cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 9 - Other Additive Devices

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Assignment 1
- Called assignment 2 in w09.
Magnifying glasses

A Generic Additive Device

Three colour channels
- usually red, green, blue
Channels combine by additive mixture
- C(RGB) ~ C1(RGB) + C2(RGB) + C3(RGB)
The components of the combination are independent of time and space.
Channels are independent of one another
- C(RGB) ~ C1(R) + C2(G) + C3(B)
Each channel provides a constant chromaticity
- C(RGB) ~ ( E1(R) * C_1 ) + ( E2(G) * C_2 ) + ( E3(B) * C_3 )
Channels track with a common exponent
- C(RGB) ~ ( e1 * (R/R0)^g * C_1 ) + ( e2 * (G/G0)^g * C_2 ) + ( e3 * (B/B0)^g * C_3 )

These assumptions must be checked and the size of deviation quantified.

Tristimulus Values of Generic Additive Devices

X   x_R x_G x_B  e_R (R/R0)^g
Y = y_R y_G y_B  e_G (G/G0)^g
Z   z_R z_G z_B  e_B (B/B0)^g

Primary Values of Generic Additive Devices

A_R           X
A_G = [M^-1]  Y
A_B           Z

R = R0 * (A_R/e_R)^(1/g)
G = G0 * (A_G/e_G)^(1/g)
B = B0 * (A_B/e_B)^(1/g)

Spatial and Temporal Non-uniformity

Not much at high spatial and temporal frequencies

Lots at low spatial and temporal frequencies.

which does not grab the viewer's attention

Spatial

Maximum at centre, falls of by 30%, 40%, 50% in the corners.
- To compare, an extended edge with 1% contrast is easy to see
Maintaining measurement geometry during measurement is very important.

Temporal

Other Additive Devices

Colour LCD

These are widely used because they occupy little volume, use little energy, and weigh little compared to CRTs. Their colour performance is comparable to CRTs, but the structure of the images they produce can be intrusive

Technology

Backlight

Should be uniform
- usually a light source covered by a diffuser
- The better the diffuser the more light is lost
- The usual compromise is that you can see the shape of the light source when you look for it, but don't see it when you don't.
Light output tends to be biased in the forward direction
- Why?
Most energy consuming part of a laptop computer
Backlight efficiency
- incandescent (100 watt bulb): 2%
- tungsten halogen in quartz envelope: 4%
- cathode luminesence (historic television): 5%
- fluorescent: 8-12%
- light-emitting diode: 8-12%
- experimental orgainic light emitting diode: 15%

Polarizers

Polaroid sunglasses
- Light reflected at grazing angle is polarized perpendicular to reflecting surface
- Polaroid sunglasses, with the plane of polarization parallel to the surface removes the reflections
What is polarization?
- In essence, the direction of oscillation of the electric field of a photon
- Can prepare materials that alter the poarization of light by lining up their dipoles
What happens when you select a single polarization?
- Exactly 50% of the light is lost to heat.

Capacitor filled with liquid crystal

Liquid crystal is naturally ordered
- oppositely-oriented layers of soap molecules
- light passes through with unchanged polarization
- polarizer perpendicular to the first one eliminates all light (in theory).
Charge the capacitor
- the orientation of the ordering twists
- the polarization of light passing through perpendicular to the plates of the capacitor rotates
- an amount of light passes through the second polarizer that is a monotonic function of the applied electric field
- there are only about 100 differentiable steps from black to white, which has an important relationship to visibility thresholds.
Because of electric leakage you have to recharge the capacitor regularly
- and the capacitor recharges through a resistance
Because of light leakage you don't really remove all the light.
- increasing contrast by reducing light leakage is an ongoing engineering challenge.

Colour filters

The liquid crystal shutters are usually arranged into groups of three
1. one covered by a red filter
2. one covered by a green filter, and
3. one covered by a blue filter
The collection of three is a pixel
- normally outlined by a rectangular black mask
- What is the purpose of the mask?
The filters have the usual trade-off
- the more light they block the dimmer the display and the larger the colour gamut
- the less light they block the brighter the display and the smaller the colour gamut
Why not have four primaries?
- Why? We are surely overdoing the blue

Colour OLED

Solid state photodetectors and LEDs are very closely related

high efficiency.

Plasma Display Panel (PDP)

Fuorescent Lights

Contrast with incandescent lights

the temporal variation of light output.

They consist of

A gas tube coated on the inside with a phosphor.
It is filled mostly with inert gases, to which a small amount of mercury vapour is added.
Electrodes at each end of the tube

They make light by

ionizing the internal gases so that current flows in the form of electrons in one direction and positively charged atoms in the other.
- This is called electrostatic breakdown.
as current flows collisions with electrons excite mercury atoms to high energy states
decay of these states emits untraviolet light at 180 nm and 240 nm.
the phosphor absorbs the ultraviolet photons going into high energy states
decay of these states emits visible photons.

PDPs

A PDP is an array of electrodes and phosphors which, in the off state maintain a gas just below it breakdown level.

Fluorescent light is turned off and on by applying varying voltages to the electrodes.

Thus, a PDP is in effect an array of tiny fluorescent lamps..

Field Effect Display (FED)

Just as the PDP is an array of tiny fluorescent lamps, an FED is an array of tiny CRTs.

A vacuum is made in a thin flat volume.
On one face is phosphor-coated glass and a transparent anode.
On the other face is an array of tiny cathodes.
Between the two is an array of gates (like the screens in an ordinary CRT), one per cathode.
The screens modulate the current deposited in the phosphor closest to its cathode.

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