question: If white reflects the whole color spectrum, why are mirrors or other reflective materials often used to reflect sun rays? e.g. in some kinds or solar plants or to keep cars cool
comment: A white surface is white because it reflects all the colors of light, but mixes them up into a big jumble of light because the surface isn't smooth. A mirror reflects all colors of light, but doesn't mix it up so the light rays leave in the same way they hit the mirror, meaning you see the individual spots of color that make an image. Imagine you have a 400x400 pixel image. Each of the 160000 pixels has its own color, and when they are properly arranged they show you a picture. If you take those exact same 160000 pixels but mix them up and arrange them randomly, you'll just get a white(ish) square.
passages: While there is no single, unique specification of "white light", there is indeed a unique specification of "white object", or, more specifically, "white surface".
  
  A perfectly white surface diffusely reflects (scatters) all visible light that strikes it, without absorbing any, irrespective of the light's wavelength or spectral distribution.
  
  Since it does not absorb any of the incident light, white is the lightest possible color.
  
  If the reflection is not diffuse but rather specular, this describes a mirror rather than a white surface.
   Section::::Promotional programs.:White Roof Project.
  
  White Roof Project is a nationwide initiative that educates and empowers individuals to coat rooftops white. The program's outreach has helped complete white roof projects in more than 20 US states and five countries, engaged thousands in volunteer projects, and sponsored the coating of hundreds of nonprofit and low-income rooftops.
  
  Section::::Urban heat island effect.
   If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared – see the diagram at the left. Most household radiators are painted white, which is sensible given that they are not hot enough to radiate any significant amount of heat, and are not designed as thermal radiators at all – instead, they are actually convectors, and painting them matt black would make little difference to their efficacy. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature (meaning the term "black body" does not always correspond to the visually perceived color of an object). These materials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence.
   Section::::Experimental considerations.
   Modified digital cameras can detect some ultraviolet, all of the visible and much of the near infrared spectrum, as most digital imaging sensors are sensitive from about 350 nm to 1000 nm. An off-the-shelf digital camera contains an infrared hot mirror filter that blocks most of the infrared and a bit of the ultraviolet that would otherwise be detected by the sensor, narrowing the accepted range from about 400 nm to 700 nm.
   In Nordic countries, this is the opposite: the houses are painted in black to better absorb the irradiation heat. Basalt is an interesting material as it is naturally black and exhibits high thermal storage capacity.
  
  Section::::Elements.:Solar tracker.
  
  Part or all of the house can track the Sun's race in the sky to catch its light. The Heliotrope, the first positive energy house in the world, rotates to catch the sunlight, converted into electricity by photovoltaic modules, heating the house through the translucent glass.
   Section::::Solar Gain in Opaque Building Components.
   The late 20th century has seen the development of adaptive optics and lucky imaging to overcome the problems of seeing, and reflecting telescopes are ubiquitous on space telescopes and many types of spacecraft imaging devices.
  
  Section::::Technical considerations.
   Other notable radiative cooling strategies include dielectric films on metal mirrors, and polymer or polymer composites on silver or aluminum films. In 2014, researchers developed a multi-layer thermal photonic structure which selectively emits long wavelength infrared radiation into space, and can achieve 5°C sub-ambient cooling under direct sunlight. Silvered polymer films with solar reflectances of 0.97 and thermal emittance of 0.96, which remain 11°C cooler than commercial white paints under the mid-summer sun, have been also been reported. Researchers have also explored designs with dielectric silicon dioxide or silicon carbide particles embedded in polymers that are translucent in the solar wavelengths and emissive in the infrared. In 2017, an example of this design with resonant polar silica microspheres randomly embedded in a polymeric matrix, was reported. The material is translucent to sunlight and has infrared emissivity of 0.93 in the infrared atmospheric transmission window. When backed with silver coating, the material achieved a midday radiative cooling power of 93 W/m under direct sunshine along with high-throughput, economical roll-to-roll manufacturing.
   A range of spectral distributions of light sources can be perceived as white—there is no single, unique specification of "white light". For example, when you buy a "white" light bulb, you might buy one labeled 2700K, 6000K, etc., which produce light having very different spectral distributions, and yet this will not prevent you from identifying the color of objects that they illuminate.
  
  Section::::Scientific understanding (Color science).:White objects.
   Section::::Surface type.
  
  Given that reflectance is a directional property, most surfaces can be divided into those that give specular reflection and those that give diffuse reflection:
  
  BULLET::::- for specular surfaces, such as glass or polished metal, reflectance will be nearly zero at all angles except at the appropriate reflected angle; that is, reflected radiation will follow a different path from incident radiation for all cases other than radiation normal to the surface;
  
  BULLET::::- for diffuse surfaces, such as matte white paint, reflectance is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to be Lambertian.
   Section::::Applications.:Technology.:Seasonal lighting.
  
  Due to its location in a steep-sided valley, the Italian town of Viganella gets no direct sunlight for seven weeks each winter. In 2006 a €100,000 computer-controlled mirror, 8×5 m, was installed to reflect sunlight into the town's piazza. In early 2007 the similarly situated village of Bondo, Switzerland, was considering applying this solution as well. In 2013, mirrors were installed to reflect sunlight into the town square in the Norwegian town of Rjukan. Mirrors can be used to produce enhanced lighting effects in greenhouses or conservatories.
  
  Section::::Applications.:Architecture.
   Surface texturing is another option for increasing absorption, but increases costs. By applying a texture to the active material's surface, the reflected light can be refracted into striking the surface again, thus reducing reflectance.For example, black silicon texturing by reactive ion etching(RIE) is an effective and economic approach to increase the absorption of thin-film silicon solar cells. A textured backreflector can prevent light from escaping through the rear of the cell.
   Metal Halide arc lamp: Primarily developed for use in film and television lighting where a high temporal stability and daylight colour match are required, metal halide arc lamps are also used in solar simulation.
  
  QTH: quartz tungsten halogen lamps offer spectra which very closely match black body radiation, although typically with a lower color temperature than the sun.
  
  LED: light-emitting diodes have recently been used in research laboratories to construct solar simulators, and may offer promise in the future for energy-efficient production of spectrally tailored artificial sunlight.
   Reflection of 100% of incident light at all wavelengths is a form of uniform reflectance, so white is an achromatic color, meaning a color without hue. The color stimulus produced by the perfect diffuser is usually considered to be an achromatic stimulus for all illuminants, except for those whose light sources appear to be highly chromatic.
   Suspended nanoparticles in stained glass prevent UV rays from causing chemical reactions that change image colors. A set of stained glass color reference chips is planned to be used to calibrate the color cameras for the 2019 ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars.
   Modified digital cameras can detect some ultraviolet, all of the visible and much of the near infrared spectrum, as most digital imaging sensors are sensitive from about 350 nm to 1000 nm. An off-the-shelf digital camera contains an infrared hot mirror filter that blocks most of the infrared and a bit of the ultraviolet that would otherwise be detected by the sensor, narrowing the accepted range from about 400 nm to 700 nm. Replacing a hot mirror or infrared blocking filter with an infrared pass or a wide spectrally transmitting filter allows the camera to detect the wider spectrum light at greater sensitivity. Without the hot-mirror, the red, green and blue (or cyan, yellow and magenta) elements of the color filter array placed over the sensor elements pass varying amounts of ultraviolet and infrared which may be recorded in any of the red, green or blue channels depending on the particular sensor in use and on the dyes used in the Bayer filter. A converted full-spectrum camera can be used for ultraviolet photography or infrared photography with the appropriate filters.
   An interference layer may be located on the first surface of the glass substrate. It can be used to tailor the reflectance. It may also be designed for diffuse reflectance of near-ultraviolet radiation, in order to prevent it from passing through the glass substrate. This substantially enhances the overall reflection of near-ultraviolet radiation from the mirror. The interference layer may be made of several materials, depending on the desired refractive index, such as titanium dioxide.
  
  Section::::Solar thermal applications.
   As a result of these favorable "differential kinetics", DSSCs work even in low-light conditions. DSSCs are therefore able to work under cloudy skies and non-direct sunlight, whereas traditional designs would suffer a "cutout" at some lower limit of illumination, when charge carrier mobility is low and recombination becomes a major issue. The cutoff is so low they are even being proposed for indoor use, collecting energy for small devices from the lights in the house.
   Fresnel reflectors are sometimes regarded as a technology with a worse output than other methods. The cost efficiency of this model is what causes some to use this instead of others with higher output ratings. Some new models of Fresnel Reflectors with Ray Tracing capabilities have begun to be tested and have initially proved to yield higher output than the standard version.
  
  Section::::Current technology.:Dish Stirling.
   Suitable digital cameras for reflected UV photography have been reported to be the (unmodified) Nikon D70 or D40 DSLRs, but many others are suitable after having their internal UV and IR blocking filter removed. The Fujifilm FinePix IS Pro digital SLR camera is purpose-designed for ultraviolet (and infrared) photography, with a frequency response rated from 1000-380 nm, although it also responds to somewhat longer and shorter wavelengths. Silicon (from which DSLR sensors are made) can respond to wavelengths between 1100-190 nm.
  
  Section::::Equipment and techniques.:UV induced fluorescence photography.
   Sunlight acts as a bleach through a similar process. High energy photons of light, often in the violet or ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless.
  
  Some detergents go one step further; they contain fluorescent chemicals which glow, making the fabric look literally whiter than white.
  
  Section::::In the natural world.
  
  Section::::In the natural world.:Astronomy.
   Digital sensors and photographic films can be made to record non-visible ultraviolet (UV) and infrared (IR) radiation. In each case, they generally require special equipment: converted digital cameras, specific filters, highly transmitting lenses, etc. For example, most photographic lenses are made of glass and will filter out most ultraviolet light. Instead, expensive lenses made of quartz must be used. Infrared films may be shot in standard cameras using an infrared pass filters, although focus must compensate for the infrared focal point.
   Protection from UV light can be accomplished by film that addresses only the UV spectrum. As UV is one of the main sources of fading, it can prolong the life of fixtures and fittings. Basic colorless clear films reject (absorbs) some UV radiation up to 380 nm, and depending on the manufacturer, UV inhibitors are added to the polyester, with specialist films offering protection to 400 nm. Tinted films are necessary for protection to 500 nm.
   Although inefficient, incandescent light bulbs have an advantage in applications where accurate color reproduction is important, since the continuous blackbody spectrum emitted from an incandescent light-bulb filament yields near-perfect color rendition, with a color rendering index of 100 (the best possible). White-balancing is still required to avoid too "warm" or "cool" colors, but this is a simple process that requires only the color temperature in kelvins as input for modern, digital visual reproduction equipment such as video or still cameras unless it is completely automated. The color-rendering performance of incandescent lights cannot be matched by LEDs or fluorescent lights, although they can offer satisfactory performance for non-critical applications such as home lighting. White-balancing such lights is therefore more complicated, requiring additional adjustments to reduce for example green-magenta color casts, and even when properly white-balanced, the color reproduction will not be perfect.
  