In the world of precision optics, the performance of an optical coating—especially its transmittance—is one of the most critical factors that determine the overall quality, efficiency, and signal-to-noise ratio of an optical system. Whether it's an anti-reflection (AR) coating, a high-reflective (HR) coating, or a bandpass filter, even slight deviations in transmittance can lead to significant degradation in system performance.
At Bena Optics, we understand that achieving optimal optical performance is not just about applying a thin film—it’s about mastering the intricate interplay between material properties, deposition processes, and coating design. In this blog, we’ll explore how these three core elements influence transmittance, with technical insights and practical considerations drawn from our advanced coating expertise.
The optical constants of coating materials—namely refractive index (n) and extinction coefficient (k)—are the first and most fundamental determinants of transmittance.
The extinction coefficient k quantifies how much light is absorbed by a material. Ideally, k = 0 for all wavelengths—but in reality, every material has some degree of absorption in certain spectral regions.
When light passes through a film, its intensity decays exponentially due to absorption: Absorption Loss ∝ 4πk / λ
This means that in shorter wavelengths (e.g., ultraviolet), even a small k value can result in significant absorption losses.
Ultraviolet Range:
TiO₂ is a high-index material widely used in visible AR coatings. While its k is <10⁻⁴ in the visible range (making it nearly transparent), it rises sharply below 380 nm. This can reduce UV AR coating performance from >99.5% to just 95–98% depending on the exact wavelength and coating complexity.
Infrared Range:
SiO, often used in near-infrared applications, becomes highly absorptive beyond 3 μm. Using it improperly in mid-IR systems may cause 5–15% transmittance loss.
Metallic Coatings (Cr, Ni):
These have very high k values and are used deliberately in neutral density filters to achieve specific attenuation levels (e.g., OD1.0 = 10% transmission, OD2.0 = 1%).
Takeaway: Always select materials with the lowest possible k values in your target wavelength range. Consult detailed n&k data from reliable suppliers before coating design.
Impurities, non-stoichiometry, or amorphous/multi-phase structures in thin films can introduce scattering, diverting light away from the intended transmission path.
Sub-oxides (e.g., TaO₂ in Ta₂O₅ films): Caused by insufficient oxygen during deposition, leading to higher k and scattering. This may degrade transmittance by 0.2–0.5%.
Crystallization (e.g., TiO₂): Can cause scattering at grain boundaries, especially in IR applications. Without proper dopants (like SiO₂ or Al₂O₃), this may reduce IR transmittance by 1–3%.
Even the best-designed coatings using ideal materials can underperform if the deposition process isn’t tightly controlled.
Film thickness dictates the optical path length and determines whether constructive/destructive interference conditions are met.
Systematic errors shift the entire spectral response (e.g., toward shorter or longer wavelengths).
Random errors distort peak shapes, reduce peak transmittance, and weaken blocking or cutoff performance.
For a standard 4-layer V-coat AR, ±1% thickness error may reduce center-wavelength transmittance from 99.8% → 99.3–99.5%.
For a narrowband filter, the same 1% error could drop peak transmittance from 90% → 85%, while also degrading FWHM and rectangularity.
Surface and interfacial roughness lead to Rayleigh scattering, particularly problematic at short wavelengths.
Roughness (measured as RMS): Advanced techniques like Ion Beam Sputtering (IBS) can achieve RMS < 0.5 nm, whereas traditional E-beam evaporation may yield 1–2 nm.
Each additional nanometer of RMS roughness can contribute 0.1–0.3% scattering loss.
Defects such as pinholes or micro-cracks not only reduce transmittance but also lower Laser Induced Damage Threshold (LIDT), a key parameter in high-power applications.
Low-temperature deposition (e.g., standard E-beam): Can result in porous films that absorb moisture, causing 0.5–1% transmittance drift post-exposure.
Ion-Assisted Deposition (IAD): Enables higher energy deposition (~200°C equivalent), producing denser, more stable films with minimal post-deposition drift (<0.1%).
An optimized coating isn’t just about piling on layers—it’s about smart design that accounts for real-world fabrication limits.
More layers enable complex spectral functions (e.g., sharp cutoffs, narrow passbands), but also increase absorption, scattering, and interface losses.
Poor material pairing (e.g., high-stress combinations) can degrade both transmission and mechanical durability.
Example:A well-engineered 25-layer bandpass filter might achieve 85% peak transmittance, but poor design or mismatched materials could drop this to just 70%.
Adjacent layers may interdiffuse slightly, creating graded-index transition zones instead of crisp interfaces.
In ultra-narrowband filters (FWHM < 1 nm), even 1–2 nm of interfacial diffusion can degrade transmittance by 2–5% and distort the spectral shape.
To ensure the highest possible transmittance, Bena Optics follows a holistic approach:
Material Selection:
We rigorously evaluate optical constants (n & k) across the target spectrum, prioritizing materials with low k values, high purity, and thermal stability.
Advanced Deposition Techniques:
Utilizing Ion Beam Sputtering (IBS) and Ion-Assisted Deposition (IAD), we achieve precise thickness control, ultra-smooth interfaces, and dense film structures.
Design-Process Integration:
Our coating designs are not created in isolation. We perform tolerance analysis, simulate process-induced variations, and optimize layer architecture to be resilient to real-world manufacturing fluctuations.
