A one-way mirror, also called two-way mirror [1] (or one-way glass, half-silvered mirror, and semi-transparent mirror), is a reciprocal mirror that appears reflective from one side and transparent from the other. The perception of one-way transmission is achieved when one side of the mirror is brightly lit and the other side is dark. This allows viewing from the darkened side but not vice versa.
The first U.S. patent for a one-way mirror appeared in 1903, then named a "transparent mirror". [2] [3]
The glass is coated with, or has been encased within, a thin and almost transparent layer of metal (window film usually containing aluminium). The result is a mirrored surface that reflects some light and is penetrated by the rest. Light always passes equally in both directions. However, when one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side. [4]
A one-way mirror is typically used as an apparently normal mirror in a brightly lit room, with a much darker room on the other side. People on the brightly lit side see their own reflection—it looks like a normal mirror. People on the dark side see through it—it looks like a transparent window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly.
When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule. These observation rooms have been used in:
Smaller versions are sometimes used in:
The same type of mirror, when used in an optical instrument, is called a beam splitter and works on the same principle as a pellicle mirror. A partially transparent mirror is also an integral part of the Fabry–Pérot interferometer.
A mirror, also known as a looking glass, is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the image in an equal yet opposite angle from which the light shines upon it. This allows the viewer to see themselves or objects behind them, or even objects that are at an angle from them but out of their field of view, such as around a corner. Natural mirrors have existed since prehistoric times, such as the surface of water, but people have been manufacturing mirrors out of a variety of materials for thousands of years, like stone, metals, and glass. In modern mirrors, metals like silver or aluminium are often used due to their high reflectivity, applied as a thin coating on glass because of its naturally smooth and very hard surface.
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. The angle is named after the Scottish physicist Sir David Brewster (1781–1868).
A retroreflector is a device or surface that reflects radiation back to its source with minimum scattering. This works at a wide range of angle of incidence, unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence. Being directed, the retroflector's reflection is brighter than that of a diffuse reflector. Corner reflectors and cat's eye reflectors are the most used kinds.
Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating. Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming or switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.
Infrared cut-off filters, sometimes called IR filters or heat-absorbing filters, are designed to reflect or block near-infrared wavelengths while passing visible light. They are often used in devices with bright incandescent light bulbs to prevent unwanted heating. There are also filters which are used in solid state video cameras to block IR due to the high sensitivity of many camera sensors to near-infrared light. These filters typically have a blue hue to them as they also sometimes block some of the light from the longer red wavelengths.
In the field of optics, transparency is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale, the photons can be said to follow Snell's law. Translucency allows light to pass through but does not necessarily follow Snell's law; the photons can be scattered at either of the two interfaces, or internally, where there is a change in the index of refraction. In other words, a translucent material is made up of components with different indices of refraction. A transparent material is made up of components with a uniform index of refraction. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. The opposite property of translucency is opacity. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.
In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a glass or plastic disk in a metal or plastic ring frame, which can be screwed into the front of or clipped onto the camera lens.
An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in the field of optics. One type of optical coating is an anti-reflective coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and camera lenses. Another type is the high-reflector coating, which can be used to produce mirrors that reflect greater than 99.99% of the light that falls on them. More complex optical coatings exhibit high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film filters.
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optical properties of filters are completely described by their frequency response, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter.
Smart glass, also known as switchable glass, dynamic glass, and smart-tinting glass, is a type of glass that can change its optical properties, becoming opaque or tinted, in response to electrical or thermal signals. This can be used to prevent sunlight and heat from entering a building during hot days, improving energy efficiency. It can also be used to conveniently provide privacy or visibility to a room.
Day for night is a set of cinematic techniques used to simulate a night scene while filming in daylight. It is often employed when it is too difficult or expensive to actually shoot during nighttime. Because both film stocks and digital image sensors lack the sensitivity of the human eye in low light conditions, night scenes recorded in natural light, with or without moonlight, may be underexposed to the point where little or nothing is visible. This problem can be avoided by using daylight to substitute for darkness. When shooting day for night, the scene is typically underexposed in-camera or darkened during post-production, with a blue tint added. Additional effects are often used to heighten the impression of night.
A window film, sometimes called tint, is a thin laminate film that can be installed on the interior or exterior of glass surfaces in automobiles and boats, and as well as on the interior or exterior of glass in homes and buildings. It is usually made from polyethylene terephthalate (PET), a thermoplastic polymer resin of the polyester family, due to its clarity, tensile strength, dimensional stability, and ability to accept a variety of surface-applied or embedded treatments.
A projection screen is an installation consisting of a surface and a support structure used for displaying a projected image for the view of an audience. Projection screens may be permanently installed on a wall, as in a movie theater, mounted to or placed in a ceiling using a rollable projection surface that retracts into a casing, painted on a wall, or portable with tripod or floor rising models as in a conference room or other non-dedicated viewing space. Another popular type of portable screens are inflatable screens for outdoor movie screening.
In photography, through-the-lens metering refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. This information can then be used to set the optimal film or image sensor exposure, it can also be used to control the amount of light emitted by a flash unit connected to the camera.
A polarizing filter or polarising filter is a filter that is often placed in front of a camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph. The rotational orientation of the filter is adjusted for the preferred artistic effect.
An electrochromic device (ECD) controls optical properties such as optical transmission, absorption, reflectance and/or emittance in a continual but reversible manner on application of voltage (electrochromism). This property enables an ECD to be used for applications like smart glass, electrochromic mirrors, and electrochromic display devices.
Large-screen television technology developed rapidly in the late 1990s and 2000s. Prior to the development of thin-screen technologies, rear-projection television was standard for larger displays, and jumbotron, a non-projection video display technology, was used at stadiums and concerts. Various thin-screen technologies are being developed, but only liquid crystal display (LCD), plasma display (PDP) and Digital Light Processing (DLP) have been publicly released. Recent technologies like organic light-emitting diode (OLED) as well as not-yet-released technologies like surface-conduction electron-emitter display (SED) or field-emission display (FED) are in development to supersede earlier flat-screen technologies in picture quality.
A see-through display or transparent display is an electronic display that allows the user to see what is shown on the screen while still being able to see through it. The main applications of this type of display are in head-up displays, augmented reality systems, digital signage, and general large-scale spatial light modulation. They should be distinguished from image-combination systems which achieve visually similar effects by optically combining multiple images in the field of view. Transparent displays embed the active matrix of the display in the field of view, which generally allows them to be more compact than combination-based systems.
The infinity mirror is a configuration of two or more parallel or angled mirrors, which are arranged to create a series of smaller and smaller reflections that appear to recede to infinity. Often the front mirror of an infinity mirror is half-silvered, but this is not required to produce the effect. A similar appearance in artworks has been called the Droste effect. Infinity mirrors are sometimes used as room accents or in works of art.
Daylight redirecting film (DRF) is a thin, flexible plastic film which can be applied to a window to refract or reflect incoming light upwards so that the deeper parts of the room are lit more evenly. It can be used as a substitute for opaque blinds. It is a form of prism lighting.
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