The improvement of transmittance in optical cylinder lenses is essentially achieved through the precise design of the surface coating, utilizing the interference effect of light to redistribute reflected and transmitted light. This process involves the selection of coating materials, thickness control, and structural optimization, ultimately reducing reflection loss through destructive interference, thereby significantly improving transmittance.
When light is incident on the surface of an optical cylinder lens, it is reflected at the interface between air and lens material (the first surface) and at the interface between lens material and coating (the second surface). These two reflected beams interfere due to the optical path difference. If the coating thickness is precisely controlled to one-quarter (λ/4) of a specific wavelength of light, and the refractive index of the coating is between that of air and lens material, the two reflected beams will undergo destructive interference, significantly reducing the intensity of the reflected light. For example, when the refractive index of the coating satisfies n_{film}}=\sqrt{n_{air}}\cdot n_{lens}}}, the destructive effect of the reflected light is optimal, and the transmitted light is significantly enhanced due to energy conservation.
The choice of coating material is crucial for improving transmittance. Alternating layers of high-refractive-index materials (such as titanium dioxide, TiO₂) and low-refractive-index materials (such as silicon dioxide, SiO₂) can form multilayer films. This structure expands the optical path difference range, allowing the destructive interference effect to cover a wider spectral band. For example, in the visible light range, multilayer films can simultaneously suppress reflected light of multiple wavelengths, making the transmittance curve flatter and avoiding transmittance fluctuations caused by wavelength selectivity in single-layer films. Furthermore, the optical stability of the material (such as resistance to deliquescence and high-temperature resistance) directly affects the long-term effectiveness of the coating; therefore, suitable materials must be selected based on the application environment (such as outdoor or high-temperature conditions).
Coating thickness control is the core element in achieving destructive interference. Taking a single-layer antireflective coating as an example, its thickness needs to be precise to the nanometer level to ensure that the optical path difference between the two reflected beams is an odd multiple of half the wavelength. If the thickness deviation exceeds the allowable range, the interference conditions will be disrupted, resulting in incomplete cancellation of reflected light. Controlling the thickness of multilayer films is more complex, requiring computer simulations to optimize the parameters of each layer so that reflected light of different wavelengths can cancel each other out synchronously at a specific angle. For example, in laser systems, the coating thickness needs to be customized for the operating wavelength to achieve ultra-low reflection loss (reflectivity can be reduced to below 0.1%).
Optimizing the coating structure can further expand the application scenarios for improving transmittance. Gradient refractive index films eliminate interface reflections in traditional multilayer films through continuously varying film refractive indices, making them suitable for broadband anti-reflection requirements. Nanostructured coatings (such as subwavelength gratings) maintain high transmittance even under non-perpendicular incident conditions by modulating light scattering through micro/nano structures. These structural innovations enable optical cylinder lenses to maintain high transmittance performance in complex optical paths (such as oblique incidence and multi-wavelength mixing).
In practical applications, coating technology must balance transmittance improvement with other performance indicators. For example, high-power laser systems require coatings with high laser damage thresholds to prevent film peeling due to thermal effects; medical endoscope lenses need coatings with fingerprint-resistant and easy-to-clean properties to maintain long-term light transmission stability. Through material modification (such as fluorine doping) or surface treatment (such as hydrophobic coatings), diverse functional requirements can be met while improving transmittance.
From single-layer λ/4 films to multilayer composite film systems, and then to nanostructure coatings, transmittance enhancement technology for optical cylinder lenses continues to push physical limits. In the future, with the development of metamaterials and intelligent coating technologies, coatings will no longer be limited to passive anti-reflection, but will achieve real-time optimization of transmittance through dynamic control of refractive index distribution, opening up a new dimension for optical system design.
