient infrared absorption within the camera’s operating wavelength range to be visualized effectively.

3. Why Does an OGI Image Look Like Smoke?

Many people seeing an OGI video for the first time ask:

“Why does the gas look like smoke?”

The answer is that it is not smoke.

The image represents a changing region of infrared absorption caused by gas movement.

Because gas is a fluid:

• Gas concentration changes continuously

• Air movement affects plume shape

• Wind influences dispersion behavior

the displayed plume appears dynamic, continuously moving and expanding.

Although it visually resembles smoke, the image represents infrared radiation changes caused by gas absorption, not visible particles.

4. What Gases Can OGI Visualize?

In principle, gases with sufficient infrared absorption characteristics within an OGI system’s operating wavelength range may be visualized under suitable conditions.

Typical examples include:

• Methane (CH₄)

• Ethane

• Propane

• Butane

• Benzene

• Toluene

• Xylene

• Other hydrocarbon-based VOCs containing C–H bonds

However, not all gases are suitable for OGI visualization.

For example:

• Nitrogen (N₂)

• Oxygen (O₂)

• Hydrogen (H₂)

have limited infrared absorption characteristics in commonly used OGI spectral ranges and therefore cannot normally be visualized using standard OGI systems.

5. Why Do Different Gases Appear Different in OGI Images?

A common misunderstanding is that all hydrocarbon gases should appear equally visible.

In reality, different gases can produce different visualization effects.

This is influenced by:

① Molecular structure

Different molecules interact with infrared radiation differently.

② Infrared absorption strength

Some gases absorb infrared radiation more strongly than others.

③ Gas concentration and distribution

Higher concentrations generally create stronger infrared absorption signals.

Under similar testing conditions:

• Methane is relatively more challenging to visualize

• Ethane may provide stronger contrast

• Propane and butane may generate more visible plumes

However, actual visualization performance depends on the complete operating environment, including leak rate, distance, background conditions, and atmospheric conditions.

Conclusion

OGI does not directly “see” gas.

Instead, it visualizes the infrared changes created when gas molecules absorb specific wavelengths of infrared radiation.

This capability enables operators to:

• Quickly locate potential leak sources

• Perform remote, non-contact inspections

• Improve efficiency when inspecting complex industrial assets

However, the quality of gas visualization is not determined only by the camera itself.

Why can the same OGI system produce a clear plume in one situation but a weak image in another?

The answer involves many factors, including background conditions, wind, optical performance, and operating environment.

Disclaimer

Optical Gas Imaging performance depends on multiple factors, including gas type, concentration, leak rate, distance, background conditions, weather conditions, and equipment specifications.

OGI visualization represents infrared absorption differences caused by gas presence and should not be interpreted as direct observation of gas molecules.

For quantitative gas concentration measurement, additional technologies such as TDLAS or other calibrated measurement methods may be required.

References

1. U.S. Environmental Protection Agency (EPA).
Leak Detection and Repair: A Best Practices Guide.

2. U.S. Environmental Protection Agency (EPA).
Method 21 — Determination of Volatile Organic Compound Leaks.

3. FLIR Systems.
Optical Gas Imaging: Infrared Cameras for Gas Detection Applications.

4. ASTM International.
ASTM E1213 — Standard Test Method for Minimum Resolvable Temperature Difference for Thermal Imaging Systems.

5. International Organization for Standardization (ISO).
ISO 17025 — General Requirements for the Competence of Testing and Calibration Laboratories.