Copyright Catherine M. Burns1 VISION. Copyright Catherine M. Burns2 The Visual System sensor system...

Copyright Catherine M. Burns 1

VISION


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

• Brightness = an intensity measurement


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


The RGB Colour Cube


Measurement of Intensity

• Depends on where the source is

• Luminous Flux: intensity of energy from a source

• Unit: candela or lumins

luminous flux



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

• 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)


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 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


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


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 VDTs

• 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

Copyright Catherine M. Burns1 VISION. Copyright Catherine M. Burns2 The Visual System sensor system...

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