A cemented prism is a composite optical assembly formed by bonding two or more individual prism elements together using a specialized transparent optical cement. This manufacturing technique allows designers to create complex optical functionalities within a single, robust component that would be impossible or inefficient to achieve with a monolithic prism or air-spaced elements.

The primary purpose of cementing prisms is to combine the optical properties of different materials or geometries while mitigating the drawbacks of air gaps:

• Reduced Reflection Losses: Cementing eliminates air gaps between surfaces, which would otherwise cause Fresnel reflections (∼4% per surface). This significantly increases light throughputand improves system efficiency.

• Aberration Correction: By combining prisms made from different types of glass (e.g., crown and flint), optical designers can correct for chromatic and other aberrations. The cement layer can also act as a beam-splitting interface if designed with a specific coating.

• Structural Integrity: The cement binds the components into a single, rigid, and mechanically stable unit, resistant to misalignment from shock, vibration, or temperature fluctuations compared to air-spaced assemblies.

• Optical Cement: The cement is a critical component. It is a transparent, thermosetting or UV-curing adhesive with specific properties: high transmission, excellent adhesion, and a refractive index often chosen to match the glass substrates to minimize reflections. It must also be durable and stable over time and under environmental stress.

• Materials: Prism elements can be made from various optical glasses, crystals (e.g., Calcite for polarizing beamsplitters), or fused silica, selected based on the required dispersion, transmission wavelength, and functional purpose.

• Precision Bonding: The process requires extreme cleanliness and precision. The surfaces to be bonded are polished to very high flatness (e.g., λ/4 or better) and are brought into contact to exclude air bubbles. The cement is then cured under controlled conditions.

• Coatings: Often, the internal interface is coated with a specific optical coating (e.g., a beamsplitter or dichroic coating) before cementing, enabling functions like wavelength separation.

• Beamsplitters: Creating polarized or non-polarized beamsplitting cubes by cementing two right-angle prisms together with a dielectric coating at the hypotenuse interface.

• Color Separation/Combination: Dichroic prisms used in high-quality color cameras and projectors to split or combine red, green, and blue light paths.

• Anamorphic Prism Pairs: Used in laser systems to circularize elliptical beams, made by cementing prisms or using them in close contact.

• Aberration-Corrected Prisms: Complex assemblies designed for high-performance imaging systems and spectrometers to provide a flat, color-corrected field.

• Thermal Sensitivity: The different thermal expansion coefficients of the glass and cement can cause stress or delamination under extreme temperature variations, limiting the operational range.

• Power Handling: Optical cements have lower damage thresholds than glass, making cemented prisms less suitable for very high-power laser applications unless designed with care.

• Chemical and Environmental Durability: The cement layer may be susceptible to degradation from humidity, solvents, or prolonged UV exposure if not properly specified and protected.