1.
Lens (optics)
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A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a piece of transparent material, while a compound lens consists of several simple lenses. Lenses are made from such as glass or plastic, and are ground. A lens can focus light to form an image, unlike a prism, devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses. The word lens comes from the Latin name of the lentil, the genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure, the variant spelling lense is sometimes seen. While it is listed as a spelling in some dictionaries. The oldest lens artifact is the Nimrud lens, dating back 2700 years to ancient Assyria, david Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight. Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, the earliest written records of lenses date to Ancient Greece, with Aristophanes play The Clouds mentioning a burning-glass. Some scholars argue that the evidence indicates that there was widespread use of lenses in antiquity. Such lenses were used by artisans for fine work, and for authenticating seal impressions, both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. The Viking lenses were capable of concentrating enough sunlight to ignite fires, between the 11th and 13th century reading stones were invented. Often used by monks to assist in illuminating manuscripts, these were primitive plano-convex lenses initially made by cutting a sphere in half. As the stones were experimented with, it was understood that shallower lenses magnified more effectively. Lenses came into use in Europe with the invention of spectacles. Spectacle makers created improved types of lenses for the correction of vision based more on knowledge gained from observing the effects of the lenses. With the invention of the telescope and microscope there was a deal of experimentation with lens shapes in the 17th. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the figure of their surfaces
2.
Image sensor
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An image sensor or imaging sensor is a sensor that detects and conveys the information that constitutes an image. It does so by converting the variable attenuation of light waves into signals, the waves can be light or other electromagnetic radiation. As technology changes, digital imaging tends to replace analog imaging, early analog sensors for visible light were video camera tubes. Currently, used types are semiconductor charge-coupled devices or active pixel sensors in complementary metal–oxide–semiconductor or N-type metal-oxide-semiconductor technologies, analog sensors for invisible radiation tend to involve vacuum tubes of various kinds. Digital sensors include flat panel detectors, today, most digital cameras use a CMOS sensor, because CMOS sensors perform better than CCDs. An example is the fact that they incorporate an integrated circuit, CCD is still in use for cheap low entry cameras, but weak in burst mode. Both types of sensor accomplish the task of capturing light. Each cell of a CCD image sensor is an analog device, when light strikes the chip it is held as a small electrical charge in each photo sensor. This process is repeated until all the lines of pixels have had their charge amplified. A CMOS image sensor has an amplifier for each compared to the few amplifiers of a CCD. Some CMOS imaging sensors also use Back-side illumination to increase the number of photons that hit the photodiode, CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors. They are also vulnerable to static electricity discharges. Another approach is to utilize the very fine dimensions available in modern CMOS technology to implement a CCD like structure entirely in CMOS technology and this can be achieved by separating individual poly-silicon gates by a very small gap. These hybrid sensors are still in the phase and can potentially harness the benefits of both CCD and CMOS imagers. There are many parameters that can be used to evaluate the performance of a sensor, including dynamic range, signal-to-noise ratio. For sensors of comparable types, the ratio and dynamic range improve as the size increases. In order to avoid interpolated color information, techniques like color co-site sampling use a mechanism to shift the color sensor in pixel steps. 3CCD, using three discrete image sensors, with the color separation done by a dichroic prism, while in general digital cameras use a flat sensor, Sony prototyped a curved sensor in 2014 to reduce/eliminate Petzval field curvature that occurs with a flat sensor
3.
International Telecommunication Union
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ITU, based in Geneva, Switzerland, is a member of the United Nations Development Group. ITU has been an intergovernmental public-private partnership organization since its inception, ITU was formed in 1865, in Paris, at the International Telegraph Convention, this makes it one of the oldest intergovernmental organizations in the world. ITU became a United Nations specialized agency in 1947, the ITU comprises three sectors, each managing a different aspect of the matters handled by the Union, as well as ITU Telecom. The sectors were created during the restructuring of ITU at its 1992 Plenipotentiary Conference, radiocommunication Established in 1927 as the International Radio Consultative Committee or CCIR, this sector manages the international radio-frequency spectrum and satellite orbit resources. In 1992, the CCIR became the ITU-R, Standardization Standardization was the original purpose of ITU since its inception. Established in 1956 as the International Telephone and Telegraph Consultative Committee or CCITT, in 1993, the CCITT became the ITU-T. Development Established in 1992, this sector helps spread equitable, sustainable and affordable access to information and communication technologies, ITU Telecom ITU Telecom organizes major events for the worlds ICT community. A permanent General Secretariat, headed by the Secretary General, manages the work of the Union. The basic texts of the ITU are adopted by the ITU Plenipotentiary Conference, the ITU is headed by a Secretary-General, who is elected to a four-year term by the member states at the ITU Plenipotentiary Conference. On 23 October 2014 Houlin Zhao was elected 19th Secretary-General of the ITU at the Plenipotentiary Conference in Busan and his four-year mandate started on 1 January 2015, and he was formally inaugurated on 15 January 2015. There are 193 Member States of the ITU, which are all UN member states, the most recent member state to join the ITU is South Sudan, which became a member on 14 July 2011. The Republic of China was blocked from membership by the Peoples Republic of China, palestine was admitted as an observer in 2010. The Summit was held as two conferences in 2003 and 2005 in Geneva and Tunis, respectively, with the aim of bridging the digital divide, in December 2012, the ITU facilitated The World Conference on International Telecommunications 2012 in Dubai. WCIT-12 was a conference to address International Telecommunications Regulations, the international rules for telecommunications. The previous conference to update the Regulations was held in Melbourne in 1988, in August 2012, ITU called for a public consultation on a draft document ahead of the conference. Telecommunications ministers from 193 countries attended the conference in Dubai, the current regulatory structure was based on voice telecommunications, when the Internet was still in its infancy. In 1988, telecommunications operated under regulated monopolies in most countries, as the Internet has grown, organizations such as ICANN have come into existence to manage key resources such as Internet addresses and Domain Names. Some outside the United States believe that the United States exerts too much influence over the governance of the Internet, current proposals look to take into account the prevalence of data communications
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Image noise
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Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the sensor and circuitry of a scanner or digital camera, image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector. Image noise is an undesirable by-product of image capture that adds spurious and extraneous information, the original meaning of noise was and remains unwanted signal, unwanted electrical fluctuations in signals received by AM radios caused audible acoustic noise. By analogy unwanted electrical fluctuations themselves came to be known as noise, image noise is, of course, inaudible. Principal sources of Gaussian noise in digital images arise during acquisition e. g. sensor noise caused by poor illumination and/or high temperature, and/or transmission e. g. electronic circuit noise. Amplifier noise is a part of the read noise of an image sensor. In color cameras where more amplification is used in the blue color channel than in the green or red channel, at higher exposures, however, image sensor noise is dominated by shot noise, which is not Gaussian and not independent of signal intensity. Fat-tail distributed or impulsive noise is sometimes called salt-and-pepper noise or spike noise, an image containing salt-and-pepper noise will have dark pixels in bright regions and bright pixels in dark regions. This type of noise can be caused by analog-to-digital converter errors, bit errors in transmission and it can be mostly eliminated by using dark frame subtraction, median filtering and interpolating around dark/bright pixels. Dead pixels in an LCD monitor produce a similar, but non-random and this noise is known as photon shot noise. Shot noise has a value proportional to the square root of the image intensity. Shot noise follows a Poisson distribution, which except at low intensity levels approximates a Gaussian distribution. In addition to shot noise, there can be additional shot noise from the dark leakage current in the image sensor. Dark current is greatest at hot pixels within the image sensor, the variable dark charge of normal and hot pixels can be subtracted off, leaving only the shot noise, or random component, of the leakage. The noise caused by quantizing the pixels of an image to a number of discrete levels is known as quantization noise. It has a uniform distribution. Though it can be dependent, it will be signal independent if other noise sources are big enough to cause dithering. The grain of film is a signal-dependent noise, with similar statistical distribution to shot noise
5.
Contrast (vision)
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Contrast is the difference in luminance or colour that makes an object distinguishable. In visual perception of the world, contrast is determined by the difference in the color and brightness of the object. The human visual system is sensitive to contrast than absolute luminance. The maximum contrast of an image is the contrast ratio or dynamic range, the human contrast sensitivity function shows a typical band-pass filter shape peaking at around 4 cycles per degree with sensitivity dropping off either side of the peak. The high-frequency cut-off represents the optical limitations of the systems ability to resolve detail and is typically about 60 cycles per degree. The high-frequency cut-off is related to the density of the retinal photoreceptor cells. The low frequency drop-off is due to lateral inhibition within the ganglion cells. A typical retinal ganglion cell presents a centre region with either excitation or inhibition, one experimental phenomenon is the inhibition of blue in the periphery if blue light is displayed against white, leading to a yellow surrounding. The yellow is derived from the inhibition of blue on the surroundings by the center, since white minus blue is red and green, this mixes to become yellow. For example, in the case of computer displays, contrast depends on the properties of the picture source or file. For some screens the angle between the surface and the observers line of sight is also important. There are many definitions of contrast. Some include color, others do not, travnikova laments, Such a multiplicity of notions of contrast is extremely inconvenient. It complicates the solution of many applied problems and makes it difficult to compare the results published by different authors, various definitions of contrast are used in different situations. Here, luminance contrast is used as an example, but the formulas can also be applied to other physical quantities, in many cases, the definitions of contrast represent a ratio of the type Luminance difference Average luminance. The rationale behind this is that a difference is negligible if the average luminance is high. Below, some definitions are given. Weber contrast is defined as I − I b I b, with I and I b representing the luminance of the features, the measure is also referred to as Weber fraction, since it is the term that is constant in Webers Law
6.
Lenses for SLR and DSLR cameras
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This article is about photographic lenses for single-lens reflex film cameras and digital single-lens reflex cameras. Emphasis is on modern lenses for 35 mm film SLRs and for DSLRs with sensor sizes less than or equal to 35 mm, most SLR and DSLR cameras provide the option of changing the lens. This enables the use of lens that are best suited for the current photographic need, DSLRs became affordable around the mid-1990s, and have become extremely popular in recent years. Others, for example Olympus, chose to create a new lens mount. The Pentax SLR camera K-mount system is compatible to all previous lens generations from Pentax, including the latest digital SLRs like the K-3. A Pentax K-mount lens from the early 70s can be used on the newest Pentax DSLR although it may not provide features that are included in newer lenses. There are a few exceptions from the MZ and ZX series of Pentax film cameras that do not work with some of the older lenses, as implied by the above, lenses are only directly interchangeable within the mount system for which they are built. Mixing mounting systems requires an adapter, and most often results in such as loss of functionality. Further, in cases the adapter will require an additional optical element to correct for varied registration distances. Adapters may not be available to every combination of lens mount. The aperture of a lens is the opening that regulates the amount of light passes through the lens. It is controlled by a diaphragm inside the lens, which is in turn controlled either manually or by the circuitry in the camera body. The relative aperture is specified as an f-number, the ratio of the focal length to its effective aperture diameter. A small f-number like f/2.0 indicates a large aperture, aperture settings are usually not continuously variable, instead the diaphragm has typically 5–10 discrete settings. The normal full-stop f-number scale for modern lenses is as follows,1,1.4,2,2.8,4,5.6,8,11,16,22,32, but many lenses also allow setting to half-stop or third-stop increments. A slow lens might have an aperture from 5.6 to 11. Fast lenses are by definition larger than slow lenses, and typically cost more, the focal length of a lens, together with the size of the image sensor in the camera, determines the angle of view. A lens is considered to be a lens, in terms of its angle of view on a camera
7.
Vignetting
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In photography and optics, vignetting is a reduction of an images brightness or saturation at the periphery compared to the image center. The word vignette, from the root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a portrait which is clear in the center. A similar effect occurs when photographing projected images or movies off a projection screen, vignetting is often an unintended and undesired effect caused by camera settings or lens limitations. However, it is deliberately introduced for creative effect, such as to draw attention to the center of the frame. A photographer may deliberately choose a lens which is known to produce vignetting to obtain the effect, when using superzoom lenses, vignetting may occur all along the zoom range, depending on the aperture and the focal length. However, it may not always be visible, except at the widest end, in these cases, vignetting may cause an exposure value difference of up to 0. 75EV. There are several causes of vignetting and this has the effect of changing the entrance pupil shape as a function of angle. Darkening can be gradual or abrupt – the smaller the aperture, when some points on an image receives no light at all due to mechanical vignetting, then this results in an restriction of the field of view – parts of the image are then completely black. This type of vignetting is caused by the dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the lens opening for off-axis incident light. The result is a decrease in light intensity towards the image periphery. Optical vignetting is sensitive to the aperture and can often be cured by a reduction in aperture of 2–3 stops. Unlike the previous types, natural vignetting is not due to the blocking of light rays, the falloff is approximated by the cos4 or cosine fourth law of illumination falloff. Here, the light falloff is proportional to the power of the cosine of the angle at which the light impinges on the film or sensor array. Wideangle rangefinder designs and the designs used in compact cameras are particularly prone to natural vignetting. Telephoto lenses, retrofocus wideangle lenses used on SLR cameras, a gradual grey filter or postprocessing techniques may be used to compensate for natural vignetting, as it cannot be cured by stopping down the lens. Some modern lenses are designed so that the light strikes the image parallel or nearly so
8.
Exposure (photography)
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In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value, in photographic jargon, an exposure generally refers to a single shutter cycle. For the same speed, the accumulated photometric exposure should be similar in both cases. A photograph may be described as underexposed when it has a loss of detail, that is. As the adjacent image shows, these terms are technical ones, there are three types of settings, manual, automatic and exposure compensation. Radiant exposure of a surface, denoted He and measured in J/m2, is given by H e = E e t, where Ee is the irradiance of the surface, measured in W/m2, t is the exposure duration, measured in s. Luminous exposure of a surface, denoted Hv and measured in lx⋅s, is given by H v = E v t, many photographic materials are also sensitive to invisible light, which can be a nuisance, or a benefit. The use of units is appropriate to characterize such sensitivity to invisible light. In sensitometric data, such as curves, the log exposure is conventionally expressed as log10. Photographers more familiar with base-2 logarithmic scales can convert using log2 ≈3.32 log10, correct exposure may be defined as an exposure that achieves the effect the photographer intended. A more technical approach recognises that a film has a physically limited useful exposure range. If, for any part of the photograph, the exposure is outside this range. In a very simple model, for example, out-of-range values would be recorded as black or white rather than the precisely graduated shades of colour and this ensures that no significant information is lost during capture. However, it is much easier to discard recorded information during post processing than to try to re-create unrecorded information. In a scene with strong or harsh lighting, the ratio between highlight and shadow luminance values may well be larger than the ratio between the maximum and minimum useful exposure values. A photograph may be described as underexposed when it has a loss of detail, that is. In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure, manual exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the Zone System. A camera in automatic exposure mode automatically calculates and adjusts exposure settings to match the subjects mid-tone to the mid-tone of the photograph, for most cameras this means using an on-board TTL exposure meter
9.
Chromatic aberration
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In optics, chromatic aberration is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light, the refractive index of transparent materials decreases with increasing wavelength in degrees unique to each. Since the focal length f of a lens is dependent on the index n. There are two types of aberration, axial, and transverse. Axial aberration occurs when different wavelengths of light are focused at different distances from the lens, transverse aberration occurs when different wavelengths are focused at different positions in the focal plane. The acronym LCA is used, but ambiguous, and may refer to either longitudinal or lateral CA, for clarity and these two types have different characteristics, and may occur together. Axial CA occurs throughout the image and is specified by optical engineers, optometrists, and vision scientists in the unit of focus known widely as diopters, transverse CA does not occur in the center, and increases towards the edge, but is not affected by stopping down. In the earliest uses of lenses, chromatic aberration was reduced by increasing the length of the lens where possible. For example, this could result in extremely long telescopes such as the very long aerial telescopes of the 17th century, isaac Newtons theories about white light being composed of a spectrum of colors led him to the conclusion that uneven refraction of light caused chromatic aberration. There exists a point called the circle of least confusion, where chromatic aberration can be minimized and it can be further minimized by using an achromatic lens or achromat, in which materials with differing dispersion are assembled together to form a compound lens. The most common type is a doublet, with elements made of crown. This reduces the amount of chromatic aberration over a range of wavelengths. By combining more than two lenses of different composition, the degree of correction can be increased, as seen in an apochromatic lens or apochromat. Similarly, the benefit of apochromats is not simply that they focus 3 wavelengths sharply, many types of glass have been developed to reduce chromatic aberration. These are low dispersion glass, most notably, glasses containing fluorite and these hybridized glasses have a very low level of optical dispersion, only two compiled lenses made of these substances can yield a high level of correction. The use of achromats was an important step in the development of the optical microscope, an alternative to achromatic doublets is the use of diffractive optical elements. Diffractive optical elements are able to generate arbitrary complex wave fronts from a sample of material which is essentially flat. Diffractive optical elements have negative characteristics, complementary to the positive Abbe numbers of optical glasses
10.
Lens flare
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Lens flare refers to a phenomenon wherein light is scattered or flared in a lens system, often in response to a bright light, producing a desirable effect on the image. This happens through unintentional image formation mechanisms, such as internal reflection, lenses with large numbers of elements such as zooms tend to exhibit greater lens flare, as they contain multiple surfaces at which unwanted internal scattering occurs. These mechanisms differ from the image formation mechanism, which depends on rays from the refraction of the image itself. Flare manifests itself in two ways, as visible artifacts, and as a haze across the image, the haze makes the image look washed out by reducing contrast and color saturation. Visible artifacts, usually in the shape of the iris, are formed when light follows a pathway through the lens that contains one or more reflections from the lens surfaces. Flare is particularly caused by bright light sources. Most commonly, this occurs when shooting into the sun, and is reduced by using a hood or other shade. For good-quality optical systems, and for most images, flare is an effect that is widely distributed across the image and thus not visible. The spatial distribution of the lens flare typically manifests as several starbursts, rings, the specific spatial distribution of the flare depends on the shape of the aperture of the image formation elements. For example, if the lens has a 6-bladed aperture, the flare may have a hexagonal pattern, such internal scattering is also present in the human eye, and manifests in an unwanted veiling glare most obvious when viewing very bright lights or highly reflective surfaces. In some situations, eyelashes can also create flare-like irregularities, although these are technically diffraction artifacts, when a bright light source is shining on the lens but not in its field of view, lens flare appears as a haze that washes out the image and reduces contrast. This can be avoided by shading the lens using a lens hoods, in a studio, a gobo or set of barn doors can be attached to the lighting to keep it from shining on the camera. Filters can be attached to the lens which will also minimise lens flare. When using a lens, as is common in analog cinematography. This is most commonly seen in car headlights in a dark scene, a lens flare is often deliberately used to invoke a sense of drama. For both of these reasons artificial lens flare is an effect in various graphics editing programs. Lens flare was one of the first special effects developed for computer graphics because it can be imitated using relatively simple means, more sophisticated rendering techniques have been developed based on ray tracing or photon mapping. S. Costume designer Rita Riggs purposefully would sometimes dress Bea Arthur in Maude in a red that would flare to make a statement, director J. J. Abrams added numerous lens flares to the 2009 film Star Trek by aiming powerful off-camera light sources at the lens
11.
JPEG
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JPEG is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between size and image quality. JPEG typically achieves 10,1 compression with little loss in image quality. JPEG compression is used in a number of file formats. These format variations are not distinguished, and are simply called JPEG. The term JPEG is an initialism/acronym for the Joint Photographic Experts Group, the MIME media type for JPEG is image/jpeg, except in older Internet Explorer versions, which provides a MIME type of image/pjpeg when uploading JPEG images. JPEG files usually have an extension of. jpg or. jpeg. JPEG/JFIF supports a maximum size of 65, 535×65,535 pixels. JPEG stands for Joint Photographic Experts Group, the name of the committee created the JPEG standard. The Joint stood for ISO TC97 WG8 and CCITT SGVIII, in 1987 ISO TC97 became ISO/IEC JTC1 and in 1992 CCITT became ITU-T. Currently on the JTC1 side JPEG is one of two sub-groups of ISO/IEC Joint Technical Committee 1, Subcommittee 29, Working Group 1 – titled as Coding of still pictures, on the ITU-T side ITU-T SG16 is the respective body. The original JPEG group was organized in 1986, issuing the first JPEG standard in 1992, which was approved in September 1992 as ITU-T Recommendation T.81 and in 1994 as ISO/IEC 10918-1. The JPEG standard specifies the codec, which defines how an image is compressed into a stream of bytes and decompressed back into an image, the Exif and JFIF standards define the commonly used file formats for interchange of JPEG-compressed images. JPEG standards are formally named as Information technology – Digital compression, ISO/IEC10918 consists of the following parts, Ecma International TR/98 specifies the JPEG File Interchange Format, the first edition was published in June 2009. The JPEG compression algorithm is at its best on photographs and paintings of scenes with smooth variations of tone. For web usage, where the amount of used for an image is important. JPEG/Exif is also the most common format saved by digital cameras, on the other hand, JPEG may not be as well suited for line drawings and other textual or iconic graphics, where the sharp contrasts between adjacent pixels can cause noticeable artifacts. Such images may be saved in a lossless graphics format such as TIFF, GIF, PNG
12.
Color
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Color or colour is the characteristic of human visual perception described through color categories, with names such as red, yellow, purple, or blue. This perception of color derives from the stimulation of cells in the human eye by electromagnetic radiation in the spectrum of light. Color categories and physical specifications of color are associated with objects through the wavelength of the light that is reflected from them and this reflection is governed by the objects physical properties such as light absorption, emission spectra, etc. By defining a color space, colors can be identified numerically by coordinates, there may also be more than three color dimensions in other color spaces, such as in the CMYK color model, wherein one of the dimensions relates to a colours colorfulness). The photo-receptivity of the eyes of species also varies considerably from our own. Honeybees and bumblebees for instance have trichromatic color vision sensitive to ultraviolet but is insensitive to red, papilio butterflies possess six types of photoreceptors and may have pentachromatic vision. The most complex color vision system in the kingdom has been found in stomatopods with up to 12 spectral receptor types thought to work as multiple dichromatic units. The science of color is sometimes called chromatics, colorimetry, or simply color science and it includes the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range. Electromagnetic radiation is characterized by its wavelength and its intensity, when the wavelength is within the visible spectrum, it is known as visible light. Most light sources emit light at different wavelengths, a sources spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, in each such class the members are called metamers of the color in question. The table at right shows approximate frequencies and wavelengths for various pure spectral colors, the wavelengths listed are as measured in air or vacuum. A common list identifies six main bands, red, orange, yellow, green, blue, Newtons conception included a seventh color, indigo, between blue and violet. It is possible that what Newton referred to as blue is nearer to what today is known as cyan, the color of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Some objects not only light, but also transmit light or emit light themselves. This effect is known as color constancy, opaque objects that do not reflect specularly have their color determined by which wavelengths of light they scatter strongly. If objects scatter all wavelengths with roughly equal strength, they appear white, if they absorb all wavelengths, they appear black. Opaque objects that reflect light of different wavelengths with different efficiencies look like mirrors tinted with colors determined by those differences
