Date post: | 13-Dec-2015 |
Category: |
Documents |
Upload: | audra-dennis |
View: | 217 times |
Download: | 1 times |
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
Copyright Catherine M. Burns 3
Visual SpectrumU
ltra
viol
et
Vio
let
Blu
e
Gre
en
Yel
low
Red
Infr
a-re
d
700nm400nm
Text Figure 4.1a
Copyright Catherine M. Burns 4
Screen Colour vs. Paint/Pigment
Screen colour is additive (adds light)
Pigment is subtractive (reflects light so decreases light)
Screens RGB Pigment CYM
Copyright Catherine M. Burns 5
Colour Maps
The CIE space -
How to make colours with Red/Green/Blue light
Screen colour palettes
Text Figure 4.1b
Copyright Catherine M. Burns 7
Measurement of Intensity
Depends on where the source is Luminous Intensity or Luminous Flux:
Actual light energy of a source Unit: candela
luminous flux
Copyright Catherine M. Burns 8
Measurement of Intensity
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
Copyright Catherine M. Burns 9
Measurement of Intensity
Luminance: Amount of light reflected back from a surface
Unit: Candela/m2 or foot-lamberts (FL)
luminous flux
illumination
luminance
Copyright Catherine M. Burns 10
Reflectance
A measure of how much a surface reflects Ratio of luminance and illuminance
Reflectance (%)
luminance (FL)
illuminance (FC)
White very reflective vs. black
Copyright Catherine M. Burns 12
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"
Copyright Catherine M. Burns 13
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
Copyright Catherine M. Burns 14
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!
Copyright Catherine M. Burns 16
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
Copyright Catherine M. Burns 17
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
Copyright Catherine M. Burns 19
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
Copyright Catherine M. Burns 20
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"
Copyright Catherine M. Burns 21
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
Copyright Catherine M. Burns 22
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
Copyright Catherine M. Burns 23
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!
Copyright Catherine M. Burns 24
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
Copyright Catherine M. Burns 25
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
Copyright Catherine M. Burns 27
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
Copyright Catherine M. Burns 28
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
Copyright Catherine M. Burns 30
Visual Illusions
Muller-Lyer Illusion: Both horizontal lines are the same length
Copyright Catherine M. Burns 34
Shade context illusions
Shockwave files
Gray blocks 1
Gray blocks 2
Whites
Gray blocks 3