3. 75-6. 25 feet
4. 25-7 feet
5. 25-8. 75 feet
6. 25-10. 5 feet
7-11. 75 feet
7. 75-13 feet
8. 75-14. 75 feet
times screen size. Although the higher
resolution of HDTV would imply that
you could move closer before seeing
its line structure, chances are you will
still be watching quite a lot of standard
definition programs. And there are
other reasons why not to get too
close (more on that in a moment).
Therefore, a good rule of thumb
would be to sit between 1-1/2 and
2-1/2 times the screen size as shown
in Table 1.
These numbers should be satisfactory for most people, although there is
still some debate about the exact
parameters. For example, one could
argue that technically you can get
closer to a TV showing 1080p material than one set for 720p display.
However, this assumes that the limiting factor is the line and/or pixel pitch;
i.e., the distance at which you start to
see the discrete structure of an image.
As we discussed last time though,
TV broadcasts (and to a similar degree
DVD pictures) are highly compressed
(roughly 100:1). Therefore, you will
start to notice compression artifacts
FIGURE 1. The three primary colors
(red, green, and blue) mix together
to form the secondary colors (yellow,
cyan, and magenta), as well as all
other colors including white.
TABLE 1. Optimum viewing
distance for a given screen size.
long before you get close enough
to see the image’s line structure.
Screen technology and
surface material — along with
lighting and screen reflections —
will also affect the results. Your
own visual acuity, display resolution, and even program material
will vary. Use Table 1 as a guide,
but the exact size TV you get may
also be based on what fits into your
existing furniture or décor.
After choosing the size for your
new HDTV, you can now narrow
down what types of screens are
available in that size. Smaller screens
will most likely be LCD or plasma.
Larger home theaters may require a
rear or front screen projection system.
Let’s take a closer look at the various
technologies used today so that you’ll
understand the pros and cons of each.
single yellow pixel. In case you don’t
remember the additive properties of
the primary light colors, take a look at
All TVs can be categorized as
either direct view or projection (front
or rear). Direct view sets use a CRT,
plasma, or LCD display. Front
projection sets throw an image from
within the viewing area onto a
reflective screen (like a movie theater)
while rear projection sets are usually
self-contained and present the image
on the back of a translucent screen
through which the picture is viewed.
The latter have become most popular
for medium to large home theater
installations. At one time, projection
sets used CRTs, but they all now
use LCD, LCOS, or DLP imagers.
Let’s examine how each of these technologies works.
Cathode Ray Tube
All color TVs rely on some form of
tri-stimulus display. This means that
they create a color image by breaking
it up into three separate monochrome
pictures — one each for the red, green,
and blue parts of the image. These
different images are then displayed
together, with the colored pixels very
close together (or rapidly in sequence
in the case of DLP sets). The eye then
merges these colors together to create
a single color based upon the mixture
of red, green, and blue.
For example, if both the red and
green pixels are displayed at full
brightness, the eye will perceive a
We’ll mention the venerable CRT
only in passing, as few sets are still
available with these bulky, heavy glass
tubes. As you probably know, the CRT
creates a TV image by scanning the
back of a phosphor-coated screen
with three separate electron beams.
When an electron strikes the
phosphor, it momentarily glows with a
color determined by its chemical
composition. Three different phosphors are used to create red, green, and
blue light (see Figure 2). The number
of electrons striking a given spot on
the screen is varied by controlling the
beam current; this, in turn, determines
the brightness and color for each spot
as the beam scans left to right and up
and down the screen.
The result is that a
complete color image is
FIGURE 2. Basic elements
of a CRT TV display: 1)
Electron guns; 2) Electron
beams; 3) Focusing coils;
4) Deflection coils; 5) Anode
(high voltage) connection;
6) Mask for separating
beams for red, green, and
blue part of displayed
image; 7) Phosphor layer
with red, green, and blue
zones; and 8) Close-up of
the phosphor-coated inner
side of the screen.