question: If you look at a dark phone screen you can choose whether you see what's happening on the screen or you can use the screen as a mirror. How does that work?
comment: Focus. You're either focusing your eyes on the screen itself and the light emitted from the screen, or you're focusing on a separate light source, i.e., the reflection.
passages: An unilluminated LCD must be lit from the front. To use ambient light, the liquid crystal itself is sandwiched between a polarization filter and a reflective surface. The mirror makes the display opaque so it cannot be illuminated from the back. Most often a light source is placed around the perimeter of the LCD.
   Visual Image screenless display includes any image that the eye can perceive. The most common example of Visual Image screenless display is a hologram. In these cases, light is reflected off some intermediate object (hologram, LCD panel, or cockpit window) before it reaches the retina. In the case of LCD panels the light is refracted from the back of the panel, but is nonetheless a reflected source. Google has proposed a similar system to replace the screens of tablet computers and smartphones.
  
  Section::::Retinal display.
   Android distributions by other manufacturers typically use different lock screen designs than what stock Android utilizes; newer versions of HTC's Sense used a metallic ring dragged from the bottom of the screen to unlock the phone, and also allows users to launch apps by dragging their respective shortcut icon into the ring instead. On recent Samsung devices, the lock screen involves dragging in any direction from any location on the screen (with TouchWiz Nature devices, such as the Galaxy S III and S4, it is also accompanied by a visual effect such as a pond ripple or lens flare); similarly to HTC's lock screen, app shortcuts can be dragged up from the bottom of the screen to unlock directly into them. 
   Simple types of LCDs such as in pocket calculators are built without an internal light source, requiring external light sources to convey the display image to the user. Most LCD screens, however, are built with an internal light source. Such screens consist of several layers. The backlight is usually the first layer from the back. Light valves then vary the amount of light reaching the eye, by blocking its passage in some way. Most use a fixed polarizing filter and a switching one, to block the undesired light.
  
  Section::::Light source types.
  
  The light source can be made up of:
   For a non-ELP backlight to produce even lighting, which is critical for displays, the light is first passed through a lightguide - a specially designed layer of plastic that diffuses the light through a series of unevenly spaced bumps. The density of bumps increases further away from the light source according to a diffusion equation. The diffused light then travels to either side of the diffuser; the front faces the actual LCD panel, the back has a reflector to guide otherwise wasted light back toward the LCD panel. The reflector is sometimes made of aluminum foil or a simple white-pigmented surface.
   Since LCD panels produce no light of their own, they require external light to produce a visible image. In a transmissive type of LCD, this light is provided at the back of the glass stack and is called the backlight. While passive-matrix displays are usually not backlit (e.g. calculators, wristwatches), active-matrix displays almost always are.
  
  The common implementations of LCD backlight technology are:
   Your Phone
  
  Your Phone is an app developed by Microsoft for Windows 10 to connect Android and iOS devices. It was first shown by Microsoft during Build 2018. It allows to see recent photos taken on a Android Phone directly on a Windows 10 PC/Tablet. It can also be used for sending SMS Messages right from a PC. It comes preinstalled with Windows 10 October 2018 Update and replaces the old Phone Companion. Your Phone app can be used to mirror the screen of an Android Phone, however currently only few phones are supported and the feature is in beta.
   Section::::Design.:Barrier position.
  
  Note that the parallax barrier may also be placed behind the LCD pixels. In this case, light from a slit passes the left image pixel in the left direction, and vice versa. This produces the same basic effect as a front parallax barrier.
  
  Section::::Design.:Techniques for switching.
   In late 2008, a new type of camera emerged, called a "mirrorless interchangeable-lens camera". It is technically a DSLR camera that doesn’t require a reflex mirror, a key component of the former. While a typical DSLR has a mirror that reflects light from the lens up to the optical viewfinder, in a mirrorless camera, there is no optical viewfinder. The image sensor is exposed to light at all times, gives the user a digital preview of the image either on the built-in rear LCD screen or an electronic viewfinder (EVF).
   BULLET::::- The newest and most convenient displays, in products such as the Nintendo 3DS, HTC Evo 3D, and LG Optimus 3D, do not have the parallax barrier in front of the pixels, but behind the pixels and in front of the backlight. The entire LCD matrix is therefore exposed to both eyes, but as seen from each eye's position only one of the interlaced images in it is backlit. Glare from the visibly lit pixel columns tends to make the adjacent unlit columns less noticeable.
  
  BULLET::::- Pros
  
  BULLET::::- Clear image
  
  BULLET::::- Largest viewing angle
  
  BULLET::::- Cons
   BULLET::::- "Tetra Lockscreen" was a Windows Phone application designed to work with the new lock screen APIs of Windows Phone 8.1, it had a stopwatch, weather, agenda, and maps widgets that provided information collected from the user's applications. Tetra Lockscreen served to show developers what they can do with Windows Phone's lock screen.
   Tru Black is a technology developed by Sony which allows a better visualization of the screen, even when there is too much light. It enables LCD screens to automatically change the display contrast in order to enhance the controlling reflectance. In other words, when light hits a display with Tru Black technology, the screen turns opaque as a means to improve the visualization of the content. 
  
  Section::::Features.:Panorama.
   A 2014 Microsoft application allowed women to "try on" clothing using a virtual mirror with the swipe of their hand.
  
  Many color contact sites feature a similar virtual try-on environments to simulate the look a user will achieve when actually wearing the contact lenses. The best product in this domain is Purple Patch from Wifin Technologies. The Freshlook color studio, and Colorful Eyes eye color changer are both examples of color contact sites that feature this cutting edge technology.
   On the Start screen, 3D Touch allows the user to hover their finger over a live tile and for that tile to "explode" revealing many smaller tiles contained within – this feature was dubbed MixView. On the near-final prototype hardware, MixView worked with very few apps (only the stock Phone app and Internet Explorer), but the original idea was to allow for developers to make use of exploding live tiles in their own apps.
  
  Other 3D Touch features include the ability to:
  
  BULLET::::- keep the screen of the phone on whilst the user is holding the device
   The phone has a stainless steel full metal body with a half-mirror screen. The front of the phone appears to be a mirror when the screen is off. When the phone is closed there is a multi-functional scroll key.
  
  The phone can also be used as a camera, with a dedicated button for taking pictures. The maximum resolution for pictures is 1600x1200 px. The camera has auto focus certified by Schneider-Kreuznach.
   The optical effect of a TN device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, TN displays with low information content and no backlighting are usually operated between crossed polarizers such that they appear bright with no voltage (the eye is much more sensitive to variations in the dark state than the bright state). As most of 2010-era LCDs are used in television sets, monitors and smartphones, they have high-resolution matrix arrays of pixels to display arbitrary images using backlighting with a dark background. When no image is displayed, different arrangements are used. For this purpose, TN LCDs are operated between parallel polarizers, whereas IPS LCDs feature crossed polarizers. In many applications IPS LCDs have replaced TN LCDs, in particular in smartphones such as iPhones. Both the liquid crystal material and the alignment layer material contain ionic compounds. If an electric field of one particular polarity is applied for a long period of time, this ionic material is attracted to the surfaces and degrades the device performance. This is avoided either by applying an alternating current or by reversing the polarity of the electric field as the device is addressed (the response of the liquid crystal layer is identical, regardless of the polarity of the applied field).
   If the phone's camera app is activated, the screen is relayed to the watch, and the user can control the shutter, and view photos on the watch. Third-party applications support using the phone camera as a streaming device, or more varied camera control.
  
  Events appear as cards on screen. "OK Google, show my agenda" will display the user's agenda. Watch faces also support marking out appointments (for instance with contrasting color to show periods with an appointment, and/or illuminating a lighted "count-down" line for upcoming appointments.
   For the Chinese market, the Nokia X6 was announced, which will get a worldwide release as the Nokia 6.1 Plus. Another Chinese market device called the Nokia X5 was later announced. Both these have bezel-less screens.
  
  In late August 2018, HMD acquired PureView branding, the famous imaging technology brand which had previously been implemented on high-end Nokia/Lumia smartphones since the year 2012 in Nokia 808 PureView.
  
  Section::::Software.
   The Hydrogen One's design is influenced by the lens mounts of Red's cinema cameras, with aluminum and Kevlar materials. scalloped grips, and texturing; hardware buttons and a fingerprint reader are integrated into some of the scallops. It features a 5.7-inch 1440p LCD with 3D display technology branded as "4V" ("4 View"). Developed by the Red-backed startup Leia, it uses nanostructures to diffract the display's backlight into a light field, creating a "holographic" depth effect.
   In late 2010, Fujifilm announced hybrid viewfinder of optical viewfinder and electronical viewfinder in one viewfinder for its highend compact cameras. There is a half mirror prism that reflect data from LCD to the optical viewfinder, so we can see both the shooting frame and the shooting data. A button can change the hybrid function to electronical viewfinder by blocking the image through the optical viewfinder with moving a half mirror prism to be a straight up mirror.
   Following the acquisition of the Nokia Devices & Services division by Microsoft, the U.S. software company acquired the PureView name and trademark, but not the related imaging technology, which remained with Nokia, who thereon licensed it to Microsoft.
  
  Since Microsoft's ownership, the technology has only been used in their late-2015 flagship Lumia 950.
  
  In late August 2018, HMD Global reportedly acquired the ownership of the PureView trademark and related assets not under the Nokia ownership.
  
  On the 24 February 2019, HMD Global, the exclusive licensee of Nokia-branded phones, launched Nokia 9 PureView at the 2019 Mobile World Congress in Barcelona.
   Sephora, the beauty specialty retailer, and ModiFace, an augmented reality virtual makeover technology provider, today announced the launch of a 3D Augmented Reality Mirror that can simulate cosmetics on a user's face in real-time and in 3D. The new patented technology, created by ModiFace, tracks the precise location of a user's facial features and applies eye shadow colors directly on the video feed from a camera.
   There are various options and filters available when taking photos or recording video. In addition to the "Normal" mode, there is a "Low-Light" option, which is useful when taking photos and recording video in dark lighting conditions. Other options include manual controls such as the color type (normal, black and white, sepia, negative or solarize), sharpness, contrast and brightness. Real-time photo filters are also available including "Sparkle", which adds moving stars to the photo, "Dream", which adds a dream-like bright light to the photo, "Pinhole", which lightens the center of the screen and darkens the edges and "Mystery", which adds a random finish to the photo. There is also a special mode called "Merge", which takes a photo of the user facing the inner camera and merges the photo from someone facing the outer cameras.
   BULLET::::- It is "virtual", meaning that the image appears to be behind the mirror, and cannot be projected onto a screen.
   Section::::Technologies.:Infrared acrylic projection.
  
  A translucent acrylic sheet is used as a rear-projection screen to display information. The edges of the acrylic sheet are illuminated by infrared LEDs, and infrared cameras are focused on the back of the sheet. Objects placed on the sheet are detectable by the cameras. When the sheet is touched by the user, the deformation results in leakage of infrared light which peaks at the points of maximum pressure, indicating the user's touch location. Microsoft's PixelSense tablets use this technology.
  
  Section::::Technologies.:Optical imaging.
  