Optical Domes

Author: the photonics expert Dr. Rüdiger Paschotta

Optical domes are a special kind of optical windows, which are strongly curved. For example, they can have the shape of a half-sphere, but domes with custom shapes are also available. Their function is generally to transmit light while protecting some optics by physically separating some environments: one prevents the penetration of materials (e.g. air flow or water, dust and humidity) and provides thermal and acoustic insulation. Domes with a wide range of sizes are available, sometimes with diameters well above 100 mm.

Plane optical windows would be simpler to manufacture than domes, having a substantial curvature. However, domes are often substantially more suitable for operation with a wide range of angles of entering light is required and typical side effects of that, for example optical aberrations and parasitic reflections, need to be minimized. The dome geometry may also help to achieve a superior structural strength. Aerodynamic considerations play a role in some applications.

Same optical domes need to transmit only visible light, for example for use with camera systems. They may be made of same high-quality optical glass like BK7 or fused silica. Glass domes are also available for the ultraviolet range, e.g. with UV-grade fused silica. For some applications, plastic optics (e.g. acrylic domes) are appropriate.

Other domes are operated in the infrared (→ infrared optics); for example, such devices are applied in heat-seeking missiles or for thermal imaging. A particularly robust infrared material is sapphire. Other possible choices, partly allowing for much longer wavelengths, are zinc selenide, germanium, silicon and certain ceramics.

A wide range of viewing directions is often important. The design of a half-sphere with the detector at its center may then be particularly appropriate because all light getting to these settings are will then have appropriately normal incidence on the dome surface. Under such conditions, there is no beam deflection, and anti-reflection coatings can reach a higher performance than for wide angular ranges.

Anti-reflection coatings are often needed, but in some cases one also tries to minimize the refractive index contrast between the dome material and the surrounding medium, for example water.

Robust Protective Domes

Optical domes are mostly used for protecting sensitive systems. For example, optical sensors for LIDAR built into airplanes must be protected with quite robust domes, withstanding the impact of sand or even birds at high velocities. Similarly, domes for submarines and underwater camera and video systems must withstand high pressures.

Due to the priority of protective features, some compromises may need to be made concerning optical performance. The sometimes harsh operating conditions may anyway not make it possible to preserve a very high optical quality of the exterior surface as dirt may be deposited to the outer surface during operation. Any optical coatings used there also need to be robust against external influences including cleaning operations. For maximum robustness, some optical domes are equipped with diamond-like hard carbon coatings. There are also other coatings for optical domes, e.g. anti-reflection coatings with particularly high durability, infrared filter coatings and metalized coatings.

Some domes have special mechanical features for precise mounting and sealing.

More to Learn

Encyclopedia articles:

• optical windows
• infrared optics