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The first mirrors were made of polished castings of solid tin, bronze, copper, gold, silver or speculum, an alloy of copper and tin. They were usually flat and were used by the ancients as cosmetic mirrors.

Today, solid-metal mirrors are used only in applications handling very hot beams of light. In these cases, the metal's high thermal conductivity enables efficient cooling. Solid-metal mirrors often contain channels for the circulation of cooling fluids. Copper is commonly used for these mirrors. Its reflecting surface can be enhanced with an overcoat of another metal.

For the vast majority of applications, thin films of metal or dielectric materials are deposited on solid-glass substrates. The advantages over solid metal are in performance and cost.

The reflectivity of metal films for use in the visible spectrum runs from about 75% to 96%. In the infrared, gold can achieve more than 98% reflectivity.

Dielectric films designed for broad spectral regions can achieve reflectivities of 99%; and when they are designed for a specific wavelength, they can achieve 99.8% reflectivity (for unpolarized light).

Some of the light that is not reflected by these reflective thin films is absorbed by the coating. This "lost" light is converted to heat through interactions with the atomic structure of the coating. Another fraction of the "lost" light is scattered by microscopic surface texture and impurities in the coating. The remaining fraction of "lost" light travels completely through the coating, to be absorbed or scattered by the underlying substrate.

Small improvements in a mirror's reflectivity can have dramatic impact upon the performance of a system. For example, consider a system of four mirrors designed for an application in white, visible light. If each mirror consists of a simple film of aluminum, then its average reflectivity across the visible spectrum is about 90%. Throughput for these four mirrors is (.90)4 = 66%. In other words, 34% of the incoming light is lost to the mirrors. That same four-mirror system, installed with highly efficient broadband dielectric coatings offering 99% reflectivity at each surface, can deliver (.99)4 = 96% of the incoming light!

All reflective coatings are shown at 45° angle of incidence unless stated otherwise.

JML can also supply a number of other neutral density filters. Please contact us with your requirements for pricing and delivery information. For your convenience, please refer to the Density to Percent Transmission Conversion Table when inquiring about their flat response inconel beamsplitter coatings.