Optical gas imaging (OGI) cameras can detect gas leaks that are invisible to the naked eye, but the smallest leaks they can reveal depend on the precision of the optical components inside them.
Fugitive emissions from valves, flanges, seals and storage tanks are hazardous, tightly regulated by the U.S. Environmental Protection Agency (EPA), and impossible to see without specialized equipment. Leak Detection and Repair (LDAR) programs rely heavily on OGI cameras to locate these otherwise invisible emissions. Within these systems, performance is ultimately determined by the lenses, windows, and filters that shape and transmit the infrared signal.
OGI instruments are specialized infrared (IR) cameras that detect hydrocarbon gases such as methane (CH₄) and propane (C₃H₈), which absorb strongly in the mid-wave infrared (MWIR) region at approximately 3.2–3.5 μm. A narrow bandpass filter restricts the imaging system to this absorption band.
Because the gas absorbs IR radiation at these wavelengths, a leak appears as a dark, smoke-like plume against a warmer background. This visibility depends on thermal contrast: the smaller the temperature difference between the gas and its surroundings, the fainter the plume becomes and the harder the imaging system must work to detect it.
For Sulphur hexafluoride (SF₆), commonly used in electrical utility applications, imaging is typically performed in the long-wave infrared (LWIR) region around 10.6 μm.
Because the spectral filter rejects most incoming light, the detector operates with a limited signal. Any light lost through reflection, absorption or scatter directly reduces detection sensitivity. High-index infrared materials such as Germanium and silicon naturally reflect a significant proportion of light at each surface, making anti-reflective (AR) coatings essential for maximizing transmission in the MWIR band.
This requirement translates directly into optical specifications. MWIR AR coatings are typically designed to achieve less than 0.25% reflectance per surface across the 3–5 μm band. Surface micro-roughness is generally maintained below 20 Å (2 nm) RMS to minimize scatter, while aspheric form error may be specified to better than 0.5λ peak-to-valley, with centration controlled to within a few arcminutes.
These tolerances directly influence a camera's noise equivalent temperature difference (NETD) - the smallest temperature difference it can distinguish. Modern handheld OGI systems typically target NETD values below 20–30 mK, and even small increases in scatter or optical error can reduce performance.
Aberration control is equally important. If an optical system blurs a small gas plume across multiple pixels, the signal may become too weak to detect. Tight surface-form tolerances and precise centration help maintain image sharpness and preserve sensitivity.
OGI cameras are deployed in demanding environments, including well pads, compressor stations, refineries and drone-mounted inspection systems. Their optics are exposed to temperature cycling, vibration, humidity and airborne contaminants throughout their service life.
To maintain long-term performance, optical components often require durable, diamond-like carbon (DLC) coatings and mechanically stable mounts capable of preserving alignment under challenging operating conditions.
Aspheric lenses follow a mathematically defined profile rather than a simple spherical shape. This allows a single aspheric element to correct aberrations that would otherwise require multiple conventional lenses.
As a result, aspheres are widely used in OGI systems, particularly where size, weight and power (SWaP) constraints are critical. Fewer optical elements mean fewer surfaces that can introduce reflection losses, while also enabling smaller, lighter optical assemblies for handheld and drone-mounted instruments. At the same time, aspheres help maintain the image quality required by the wide apertures commonly used in OGI cameras.
However, these benefits depend on manufacturing quality. Diamond-turned aspheric surfaces in materials such as chalcogenide and silicon can introduce mid-spatial-frequency tooling marks that increase scatter and reduce image contrast. For this reason, aspheric surfaces undergo rigorous metrology testing to verify surface quality and ensure that performance gains are not compromised by manufacturing errors.
Aspheres for OGI cameras are commonly manufactured from germanium, zinc selenide, chalcogenide and silicon, because of their high transmission in the MWIR band. Chalcogenide also exhibits lower thermal sensitivity than germanium, helping optical systems maintain focus across changing environmental conditions.
In practical terms, germanium's thermo-optic coefficient (dn/dT) is approximately 4 × 10⁻⁴/K, compared with roughly 1–2 × 10⁻⁴/K for many chalcogenide glasses. This difference becomes significant when cameras operate across temperature ranges exceeding 60°C. Where germanium is selected for its higher refractive index and mechanical durability, designers often incorporate athermalized mounts or compensating optical elements to minimise focus shift across the operating temperature range.
Read more: How high-precision aspheric lenses are made
The performance of an OGI camera depends on more than its detector alone. Coatings, surface finish, form accuracy and material selection all contribute to sensitivity, image quality and long-term reliability.
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If you’re working on an OGI system that may benefit from our expertise, please contact our technical sales team to discuss your requirements.