copyright catherine m. burns 1 vision text chapter 4 p. 61-89

Copyright Catherine M. Burns

1

VISION

Text Chapter 4 p. 61-89

Copyright Catherine M. Burns 2

The Visual System

sensor system for electro-magnetic radiation typically 400nm (blue-violet) to 700nm (red) Hue = colour (mix of red, green, blue) Saturation = purity of wavelength in terms of

red, green, blue Pure red=Saturated Brightness = an intensity measurement


Visual SpectrumU

ltra

viol

et

Vio

let

Blu

e

Gre

en

Yel

low

Red

Infr

a-re

d

700nm400nm

Text Figure 4.1a


Screen Colour vs. Paint/Pigment

Screen colour is additive (adds light)

Pigment is subtractive (reflects light so decreases light)

Screens RGB Pigment CYM


Colour Maps

The CIE space -

How to make colours with Red/Green/Blue light

Screen colour palettes

Text Figure 4.1b


The RGB Colour Cube

Adds saturation (white-black axis)


Measurement of Intensity

Depends on where the source is Luminous Intensity or Luminous Flux:

Actual light energy of a source Unit: candela

luminous flux



Illumination or illuminance: Amount of energy that lands on a surface

Declines with the square of the distance from the source Units: foot candle or lux

luminous flux

illumination



Luminance: Amount of light reflected back from a surface

Unit: Candela/m2 or foot-lamberts (FL)

luminous flux

illumination

luminance


Reflectance

A measure of how much a surface reflects Ratio of luminance and illuminance

Reflectance (%)

luminance (FL)

illuminance (FC)

White very reflective vs. black


The Eye

www.eyenet.org/public/anatomy/anatomy.html


Elements of the Visual System

Cornea - performs 1/3 of total refraction Pupil - controllable, an aperture which controls the

amount of light energy entering Lens - complex refraction (gradient refraction

optics) shape is controllable, "flexes", accommodation to near

and far doesn't stop growing loses flexibility around your 40s. "reading glasses"


Retina flat surface of receptor cells 130 million rods and 7 million cones cones provide colour vision during daylight rods are more sensitive (light level activation) but don't

provide colour for dim light, night vision photochemical reactions to produce nerve impulses


fovea entirely cones, on the optic axis of the eye most dense region move eyes to look at things sharpest vision each foveal cone has its own nerve fibre other areas of the retina don't - think data

transfer, data bus!


Visual Angle

Height H

Distance D

VA=arctan(H/D)


The Visual Field

sharp vision along 1 degree v.a. (fovea) middle field: mix of rods and cones, up to 40

degree angle periphery: primarily rods


Sensory Processing Limitations

Contrast sensitivity Uses luminance levels of two areas “L” light and

“D” dark Ratio of the difference over the sum C=(L-D)/(L+D) People each have a minimum contrast level they

can detect


Contrast Sensitivity Measures


Color “Blindness”

7% of male population is color deficient Most common is red-green Design advice

Design for monochrome first and add color later for emphasis


Accommodation

lens continually readjusts to keep objects at different distances in focus. autofocus natural "resting" accommodation is about 1m close work can cause it to be difficult for the lens

to readjust afterwards to longer distances "temporary myopia"


The Near Point

near point - closest distance you can focus on.

indicates lens flexibility moves out with age moves farther out with fatigue -> visual

fatigue


Presbyopia

lens loses its ability to flex close work becomes difficult need for reading glasses everyone goes through it starting at 40 accommodating requires muscular work and

can create fatigue


Pupil changes

a control system - more light, smaller diameter, etc.

takes a measurable length of time to adjust rapid light changes can cause retinal damage

because pupil doesn't adjust quickly enough physical discomfort!


Pupil Size

Regulated by: brightness of the visual field focus distance (contracts for near) emotional states - dilates with alarm, happiness,

mental work contracts with fatigue and sleepiness


Retinal Adaptation

retina is also a control system darkness rods come out, bright light cones come out see more in a dark room 10 to 15 minutes later complete dark adaptation takes 45 minutes adaptation dark to light much more quickly, a minute or

two dark workplaces typically use red light to maintain

adaptation


Glare

excessive brightness which wipes out visual detail


Visual Fatigue

irritation, burning , tearing watering of eyes, red eyes

double vision headaches reduced acuity and accommodation Work Impact: mistakes, loss of productivity, lower

quality work, complaints, higher accident rate


Ergonomics for Computer Screens

refresh rate high enough to remove flicker use larger characters for wider population, older

people remove sources of reflection on screen don't have bright lights directed into screen, washes

out screen dark room versus light room a personal preference

or depends on need to switch work


Visual Illusions

The Ponzo Illusion: Both lines are the same length


Visual Illusions

Muller-Lyer Illusion: Both horizontal lines are the same length


Visual Illusions

Both rectangles are the same size


Visual Illusions

Center shape is a square


Shade context illusions

Shockwave files

Gray blocks 1

Gray blocks 2

Whites

Gray blocks 3


Center surround effects

Haze Vasarely

copyright catherine m. burns 1 vision text chapter 4 p. 61-89

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