{"id":610894,"date":"2026-08-30T03:10:43","date_gmt":"2026-08-30T03:10:43","guid":{"rendered":"https:\/\/www.olympiajournal.com\/news\/story\/610894\/infrared-passes-through-fog-but-not-through-glassthe-physics-that-defines-infrared-system-design.html"},"modified":"2026-08-30T03:10:43","modified_gmt":"2026-08-30T03:10:43","slug":"infrared-passes-through-fog-but-not-through-glassthe-physics-that-defines-infrared-system-design","status":"publish","type":"post","link":"https:\/\/www.olympiajournal.com\/news\/story\/610894\/infrared-passes-through-fog-but-not-through-glassthe-physics-that-defines-infrared-system-design.html","title":{"rendered":"Infrared Passes Through Fog, But Not Through Glass – The Physics That Defines Infrared System Design"},"content":{"rendered":"

Hangzhou, Zhejiang, China – August 30, 2026 –<\/strong><\/p>\n

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Someone once ran a test: using a thermal imager to capture a person indoors through an ordinary glass window—the thermal image showed nothing. Remove the glass, same distance, and the human silhouette appeared clear. Same device, add a pane of glass, and it becomes completely useless. Yet that same thermal imager works perfectly outdoors in heavy fog, capturing human thermal signatures through tens of meters of water vapor. Passes through fog, blocked by glass—this counterintuitive phenomenon is rooted in a fundamental physical concept that underpins the entire field of infrared optics: the atmospheric window.<\/p>\n

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The Interaction Between Electromagnetic Waves and Matter Determines Which Wavelengths “Make It Through”<\/p>\n

Infrared radiation is a band of the electromagnetic spectrum, spanning roughly from 0.75μm to 1000μm. Within this range, different wavelengths encounter vastly different levels of “resistance” when passing through the atmosphere. Molecules in the air—water vapor (H\u2082O), carbon dioxide (CO\u2082), ozone (O\u2083), and others—strongly absorb specific infrared wavelengths. Radiation comes in, molecular vibrations are excited, energy converts to heat, and the radiation disappears. Which wavelengths are strongly absorbed depends on the vibrational frequencies of these molecules—an inherent physical property determined by molecular structure that does not change with environmental conditions. Plot the transmittance of the atmosphere versus wavelength, and several high-transmittance bands emerge—these are called atmospheric windows.<\/p>\n

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The three main atmospheric windows are: the near-infrared window (around 0.75–2.5μm, with multiple sub-windows), the mid-wave infrared window (3–5μm), and the long-wave infrared window (8–14μm). Inside these windows, atmospheric transmittance is high and infrared radiation travels efficiently. Outside the windows, absorption bands from water vapor and CO\u2082 attenuate infrared radiation to near-zero over short distances.<\/p>\n

Why Fog Doesn’t Block Infrared, but Glass Does<\/p>\n

Fog consists of tiny water droplets suspended in the air, typically ranging from a few micrometers to tens of micrometers in diameter. Visible light has wavelengths of 0.4–0.7μm—far smaller than fog droplet diameters, resulting in strong scattering and reduced visibility. But long-wave infrared at 8–14μm has wavelengths comparable to or even longer than fog droplet diameters, where scattering effects are substantially weaker. Radiation bends around the droplets and continues propagating rather than being scattered away. This is the physical reason thermal imagers retain effective detection capability under light-to-moderate fog and haze.<\/p>\n

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Glass is a different matter entirely. Ordinary glass is primarily composed of SiO\u2082. The vibrational frequency of the Si–O bond falls right in the mid-to-far-infrared range. Long-wave infrared (8–14μm) striking glass is almost completely absorbed, with transmittance near zero. From the thermal imager’s perspective, glass is a strong thermal radiator, not a transparent medium—what it sees is the glass itself’s thermal radiation, not the scene behind it. This property is wavelength-specific: glass is opaque to long-wave infrared. But ordinary glass has reasonably high transmittance in the near-infrared (0.75–2.5μm), exceeding 85% around 1μm. So with a near-infrared camera, you can see through glass and capture the scene behind it.<\/p>\n

How the Atmospheric Window Determines Infrared System Design<\/p>\n

Understanding the atmospheric window is essential to understanding why different infrared applications use completely different wavelength bands—rather than just picking any wavelength that seems convenient.<\/p>\n

Human body thermometry and security thermal imaging use 8–14μm long-wave infrared. The thermal radiation peak of room-temperature objects (20–40°C) falls squarely within this window, and atmospheric transmittance is high, enabling long detection ranges.<\/p>\n

Industrial high-temperature thermometry (hundreds to thousands of degrees Celsius) uses the 3–5μm mid-wave infrared window. The radiation peak of high-temperature objects shifts to the mid-wave band, and detectors in this band offer higher sensitivity for precision high-temperature measurements.<\/p>\n

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Gas detection selects the band corresponding to the target gas’s absorption peak—CO\u2082 at 4.26μm falls within the mid-wave window; methane at 3.3μm similarly falls within the mid-wave window. If a target gas’s characteristic absorption peak lies in a region of strong atmospheric absorption, open-path detection is not feasible—closed-cell methods are required.<\/p>\n

Laser ranging and near-infrared communications use the near-infrared window around 1μm and 1.55μm. These bands experience low atmospheric attenuation and are relatively eye-safe at higher power levels.<\/p>\n

The Role of Filters in the Atmospheric Window<\/p>\n

The atmospheric window defines the usable wavelength range. The filter’s job is to precisely extract the signal within the window while blocking interference at the window edges and outside it. This task involves two engineering challenges:<\/p>\n

First, the cutoff at the window edge must be clean. The window boundaries are not sharp—transmittance transitions gradually. If the filter’s cutoff edge is not precisely designed, wavelengths from the adjacent atmospheric absorption regions can leak into the detector, introducing uncontrollable background variations with changing humidity and CO\u2082 concentration, causing temperature measurement accuracy to fluctuate with weather.<\/p>\n

Second, transmittance within the window must be flat. Even within the 8–14μm window, atmospheric transmittance is not entirely uniform—there is an ozone absorption band around 9.6μm and a water vapor absorption band near 11μm. For precision thermometry applications, these localized absorption features may need to be addressed through filter design.<\/p>\n

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MULTI IR’s long-wave infrared longpass and bandpass filter series are specified with quantifiable metrics in both cutoff slope control and transmittance flatness, matching the requirements of thermal imaging and thermometry applications across different precision levels.<\/p>\n

Fog can be penetrated; glass cannot. This is not a problem with the thermal imager, nor a problem with the glass—it is the physics of electromagnetic wave interaction with different materials at work. These laws determine both what infrared technology can do and what it cannot. Within its domain, pushing to the limit is what this industry has always done.<\/p>\n

About Us<\/strong><\/p>\n

Founded in 2007, Hangzhou MULTI IR Technology Co., Ltd. is an optoelectronic technology enterprise integrating R&D, production, and sales. Its products are widely applied in aerospace, medical care, AR\/VR, display imaging, photography, and other fields, steadily holding the position of the world’s largest spot supplier of optical components.<\/p>\n

Media Contact<\/span>
Company Name:<\/strong> HANGZHOU MULTI IR TECHNOLOGY CO., LTD.<\/a>
Contact Person:<\/strong> Media Relations
Email:<\/strong> Send Email<\/a>
Country:<\/strong> China
Website:<\/strong> https:\/\/www.miroptech.com\/<\/a><\/p>\n

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Hangzhou, Zhejiang, China – August 30, 2026 – Someone once ran a test: using a thermal imager to capture a person indoors through an ordinary glass window—the thermal image showed<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts\/610894"}],"collection":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/comments?post=610894"}],"version-history":[{"count":0,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/posts\/610894\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/media?parent=610894"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/categories?post=610894"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.olympiajournal.com\/news\/wp-json\/wp\/v2\/tags?post=610894"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}