I have purchased a display model Sony 23" LCD HDTV monitor. There are
some light green blemishes that only show up when there is a blank
screen transmission. It does not show during picture transmissions or
when the TV is off.
My only concern is that this may be a sign of eminent failure. I have
7 days to return the set.

On a CRT that would usually indicate a magnetic field, but that
shouldn't apply to an LCD. Bring it back and let the seller tell you
what it is. Get it in writing that you brought it back under the
return period.

I bought it from Sam's Club. I doubt they have anyone who could tell me what it is.

It's hard to tell from your description, but I suspect what you are
seeing are "stuck" pixels.  See
http://compreviews.about.com/od/multimedia/a/LCDPixelDefects.htm.

LCD screens commonly have a few pixels that are either permanently off
(dead pixels) or permanently on (stuck pixels).  Most manufacturers
set some limit on the number of such bad pixels that must be exceeded
before the display is considered "defective".  I'm not sure what
Sony's policy is for TV's, but for PlayStation Portables, it's 10
pixels. (See http://www.cnet.com/4520-6033_1-6023185-1.html).

If it is stuck pixels you are having a problem with, then I'd return
the unit  to the store and ask for a new unit (regardless of how many
pixels are involved).

Most manufacturers will hide behind ISO13406-2, which is a standard
that defines an "acceptable" number of defective pixels in an LCD
screen.

Ian G.

Doesn't a standard for an acceptable number of defective pixels sound
almost as bad as a standard for an acceptable number of insect parts
in canned corn?

But maybe not as bad as a standard for the maximum ammount of whatever
killed the insect in the first place :-)

In fairness to the LCD manufacturers, guaranteed perfect screens would
be prohibitively expensive - so long as the defects aren't noticeable
in normal use then does it matter?  Also, the standard cuts both ways
- if the number of defective pixels exceeds the allowed value then the
screen's defective - no arguments!

Ian G.

How do I count defective pixels

On LCD panels, there may be several defects.

The defect you are describing is quite typical.

A typical LCD panel has a backlight (CCFL light) as source of light.
This backlight is On when the TV is On. On top of the backlight there
is a "glass sandwich" which contains in between the liquid crystal.
Top and bottom glasses are polarized. The liquid crystal "rotates" the
light so that it can pass through the second polarizer with a certain
intensity, depending on the electrical field.

Every pixel has three sub-pixels of primary colours (red, green and blue). 

Every sub-pixel has a liquid crystal cell. 

Every sub-pixel has associated a transistor. This means that in a WXGA
panel (1280 pixels x 768 pixels) there are 1280 x 768 x 3 = 2949120
transistors.

In semiconductor manufacturing process, it is impossible to reach
zero-defect production. In normal semiconductors, this is not a
problem, because the defective devices are rejected and don't reach
the market.

But in LCD panels, all transistors are joined in the same panel.
Unfortunately, a defective transistor cannot be repaired nor changed.
Rejecting every screen with one or more damaged sub-pixels would make
a too expensive product. The LCD screen makers has it's own criteria
to reject or accept the panel. That's why "defective" screens reach
the market.

On other hand, semiconductors have child death effect. This means that
during the beginning of the screen life, some transistors (sub-pixels)
can die. Once the initial period passes, it is more difficult to get
more damaged transistors. This means that your new TV set can show new
defective sub-pixels during the initial period.

In your panel, you have some sub-pixels damaged, in lit condition.
This means that these sub-pixels (green in your case) are always
transmitting light.
When picture is white (or has some green in your case), you cannot (or
can hardly) notice the defect, but when the picture is dark or black,
as the backlight is still turned On it is quite easy to see the
problem.
When the TV set is turned Off, you cannot notice the defect because
the backlight is turned Off.

My advice is to return the TV set to the shop if you have several lit
dots, and get another one which may have less defects (or zero-defects
if you are very lucky). But consider that the seller can argue that
the shop cannot  replace the TV set for this reason.
If you have problems with the shop, I suggest to read standard
ISO13406-2 and check if your set meets this standard.

For more explanations about LCD technology, please visit
http://plc.cwru.edu/tutorial/enhanced/files/textbook.htm