Use Parallel Viewing Method for Easy 3d Without Eye Strain

Various 3D media all work by giving the illusion of depth by presenting offset images separately to each eye. The human brain then combines these images to form a 3D image. Techniques for achieving this include glasses to combine two images, glasses which filter offset images from a single source or to have the light source split so that each eye sees a different offset image.

Separate Images:

This technique used two different images, each shown to one eye to achieve stereoscopy. This technique was invented y Sir Charles Wheatstone in 1838. This has been used in early 3D picture cards, penny arcade machines, Viewmaster toys and world war two surveillance pictures. The viewers used are normally designed for one person to view the image at a time.

Using positive curvature (magnifying) lenses, the focus point of the image is changed from its short distance (about 30 to 40 cm) to a virtual distance at infinity. This allows the focus of the eyes to be consistent with the parallel lines of sight, greatly reducing eye strain. The card image is magnified, offering a wider field of view and the ability to examine the detail of the photograph. The viewer provides a partition between the images, avoiding a potential distraction to the user.

Disadvantages of stereo cards, slides or any other hard copy or print are that generally only one person at a time can view the media and the two images are likely to receive differing wear, scratches and other decay. These result in stereo artefacts when the images are viewed which compete in the mind resulting in a distraction from the 3D effect, eye strain and headaches. Because of these disadvantages, this system is not widely used.

Figure 1 shows some examples of side-by-side viewers and figure 2 is an example of a side-by-side picture.

Figure 1: Side-by-side viewers

Figure 2: Exmple of a side-by-side picture.

Filtered Offset Images:

This technique relies on the separation of two images superimposed on top of each other. This is done using colour (anaglyphs and Dolby 3D), polarized light (RealD and Imax 3D) or by using synchronized shutters (Active 3D).

Anaglyphs:

This technique uses colour layers which are filtered using coloured glasses. Anaglyph images are made up of an image in one colour for one eye overlaid with an offset image in another colour for the other eye. For example to produce a red/blue picture, two cameras shoot a scene simultaneously. They can either have a red filter, and a cyan filter, or the green and blue can be removed from the left picture and the red from the right picture. The two pictures are then superimposed to produce the anaglyph image. When viewed through red-cyan anaglyph glasses, the red blue and green light appears dark in the left eye and the red lens over the left eye allows only the red part of the anaglyph image through to that eye, while the cyan (blue/green) lens over the right eye allows only the blue and green parts of the image through to that eye. Portions of the image that are red will appear dark through the cyan filter, while portions of colours composed only of green and blue will appear dark through the red filter. Each eye therefore sees only the perspective it is supposed to see. The brain then stitches the two images together to form a 3D image. An example of an anaglyph image is shown in figure 3.

Figure 3: A red/cyan anaglyph picture, taken outside my spare room in Carterton, UK.

This technique was first developed in 1853 by Wilhelm Rollmann in Leipzig who used red and blue lines on a black background to produce 3D images viewable using red and blue glasses. In 1858 Joseph D'Almeida produced an anaglyph magic lantern show using red and green filters and the audience wearing red and green goggles. The first printed anaglyphs were produced in 1891 by Louis Ducas Hauron by printing two negatives on the same paper, on in red and one in either green or blue. The first anaglyph film was presented on June 10th, 1915 by Edwin S. Porter and William E. Waddell in the Astor Theatre in New York using red-green glasses. It consisted of three reels of films showing rural scenes, test shots of Marie Doro, a segment of John Mason playing a number of passages from Jim the Penman (a film released by Famous Players-Lasky that year, but not in 3-D), Oriental dancers, and a reel of footage of Niagara Falls. The first commercially produced anaglyph films was The Power of Love, which premiered at the Ambassador Hotel Theatre in Los Angeles on September 27, 1922. Further films were produced in the early 1920s, but fell out of fashion in the late 20's. 3D films made a comeback in the early 1950s, but by this time the polaroid filters had become popular, although many were also release as anaglyph as this required only one projector whereas the polaroid required two. Anaglyphs were also used in comic books which used disposable cardboard glasses.

A second wave of anaglyph movies occurred in the early 1980s with such films as Jaws 3D, , Amityville 3D and Friday 13th Part III. Anaglyphs also have the advantage that they can be shown on any colour TV and in the 1980s some 3D films were shown as anaglyphs. Since then, polaroid systems have largely taken over the 3D format in cinemas and traditional anaglyph pictures and movies can be found in places such as children's comics and books, molecular modelling software and on the internet. An exception to this is the Dolby 3D and Infitec systems. These use red, green and blue light of different frequencies for each eye. This makes it possible to have full colour images shown to each eye and the quality is comparable to the polaroid systems in terms of picture quality.

The colours schemes used are given below. Most use one of the six possible combinations of colours possible for pure anaglyphs: red-green, red-blue, green-blue (extremely rare), red-cyan (most common), green-magenta or blue-yellow. These all depend on not allowing light of the same colour into both eyes, which would cause 'ghosting'. Some later propriety technique deliberately allow some ghosting to occur as this supposedly makes the image more pleasant to view, but they are all based on the six basic combinations. In the cases of lighter colours being paired with darker ones (e.g. red-cyan, red-green or especially yellow-blue), the lighter colour may need to be darkened, both to allow both eyes to work equally and to avoid introducing a possibly undesired Pulfrich effect (a psychophysical percept wherein lateral motion of an object in the field of view is interpreted by the visual cortex as having a depth component, due to a relative difference in signal timings between the two eyes).

A more advanced form of anaglyphs is wavelength multiplex visualisation, also known as super-anaglyph. This uses 6 narrow bands of the visible spectrum, 2 red, 2 green and 2 blue. Two images are overlaid with the right eye seeing bands of red, green and blue light of slightly shorter wavelength than the three bands seen by the left eye. The viewer wears glasses which filter out either set of wavelengths giving the viewer a fully colour 3D image. This system is used by the Dolby 3D system and the Infitec Dualcolor3D.

Scheme	Left eye	L	R	Right eye	Colour Rendering	Description
red- green	pure red			pure green	monochrome	The predecessor of red- cyan; ghosting on computer screens as the green filter lets too much red through. Used for printed materials, e.g. books and comics.
red- blue	pure red			pure blue	monochrome	No ghosting on screens as there is no overlap between the colours the filters let through. Often used for printed materials.
red- cyan	pure red			pure cyan (green+blue)	colour (poor reds, good greens)	Patent- free, limited colour perception, currently the most common in use; images available in full version (red channel has only red colour) and half version (red channel is a red- tinted grayscale image, yields worse colours but less retinal rivalry)
anachrome	dark red			cyan (green+blue+ some red)	colour (poor reds)	A variant of red- cyan; left eye has dark red filter, right eye has a cyan filter leaking some red; better colour perception, can show red hues better than red- cyan
mirachrome	dark red+lens			cyan (green+blue+ some red)	colour (poor reds)	Same as anachrome, with addition of a weak positive correction lens on the red channel to compensate for the chromatic aberration of eyes.
Trioscopic	pure green			pure magenta (red+blue)	colour (better reds, oranges and wider range of blues than red/cyan)	Same principle as red- cyan, somewhat newer. Less chromatic aberration, as the red and blue in magenta balance well with green.
INFICOLOUR	complex magenta			complex green	colour (almost full and pleasant natural colours with excellent skin tones perception)	Developed by the TriOviz company, INFICOLOUR 3D is a newer, patent pending stereoscopic system first demonstrated at International Broadcast Convention in 2007 and deployed in 2010. It works with traditional 2D screens and TV sets (LCD, Plasma) and uses glasses with brand new complex colour filters and dedicated image processing that allows a natural colour perception with a pleasant 3D experience. When observed without glasses only some slight doubling can be noticed in the background of the action which allows watching the movie in 2D without the glasses. This is not possible with traditional brute force anaglyphic systems.
ColourCode 3D	amber (red + green + neutral grey			pure dark blue (+optional lens)	colour (almost full- colour perception)	Also named yellow-blue, ochre-blue, or brown-blue, this is a newer system deployed in 2000s; better colour rendering, but dark image, requires dark room or very bright image. Left filter darkened to equalize the brightness received by both eyes as the sensitivity to dark blue is poor. Older people may have problems perceiving the blue. Like in the mirachrome system, the chromatic aberration can be compensated with a weak negative correction lens (-0.7 diopter) over the right eye. ]  Works best in the RG colour space. The weak perception of the blue image may allow watching the movie without glasses and not seeing the disturbing double-image.
magenta- cyan	magenta (red+blue)			cyan (green+blue)	colour (better than red- cyan)	Experimental; similar to red- cyan, better colour rendering and less retinal rivalry. Blue channel is blurred horizontally by the amount equal to the average parallax, and visible to both eyes; the blurring prevents eyes from using the blue channel to construct stereoscopic image and therefore prevents ghosting, while supplying both eyes with colour information.
Infitec	white (Red 629 nm, Green 532 nm, Blue 446 nm)			white (Red 615 nm, Green 518 nm, Blue 432 nm)	colour (full colour)	Uses narrow- band interference filters, requires corresponding interference filters for projectors, technical requirements comparable with polarization- based schemes. Not usable with standard CRT, LCD, etc. displays.

Polaroid Systems:

Polaroid systems work by exploiting the polarisation of light to achieve the 3D effect. This is achieved by projecting two images superimposed over each other, each image having a different polarisation. The Viewers wear glasses which contain polarised filter which blocks the unwanted image from each eye.  Two types of polarised system exist, linear and circular. The main advantage of polarisation system is that full colour is retained for both eyes.

Linear polarised systems

This is the original system, developed in 1936 by Edwin H. Land. This technique was used in the first 3D boom in the early 1950's and is still used in Imax cinemas. Two images are projected superimposed onto the screen through orthogonal polarising filters, usually at 0º and 90º, shown in figure 4. The glasses worn by the viewers also contain a pair of orthogonal polarising filters oriented the same as the projected images. Since each filter only allows light similarly polarised through, each eye only sees one of the two superimposed images. The main disadvantage of this technique is that the viewer must keep their head still otherwise the unwanted image will bleed into the other image causing ghosting. This can make prolonged viewing uncomfortable.

Figure 4: Linear polarised light

Circular Polarised Systems

This method uses clockwise and anticlockwise circular polarised light to achieve the same effect as linear polarisation. It has the advantage that tilting the viewer's head does not cause the images to bleed into each other.

Light consists of two perpendicular waves, one magnetic and the other electric as illustrated in figure 5.

Figure 5: Electromagnetic waves

Circularly polarized light consists of two perpendicular electromagnetic plane waves of equal amplitude and 90° difference in phase. The light illustrated is right- circularly polarized.

Figure 6: Anticlockwise circular polarised light

If light is composed of two plane waves of equal amplitude but differing in phase by 90°, then the light is said to be circularly polarized. If you could see the tip of the electric field vector, it would appear to be moving in a circle as it approached you. If while looking at the source, the electric vector of the light coming toward you appears to be rotating anticlockwise, the light is said to be right-circularly polarized. If clockwise, then it is left-circularly polarized light. The electric field vector makes one complete revolution as the light advances one wavelength toward you. Another way of saying it is that if the thumb of your right hand were pointing in the direction of propagation of the light, the electric vector would be rotating in the direction of your fingers.

Circularly polarized light may be produced by passing linearly polarized light through a quarter-wave plate (plate of material which has a different index of refraction) with a refraction angle of 45° to the optic axis of the plate.

The RealD system use clockwise for the right eye and anticlockwise for the left and, in the UK, is the most common type of 3D projection. The glasses have opposite polarised lenses so that each eye sees only its designated image, even if the head is tilted. The images are projected onto a silver screen which preservers the polarisation.

Active 3D Displays

This technique uses a shutter system which is synchronised with the display so that each eye sees a different image and is mainly used in 3D televisions. Liquid crystal shutter glasses most commonly used in these displays was invented in the mid 1970's. The prototype had the LCDs mounted to a small cardboard box using duct tape. The glasses were never commercialized due to ghosting, but E&S was a very early adopter of third-party glasses such as the StereoGraphics CrystalEyes in the mid-1980s. This technology was most used at first for the computer and console games in the 1980s. In 1982 the Sega arcade game, SubRoc-3 came with a viewer with spinning discs to alternate the left and right images. Sega also produced active 3D glasses for the Sega Master System. In the late 90's several companies produced active 3D glasses for using with Windows PCs. These all worked with CRT monitors and became redundant in the early 2000's when CRT monitors were replaced with LCD ones. It was not until the late 2000's that 3DTVs started appearing. Until recently these were all active 3D displays and require active 3D glasses with LCD shutters (a few passive displays are now available such as LG's 47LD950 Passive 3DTV). It is also used in some cinemas with the XpanD 3D system. This has been installed in only about 3500 cinemas worldwide. It is not very common as the glasses cost about 50-100 times the cost for the polarised glasses used by RealD.

Autostereoscopy

Autostereoscopy is any method of displaying 3D images without the need for 3D glasses or other viewers. Because glasses are not needed, it is also called "glasses-free 3D" or "glasses-less 3D". The approaches used to achieve this include parallax barrier, lenticular, volumetric, electro-holographic and light field displays, with the former two being the main techniques.

Parrallax Barrier

A parallax barrier consists of a barrier which has a series of slits cut out is placed in front of the display, LCD being the most common displayed used. At one of the correct viewing angles the holes allows the light from one image to be seen by one eye and the light from the other image is seen by the other eye. The barriers block the light from each of the images from being seen by the other eye. This is illustrated below.

This method is used in a navigation system installed in the 2010-model Range Rover which allows the driver to view GPS directions whilst the passenger watches a movie. It is also used in the Nintendo 3DS hand-held console, LG's Optimus 3D smartphone and a Toshiba 21-inch 3DTV. The latter has 9 pairs of images to cover a viewing angle of 30º.

Lenticular Lens

A lenticular lens is an array of magnifying lenses, designed so that when viewed from slightly different angles, different images are magnified. The most common example is the lenses used in lenticular printing, where the technology is used to give an illusion of depth, or to make images that appear to change or move as the image is viewed from different angles. This is mainly used for printed images, but is also used for projected TV systems and glass-free 3D TV.

Lenticular printing is a multi-step process consisting of creating a lenticular image from at least two existing images, and combining it with a lenticular lens. This process can be used to create various frames of animation (for a motion effect), offsetting the various layers at different increments (for a 3d effect), or simply to show a set of alternate images which may appear to transform into each other. There are three distinct types of lenticular print, distinguished by how great a change in angle of view is required to change the image:

Transforming prints

Here two or more very different pictures are used, and the lenses are designed to require a relatively large change in angle of view to switch from one image to another. This allows viewers to easily see the original images, since small movements cause no change. Larger movement of the viewer or the print causes the image to flip from one image to another. (The "flip effect".)

Animated prints

Here the distance between different angles of view is "medium", so that while both eyes usually see the same picture, moving a little bit switches to the next picture in the series. Usually many sequential images would be used, with only small differences between each image and the next. This can be used to create an image that moves ("motion effect"), or can create a "zoom" or "morph" effect, in which part of the image expands in size or changes shape as the angle of view changes. The movie poster of the film Species II, shown in this article, is an example of this technique.

Stereoscopic effects

Here the change in viewing angle needed to change images is small, so that each eye sees a slightly different view. This creates a 3D effect without requiring special glasses.

Lenticular lenses are used to produce a bright picture on projected TV screens. It is not used in this way for 3D projected television. It is used in some glasses-free 3D TV such as TCL's 42" LCD model – the TD-42F – available in China for $20 000.

Summary Table

Comparison between Parallax Barrier and Lenticular lenses

Technique	Advantages	Disadvantages	Uses
Side-by-Side	Full colour image Full brightness Reduced Eyestrain	Only one person can see image at a time Differing wear can distort 3D	Stereographic postcards Aerial photo reconnaissance
Anaglyphs	Cheap Simple Lightweight glasses	Colours appears distorted	Comics and children's books 3D films on the internet
Wavelength Multiplex Visualisation	Full colour Brighter than other 3D systems Expensive silver screens not needed	Glasses more expensive and fragile than polarisation techniques	3D cinemas (Dolby 3D and Infitec Dualcolor3D)
Linear Polarisation	Cheap glasses Lightweight glasses Full colour images	Ghosting can occur if views head is tilted Reduced image brightness	3D cinemas (Imax 3D)
Circular Polarisation	Cheap glasses Lightweight glasses Full colour images Reduced resolution (for TVs)	Incorrect parallax can occur if head is tilted Reduced image brightness	3D cinemas (RealD and MasterImage 3D) Passive 3DTV (LG's 47LD950 3DTV)
Active 3D	Full high definition image (for 3DTV) Lower cost 3DTVs	Expensive glasses Heavy glasses Complicated glasses Reduced refresh rate (for 3DTV) Image can seem to flicker by some people	3DTV Cinemas (XpanD 3D)
Autostereoscopy	Glasses free	Limited viewing angle Multiple views can be jarring when moving from one view to the next	3D posters Heldheld 3D displays (Nintendo 3DS)

Further Reading

Wikipedia pages:

Stereoscopy

Anaglyph Image

3D Film

Digital 3D

Dolby 3D

Polariser

Polarised 3D glasses

Imax

RealD Cinema

MasterImage 3D

3D Television

Liquid Crystal Shutter Glasses

Autosteroscopy

Parallax Barrier

Lenticular Lens

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Source: https://optics3d.co.uk/how_3D_works