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Human Visual System Eye Perception of colour Display types Display terminology

Human eye and Displays. Human Visual System Eye Perception of colour Display types Display terminology. Metropolia University of Applied Sciences, Jutta Jokela. 6 May 2010. Introduction. Human Visual System Eye Brains Camera and HVS HVS and displays.

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Human Visual System Eye Perception of colour Display types Display terminology

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  1. Human eye and Displays • Human Visual System • Eye • Perception of colour • Display types • Display terminology Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  2. Introduction • Human Visual System • Eye • Brains • Camera and HVS • HVS and displays Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  3. The eye http://www.eyemdlink.com/Anatomy.asp Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 3

  4. Seeing process Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 4

  5. Seeing process Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 5

  6. Seeing process http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.aspx Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 6

  7. Eye • Retina • Three layers – one converts light to neural signals, one does the image processing and one transfers information to the brain • Rods and cones • Rods see in black and white • Cones create the colour images • Red cone detects light around 564 nm, green at 533 nm and blue at 437 nm http://askabiologist.asu.edu/rods-and-cones Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  8. Eye Fovea In the center of the retina 1 mm in diameter Used for high resolution vision No rods around Makes it easier to see in dark when looking to the side rather than forward Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  9. Summary of the eye http://www1.curriculum.edu.au/sciencepd/readings/ligh_how.htm#1 Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 9

  10. Camera versus HVS (1) Camera and eye are built the same way Image based on light Lens and image sensor Two adjustable parameters Focus With camera: manually moving lens With eye: Cornea let's light in but lens behind it changes in shape and size Aperture Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  11. Brightness Eye Subjective measurement Display Amount of light a display is capable of emitting Black should look black – can change as ambient light varies With LCD displays usually affects to backlight luminance Measured with candelas per square metre Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  12. Contrast Luminance levels within an image – difference between brightness of objects in an image In displays contrast affects to the luminance that is meant for reproducing a full white pixel Contrast and brightness must be in decent level to make image viewable Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  13. Dynamic range and Contrast ratio (1) In display means the maximum and minimum intensities that a display can generate Brightest white and darkest black a display can display No common way of measuring – measures are not comparable Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  14. Dynamic range and Contrast ratio (2) Dynamic contrast ratio is difference in intensity over time Static contrast ratio is a difference between darkest and brightest pixels displayed by the display Intensity of the backlight affects as well Different areas of the screen may be illuminated differently With OLED only static contrast ratio matters Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  15. Visual acuity Eye Clearness of the vision Light enters the eye Through pupil, then lens and is projected on retina Muscles move eyeball so that image is in the fovea Display Level of detail eye can actually see from pattern of pixels on a screen Not usually counted separately – 200 ppi good enough Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  16. Angle of viewing Eye The angle in which eye tries to get the point of interest reflected to the foval area Usually around 155 degrees vertically and 185 degrees horizontally Camera Means the scene that can be imaged Display Angle in which you are still able to see the content of the display – usually 120-170 degrees with LCD displays Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  17. Perception of colour Very often a subjective matter Hue – dominant wavelength of the colour Brightness/lightness – luminosity of the colour If an object reflects light, this is lightness, if it emits light, this is brightness Saturation – pureness of the colour Can be called as white light mixed with hue Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  18. Colour • In additive colour mixing light is added to a dark background • Example RGB • In subtractive colour mixing a certain method is used for blocking white pixels • Example CMYK Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  19. Colour gamut • A certain part of the colour map that a device can present • In 1931 Commission Internationale de l'Eclaraige (CIE) developed the chromaticity diagram http://www.pcmonitors.org/articles/monitor-technology Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  20. Colour in displays Displays emit or reflect light with different strengths of three primary colours Ambient light has always an effect as well as it reflects off the display With LCD displays the gamut is related to the emitted spectrum of the backlight Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  21. Colour depth The number of colours the display can display Bits LCDs 6-bits per pixel causing a 18-bit colour image Dithering increases the amount of colours Dithering – artificially changing shade of a colour during a refresh of the display Causes 24-bit image Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  22. Refresh rate (1) This means the number of times a display is repainted per second Expressed in Hertz Should be twice as much as the viewer's refresh rate Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  23. Refresh rate (2) Eye Detects and processes a moving object in about 120 ms – requires 20 images per second for blurring to start Display Too slow refresh rate causes flicker effect Meaningful with CRTs With LCDs flickering does not occur because liquid crystals merely act as shutters against the backlight Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  24. Introduction • Typical display types • Terminology related to displays and visual system Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  25. Cathode Ray Tube (CRT) The most common display until last years No fixed amount of pixels Image can be very good Usually big in size Not as bright as LCD displays Technology Electron guns send beams of electrons Stimulating a layer of phosphor that are ”the pixels” of a CRT (red, green and blue) Magnetic field and deflection of the electrons form the image Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  26. Liquid Crystal Display (LCD) Liquid crystals are sandwiched between two polarizing filters Backlight travels through the first filter while electrodes create a current that travels through liquid crystals and forces those to align in a certain way Alignment of the liquid crystals creates the intensity and the colour of light visible finally on the screen Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  27. Plasma Display Panel High contrast ratio, panel can be big Popular in public places Contains cells that are filled with ionized gas Glows when electrical field around it is charged Pixels can be only on or off so no grey levels are possible to create Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  28. Organic Light Emitting Diode (OLED) Yet quite small displays Can be even flexible Do not use backlight Self illuminated materials Less power consumption Very good contrast ratio, colour reproductions and response time Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  29. Resolution CRT – no resolution LCD, OLED, Plasma – amount of pixels is fixed (pixels horizontally and vertically available) HDTV has made this especially important Resolution is not fixed quantity For that reason displays with same resolution but with different size are not comparable Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  30. Response time A time it takes for pixels to move from one state to another Faster response time means smoother image transitions e.g. In sports Before this measure measured time for turning display on, off and on again 2005 manufacturers changed this for considering grey-to-grey changes Good LCD displays claim to have 2 ms response time Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010

  31. Screen size Diagonal size of the screen in inches With CRTs there was screen size and viewable area separately Different LCD display manufacturers handle this differently EITHER viewable area OR actual screen size Metropolia University of Applied Sciences, Jutta Jokela Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 6 May 2010

  32. Summary • Eye is a very complex system • Camera is a simpler but similar system • Different aspects have to be taken into account when designing displays • Depends on technology Metropolia University of Applied Sciences, Jutta Jokela Metropolia University of Applied Sciences, Jutta Jokela 6 May 2010 6 May 2010

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