Mirrors are in virtually every room of our homes, in our cars, in our purses and pocketbooks, and part of decorations. Given their ubiquity, it’s sometimes easy to overlook the qualities of mirrors that make them high-tech components critical to the design of sensitive optical systems. And, nearly every optical system uses mirrors to direct and steer light waves, fold light beams along prescribed paths, focus light and images, collimate point light sources into beams, or project images onto screens. They are versatile components that can be designed and shaped for specific applications. The surface of the substrate to which the dielectric coating, is applied classifies them as a first surface or second surface mirror.
First Surface vs Second Surface Mirror
The most significant difference between the inexpensive wall mirror (second surface mirror) and one used in an optical system (first surface mirror) is the surface of the reflective coating. Most mirrors we use in everyday life are called a second surface or back surface mirror. These mirrors have a reflective coating behind a glass substrate. Ans although silver is not typically used, the reflective layer is sometimes referred to as silvering.
Second Surface Mirror
In a second surface mirror the incident light enters the front or first surface, travels through the substrate, and reflects off the coating on the back or second surface. The substrate’s primary function is to provide a smooth, transparent surface to carry and protect the reflective coating. Glass is often used since it is rigid and remains relatively flat. When you hold an object, like a pen tip, on the surface of a second surface mirror, the tip of the pen will appear to float above its reflection.
Second surface mirrors have drawbacks, and they are not typically used for optical systems. Light can reflect off the first surface, and the substrate causes refraction. First surface reflection and refraction are also depending on the angle of incidence. Second surface mirrors can cause distortion and ghosting as depicted in Figure 1 above.
There is also a loss of energy with second surface mirror, which many precision applications cannot tolerate. A second surface mirror typically reflects 80% to 85% of incoming light.
First Surface Mirror
Similar to second surface mirrors, first surface mirrors have a reflective coating on a substrate. In this case, the reflective layer is on the front of the substrate so that incoming light reflects directly off the mirror’s face. This eliminates the ghosting and distortion of second surface mirrors, which makes them appropriate for optical applications. When you hold the pen tip on the surface of a first surface mirror, the tip of the pen will appear to touch its reflection with no sign of ghosting.
Typical reflective coatings include protected and enhanced aluminum, protected gold, or silver. Other materials such as titanium, chromium, copper, nickel, and rhodium can be used for specific application needs. For example, rhodium is often used for high-performance dental mirrors used in intra-oral photography. Compared to the alternatives (such as polished stainless-steel mirrors and chromium-coated mirrors), rhodium offers better clarity and higher reflectance for this application. The graph in Figure 2 compares the reflectance curves for a gold, aluminum, and silver of first surface mirrors with different thin film protective coatings.
A thin layer of reflective coating is applied to a substrate using vapor deposition. Float glass and soda lime float glass are common substrates, but other glasses like chalcogenides and rigid and flexible plastic substrates can also be coated.
Since the coating adheres closely to the surface, a highly polished and flat substrate is necessary to meet the undistorted, low-loss requirements for most optical systems. Many off-the-shelf first surface mirrors offer a 4-6λ surface flatness, but custom, high-performance mirrors may have flatness as high as ¼-½ λ. A first surface mirror can reflect up to 99% of incoming light.
Since the reflective coating is on the front surface, it is susceptible to damage. First surface mirrors are typically coated with a dielectric thin-film layer to increase the durability of the metal coating and provide a barrier to help prevent oxidation. The overcoat has little impact on the performance of the metal coating.
It is also possible to design the dielectric coating to enhance reflectance for specific wavelengths or spectral regions. For example, an enhanced aluminum coating uses a multi-layer film of dielectrics on top of aluminum to increase the reflectance in the visible region between 400nm and 650nm.
Applications of First Surface Mirrors
Just as second surface mirrors are found in nearly every room in our homes, first surface mirrors are found in almost every subsystem of optical applications. They have various uses throughout laser applications, projection and imaging, telescopes, microscopes, spectrometry and more. Figure 3 shows schematic examples of a few ways mirrors may be used in optical systems.
Beam Deflection — Using a first surface mirror to bend light along a prescribed path. Commonly found in laser systems, beam steering, overhead projection systems, spectrometer, interferometers. In a LIDAR system, a mirror mounted on a rotating axle deflects a light beam outward to sweep the surroundings.
Beam Folding — Similar to beam deflection, first surface mirrors are used to fold a beam to make the optical path longer than the size of the system. It is used for high-resolution cameras, back projection systems, flight simulators, and heads-up displays (HUDs).
Focusing Mirror — A curved mirror can be used to direct an image to a focal point. These mirrors are standard throughout telescopes, cameras, flight simulations, and other imaging systems. Figure 4 shows an example of a mirror used in a flight simulator system; its shape both focuses and deflects the incoming image.
Collimating Mirror — These are curved mirrors, similar to a focusing mirror, but used in the opposite orientation. Here, a light source placed at the focal point is redirected to create a collimated beam. Lamp reflectors in flashlights use a low-quality collimating mirror, while many optical instruments use similar high-quality collimating mirrors.
While inexpensive second surface mirrors might be ubiquitous throughout everyday life, first surface mirrors are critical for many high-tech optical systems. Precision, high-performance mirrors are found in everything from high-power telescopes probing the furthest reaches of the universe to scanning electron microscopes taking images of the smallest building blocks of nature. Mirrors found in these instruments are specialized components that are vastly different than the mirror inside a car’s visor.