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

3. Metropolia University of Applied Sciences,
The eye
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010 3

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

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

6. Metropolia University of Applied Sciences,
Seeing process
pticalIllusions/illusions.aspx
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010 6

7. Metropolia University of Applied Sciences,
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
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010

8. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
Summary of the eye
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010 9

10. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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 degrees with LCD displays
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010

17. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
Colour gamut
A certain part of the colour map that a device can present
In Commission Internationale de l'Eclaraige (CIE) developed the chromaticity diagram
Metropolia University of Applied Sciences,
Jutta Jokela
6 May 2010

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

25. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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. Metropolia University of Applied Sciences,
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