Transmission – Passage of Light Through Material

Introduction

Transmission in optics is the foundational process by which electromagnetic radiation, especially visible light, passes through a material. In practical terms, it quantifies how much of the incident light on a surface exits on the opposite side, after accounting for the inevitable losses due to reflection and absorption. The degree of transmission a material offers is pivotal in shaping its use in windows, lenses, filters, fiber optics, and various advanced optical devices.

Transmission is not only a qualitative property but is also precisely quantified to ensure materials perform as required in critical applications. For instance, cockpit windows must allow pilots to see clearly regardless of lighting conditions, while also protecting them from ultraviolet (UV) and infrared (IR) radiation. To this end, industry standards—such as those set by the International Civil Aviation Organization (ICAO)—define specific transmission criteria for aircraft transparencies, balancing clarity, comfort, and safety.

Understanding how light propagates through matter is crucial for a wide array of scientific, engineering, and everyday applications, ranging from architectural daylighting to the development of high-performance sensors and energy-efficient solar panels. This glossary entry offers a comprehensive overview of transmission and its related concepts within optics.

Transmission: The Fundamentals

In optics, transmission describes the process by which electromagnetic waves, primarily light, continue their trajectory through a medium instead of being reflected or absorbed at its surface. The transmitted fraction of incident light is determined by:

• The atomic and molecular structure of the material
• Thickness and surface quality
• Wavelength of the incoming light

When light encounters a material, three primary processes occur:

1. Reflection – some light bounces off the surface.
2. Absorption – some light is absorbed by the material and converted to other energy forms (like heat).
3. Transmission – the remainder passes through, emerging on the other side.

The sum of these three processes always equals the total incident light energy:

[ T + R + A = 1 ]

where T is transmittance, R is reflectance, and A is absorptance.

Types of Transmission

• Regular (Specular) Transmission: Light passes directly through a transparent, homogeneous material without significant deviation. This is essential for applications requiring clear imaging, such as aircraft windshields, camera lenses, and high-precision optical instruments.
• Diffuse Transmission: Light is scattered as it passes through the material, often due to internal microstructures or surface texture. This results in a blurred appearance but is useful in applications like privacy glass, lighting diffusers, or decorative panels.

Transmittance

Transmittance (T) is the ratio of the transmitted light intensity ((I_{transmitted})) to the incident light intensity ((I_{incident})):

[ T = \frac{I_{transmitted}}{I_{incident}} ]

• Expressed as a value between 0 (no transmission) and 1 (perfect transmission), or as a percentage.
• Spectral Transmittance: Measured at specific wavelengths, crucial for color management, UV/IR protection, and sensor applications.
• Integral Transmittance: Measures the overall passage of light across a range of wavelengths, such as total visible or solar transmittance.

Application Example:
Aircraft cockpit transparencies must meet ICAO-specified minimum visible transmittance to ensure pilots have unobstructed views.

Reflection

Reflection occurs when incident light bounces off the surface of a material. It is governed by the law of reflection (angle of incidence equals angle of reflection) and depends on factors like:

• Material’s refractive index
• Surface smoothness
• Angle of incidence

Types of Reflection:

• Specular Reflection: Maintains image clarity (e.g., mirrors).
• Diffuse Reflection: Scatters light in many directions (e.g., matte surfaces).

Significance in Aviation:
Anti-reflective coatings on cockpit windows minimize glare, improving pilot visibility and safety.

Absorption

Absorption is the conversion of incident light energy into other forms, typically heat, within a material. The degree of absorption depends on:

• Material composition
• Light wavelength

Absorptance (A) quantifies absorbed light. For critical optical applications, the goal is often to minimize absorption in the visible range (to prevent heating and distortion) and maximize it where protection is needed (such as in UV-blocking windows).

Scattering

Scattering refers to the deviation of light from its straight path due to imperfections, particles, or structural inhomogeneities in or on a material.

• Rayleigh Scattering: From particles much smaller than the wavelength (explains blue sky).
• Mie Scattering: From particles similar in size to wavelength (clouds, haze).
• Non-selective Scattering: From larger particles.

In Optics:
Excessive scattering reduces clarity and contrast. In aviation, ICAO limits allowable haze and scattering in cockpit transparencies.

Transparent, Translucent, and Opaque Materials

• Transparent Materials: Allow light to pass with minimal scattering or absorption (e.g., pure glass, acrylic). Used where clarity is vital.
• Translucent Materials: Transmit light but scatter it, blurring images (e.g., frosted glass, diffusers).
• Opaque Materials: Block all light; only reflection and absorption occur (e.g., metals, wood).

Aviation Example:
Cockpit windows must be highly transparent; cabin dividers may be translucent for privacy; structural parts are often opaque.

Regular (Direct) vs. Diffuse Transmission

• Regular Transmission: Light maintains its path, enabling clear imaging. Critical in applications like windshields and camera optics.
• Diffuse Transmission: Useful in ambient lighting and privacy applications but problematic for vision-critical components.

Spectral Transmission & Wavelength Dependence

Spectral Transmission profiles show how much light at each wavelength passes through a material. Most materials transmit certain wavelengths better than others, allowing for:

• UV Blockage: To protect against harmful radiation.
• IR Filtering: Managing heat from sunlight.
• Color Management: Customizing appearance and functionality.

Wavelength dependence is harnessed in smart windows, optical filters, and solar control glazing.

Surface Quality

Surface smoothness, cleanliness, and absence of defects are critical in minimizing unwanted scattering and reflection. High-quality surfaces are essential for:

• Cockpit windows and windshields
• Camera and sensor lenses
• Laboratory optics

Angle of Incidence

The angle of incidence affects the proportions of transmitted, reflected, and absorbed light. At steeper angles, transmission often decreases, while reflection increases. This is especially important for:

• Cockpit windows (to maintain clear vision under varying sun angles)
• Sensors and instruments facing multiple directions

Total Internal Reflection

Total Internal Reflection (TIR) occurs when light in a higher-index medium hits a boundary at an angle greater than the critical angle, causing all light to reflect internally. TIR is the working principle of:

• Optical fibers (for data and sensor signals)
• Certain periscopes and light guides

Optical Density

Optical Density (OD) expresses how much a material attenuates light:

[ \text{OD} = -\log_{10}(T) ]

High OD filters are used for laser safety and protective visors in aviation.

Haze

Haze quantifies the percentage of transmitted light scattered at wide angles, resulting in a milky or blurred appearance. Low haze is essential for clear vision through cockpit windows and instrument covers.

Integrating Sphere

An integrating sphere is used to measure total transmitted, reflected, or absorbed light, capturing both direct and scattered components. It ensures accurate assessment of optical properties for quality control and compliance.

Spectrophotometer

A spectrophotometer measures the spectrum of transmitted (or reflected) light as a function of wavelength, providing key data for material certification and product design in optics, aviation, and manufacturing.

Optical Fiber

Optical fibers are thin, flexible strands of glass or plastic that transmit light using total internal reflection. In aviation, they are used for:

• Data communications
• In-flight entertainment
• Lighting and sensor systems

Anti-Reflective Coating

Anti-reflective (AR) coatings dramatically reduce unwanted reflections and maximize transmission. They are vital for cockpit windshields, instrument covers, and camera lenses.

UV and IR Transmission

UV and IR transmission control the passage of ultraviolet and infrared light. Aviation standards require:

• High visible light transmission for clarity and safety
• Low UV and IR transmission for protection and thermal comfort

Photometric and Radiometric Quantities

• Photometric: Relate to visible light as perceived by humans (lumens, lux).
• Radiometric: Concern all electromagnetic energy (watts).

Transmission evaluation uses both, depending on whether human vision or instrument response is the priority.

Lambert’s Law (Beer-Lambert Law)

Describes how light intensity decreases exponentially as it passes through an absorbing medium:

[ I = I_0 \cdot e^{-\alpha x} ]

where:

• (I) = transmitted intensity
• (I_0) = incident intensity
• (\alpha) = absorption coefficient
• (x) = thickness

This law is central to calculating transmission through materials of varying thickness and absorption.

Solar Transmission

Solar transmission is the fraction of total solar energy (visible + near-UV + near-IR) that passes through a material. Materials with tailored solar transmission are used for:

• Daylighting (high transmission)
• Thermal management and glare reduction (low transmission)

Aviation and Regulatory Considerations

In aviation, ICAO and other organizations specify:

• Minimum visible transmittance for cockpit windows and windshields
• Maximum allowable haze and scattering
• UV and IR transmission limits for crew and passenger protection
• Surface quality standards for optical clarity and durability

Compliance ensures safety, comfort, and operational efficiency under diverse environmental conditions.

Conclusion

Understanding and controlling the transmission of light through materials is fundamental to the safety, efficiency, and performance of countless optical systems. From the clarity of cockpit windows to the precision of scientific instruments and the comfort provided by architectural glazing, transmission stands at the heart of modern optics. Engineers and designers must consider transmittance, spectral properties, surface quality, and regulatory requirements to optimize materials for their intended applications.

For aviation, in particular, maintaining optimal transmission characteristics under all operational conditions is not just a matter of performance—it’s a matter of safety.

Related Terms

• Transmittance
• Reflection
• Absorption
• Scattering
• Transparent/Translucent/Opaque Materials
• Spectrophotometer
• Integrating Sphere
• Optical Fiber
• Anti-Reflective Coating
• UV and IR Transmission
• Photometric/Radiometric Quantities
• Surface Quality

For more details or to discuss how advanced transmission control can benefit your application, contact our specialists or schedule a demo .