The EYE and the Television Display

1
The EYE and the Television Display
A brief consideration of the eyes viewing
potential and the relationship to a Television
display is given. Discussion is encouraged on
this complex subject which can only have a
subjectively based conclusion.
Compiled by Wayne Dickson SMIREE MIEAust. CPEng.
Member SMPTE in consultation with Bryan Powell of
the ABC
13 October 98
2
The Normal EYE

• What is 20/20 Vision ?
• Being able to, at a distance of 20 feet, read a
  character, eg the letter E, which is meant to be
  read at a distance of 20 feet.
• The size of the letter E for example which can be
  read by the eye with 20/20 vision is
• 20/15 vision is better than 20/30 vision.

1
1
5 minutes of arc
1
1
1
1
5
3
The size of the charactersto test 20/20 vision ?

• 1 minute equals 1/60 of a degree
• This is the normal minimum discernible
  resolution for a human eye when tested for
  character recognition.

O
D
20 feet
O 1 minute D 2 x 20 feet x Tan (O/2)
0.07 inches ( 1.8mm)
Hence the letter E will be 9 mm x 9 mm if
intended to be read by a person with 20/20 vision
at a distance of 20 feet or 6 metres.
4
Maximum resolving power

• The eye under some conditions may provide
• Vernier Acuity of 8 seconds of arc. Eg. Offset
  of two lines placed end to end.
• Detection can go down to 1 second, maybe 0.5
  second Eg. single power lines in the sky.
• With more complex patterns like a matrix of
  pixels in a colour television display, a probable
  best resolving power of around 30 seconds is
  thought to be typical.
• Note that the eye can discriminate down to 1 in
  intensity between overlapping light sources.
• The eye has a greater resolution for vertical
  separated lines compared to horizontal separated
  lines.

8 sec.
Hence approx. 1mm differences between objects
maybe detectable, at a viewing distance 6 metres
from a display, by a typical eye.
5
Maximum resolving power related to a Display size

• With a viewing distance of 6 H (6 x height of
  picture) for SDTV and 4H for HDTV, and a typical
  best resolving power of the eye for a complex
  pattern of pixels of 30 sec. of arc, the minimum
  discernible object separation is

Display diagonal dim.
Display height
Minimum discernible object separation
Viewing Distance
5 12 18 22 27 32 42 (169) 10ft
(169) 20ft (169)
0.07 mm 0.16 mm 0.24 mm 0.29 mm 0.36 mm 0.43
mm 0.38 mm 1.1 mm 2.1 mm
76 mm 183 mm 274 mm 335 mm 412 mm
488 mm 640 mm 1830 mm 3658 mm
0.5 m 1.1 m 1.6 m 2 m 2.5 m
2.9 m 2.6 m 7.3 m 14.6 m
The minimum pixel separation on a television
display is currently 0.21 mm for a
Professional Monitor to 0.35 mm for a
consumer display.
_at_ 6H
_at_ 4H
6
Pixel separation in a display
4 (P)
Horizontal pixel separation 4D/(P(42
32)0.5) 4D/(P5) Vertical line separation
3D/(L5)
D
3 (L)
Where P horizontal Pixels L vertical Lines
16 (P)
Horizontal pixel separation 16D/(P(162
92)0.5) 16D/(P(337)0.5) Vertical line
separation 9D/(L(337)0.5)
D
9 (L)
7
43 Display pixel size
4 (720)
Horizontal pixel separation 0.51 mm Vertical
line separation 0.48 mm
18
3 (576)
The eye discernability is possibly 0.24 mm _at_ 6H
4 (720)
Horizontal pixel separation 0.76 mm Vertical
line separation 0.71 mm
3 (576)
27
The eye discernability is possibly 0.36 mm _at_ 6H
8
169 Display pixel size
16 (720)
Horizontal pixel separation 1.2 mm Vertical
line separation 1.1 mm
42
9 (576)
16 (1920)
Horizontal pixel separation 0 .48 mm Vertical
line separation 0.48 mm
42
9 (1080)
For robustness, variable pitch is sometimes used.
The eye discernability for a 42 display is
possibly 0.4 mm _at_ 4H
9
Large 169 Display pixel size
16 (1920)
10 foot
9 (1080)
Note pixel count is 2,073.600 pixels and dot
count is 6,220,800 dots.
Horizontal pixel separation Vertical line
separation 1.4 mm 1.4 mm
The eyediscernability for a 10ft display is
possibly 1 mm _at_ 4H
10
Pixel separationversusPicture resolution
Horizontal
Pitch of the Resolution lines
(some examples)
Mask pattern
R
G
B
R
B
G
R
B
R
G
B
R
G
B
R
G
B
R
G
B
R
G
B
or
or
G
R
B
G
R
G
B
Pixel B
Pixel A
Pixel C
Pixel B
Pixel A
Pixel C
Pixel A
Pixel C
Pixel B
Trinitron or Plasma
Slot
Dot
Pixel separation
Pixel separation
Pixel separation
That is 1920 pixels horizontally gives 960 lines
resolution and 720 pixels horizontally
gives 360 lines resolution
11
Pixel separationversusPicture resolution
Vertical
Pitch of the Resolution lines
(some examples)
on
on
R
B
Pixel A
Pixel A
R
G
B
G
R
B
Pixel B
Pixel B
R
G
B
or
off
off
G
Line Pixel separation
Line Pixel separation
R
B
Pixel C
Pixel C
R
G
B
G
on
on
Note stripes are continuous for a Trinitron
That is 1080 pixels vertically gives 540 lines
resolution and 576 pixels vertically
gives 288 lines resolution
12
Other Considerations

• The other dominant factors which may influence
  whether the line structures or the increased
  resolutions can actually be seen are
• Kel factor associated with interlace scans.
  (works to decrease the observed resolution both
  vertically and horizontally by 0.7
  to 0.8)
• Interlace artefacts such as pairing in displays
  (H to V xtalk)
• Room lighting conditions.
• Programme material type.
• Programme production and coding artefacts
• Display structures - dot, line, rectangular,
  black matrix, tube, plasma etc.
• Display dynamics, persistence, dynamic focus, and
  shading correction.
• Viewing distance - this analysis is based on 6H
  for SDTV and 4H for HDTV.
• Variability of the human observer from the
  typical normal vision for a young observer.

13
Concluding analysis

• By analysing the eyes potential and the
  displays mechanics to see if
• the number of vertical lines is too low and hence
  is observable
• or the number of vertical lines is too high and
  is wasted
• the displayed horizontal resolution can be seen
• or the displayed horizontal resolution is below
  what could be resolved by the observer
• Is the eye able to see the separation between
  the pixels or is the eye only able to detect the
  separation between alternate pixels, that is the
  resolution lines ?
• I suspect the answer is dependent upon the other
  considerations, in particular the display
  structure and technology used.
• If the perception of the eye is 30 sec. of arc
  and if the variation between adjacent pixels can
  be detected the followings conclusions are
  reached

14
Conclusions

• The following conclusions can generally be made
  at a viewing distance of 6H for SDTV and 4H for
  HDTV from the display
• A SDTV (720 x 576) displays line structure can
  potentially be observed and the resolving power
  of the eye is not reached with the displayed
  horizontal resolution. ( possibly half the eyes
  potential)
  
  
• A HDTV (1920 x 1080) displays line structure is
  probably not observable and the resolving power
  of the eye is probably matched with the displayed
  horizontal resolution.