Spherical Concave

The optical power of a concave mirror is positive, like that of a convex refractive surface. Like a singlet lens, a concave mirror can produce a magnified or reduced inverted image.

An important application can be found in condensing systems. When the light-emitting filament of a bulb is placed near the center of curvature of a spherical, concave mirror, the reflecting surface creates a well-corrected image of the filament. That image is located in the same plane but slightly displaced from the filament itself. Thus, the apparent brightness of the source is doubled (Figure C-3).

Figure C-3: A spherical mirror nearly doubles the apparent brightness of a source through its return of backward-propagating rays of light to the condensing lens.

Some projection lamps contain a spherical reflector inside the glass bulb. With this reflector, light that would have been lost to the projector’s housing is collected and sent back to its point of origin, where it can be collected by the condensing lenses. In fact, filaments of these lamps are designed with an open geometry to minimize blocking of the retro reflected light.

Indicator light sockets will often employ a spherical reflector to maximize apparent brightness for an observer at any viewing angle. Manufacturers of LEDs often build the reflector into the electronic package.

Another application for spherical concave mirrors is spectrometry. A spectrometer is an instrument designed to divide light into its component colors and measure their relative intensities. Spectrometers are used as tools for many kinds of fundamental research in the sciences. More commonly, spectrometers are vital components in color analyzers. Examples include color matching machines, which identify the exact color of a paint chip; color analysis machines, which provide information for the quality control of color TV tubes; and blood analyzers, which measure the color of blood samples when exposed to various chemical agents.

Many spectrometers incorporate spherical concave mirrors rather than lenses to form and focus the beam of light originating at the sample. Lenses would introduce too many aberrations of color and complicate both system design and the interpretation of data. Lenses also have limited transmission in the ultraviolet and far infrared regions of the spectrum.

A mirror with enhanced imaging quality can be designed by coating the back surface of a lens, rather than a bowed sheet of planar glass, with a reflective finish. It is similar to the cosmetic magnifying mirror, but is called a "Mangin" mirror after the man who first designed such a lens-mirror hybrid (Figure C-4).

Figure C-4: A Mangin mirror is a lens-mirror hybrid.

The image is formed by reflection and refraction in the same optical element; hence, a Mangin mirror is a catadioptric element. "Catadioptric" originates from two Greek prefixes: "kata-," meaning backwards or reflective, and "dia-," meaning through or refractive.

Gas and solid-state laser manufacturers use large numbers of spherical concave mirrors. The heart of every laser is a "cavity" or "resonator" created with mirrors that face each other and reflect laser energy back and forth through an amplifying medium. Usually, one of these mirrors is spherically concave to create a stable laser beam.

The quality of the image produced by a spherical concave mirror is comparable to that obtained with a single lens element. The mirror surpasses the lens in its color performance (a mirror has no aberrations of color since all colors experience the same geometry of reflection), but a mirror, like a lens, introduces spherical aberration, coma, astigmatism and curvature of field.

Limitations of image quality produced by a single spherical concave mirror can be overcome by using several complementary mirrors in a system. This is the same philosophy by which limitations of a single lens element are transcended when a designer creates a compound lens. Also, catadioptric systems contain mirrors and lenses to help balance aberrations.

JML offers two grades of spherical concave mirrors: precision and commercial quality. Please see the following product list for both types of concave mirrors.