Perhaps this is overdoing it, but you could express the minimum black level
M with a formula like this:
M = E + R
In that E is the emitted light and R the reflected ambient light. You could
then define R as A/k, where A is the ambient light level (let's assume
omnidirectional even distribution) and k is a "filtering constant". That is,
the higher the value of k, the less ambient light is reflected (i.e., more is
filtered). So we have:
M = E + A/k
A plasma display will have lower values for both E and k than an LCD
display. So, when A is zero (dark room) we get:
M = E
and the plasma's blacks are darker. But as A increases the second term A/k
also increases relative to E and becomes the dominant term, because E is fixed.
Since k for plasma displays is relatively small, A/k for plasma displays
increases more rapidly than it would do for LCD displays.
Contrast is also affected. Suppose T is the maximum amount of light emitted
by a display, then we can define contrast as:
(T + A/k)/(E + A/k)
For A = 0 (again, dark room), that reduces to T/E. But for positive non-zero
A, the value of the above expression is always lower than T/E.
For example, suppose your display emitted a minimum of 1 unit of light and a
maximum of 10. Then the contrast ratio would be 10:1, or you could just say
it has a contrast of 10. But then suppose that 1 unit of ambient light is
reflected in addition to the light emitted. Then the contrast ratio is
(10+1) / (1+1). So the contrast, as a number, is 11/2 = 0.55. In other words,
by reflecting some ambient light, the contrast has been almost halved.
Similar arguments can be made for saturation. Saturated colours are,
essentially, produced by a mix of two colours from red, green or blue with
the remaining colour being held low. If the remaining colour cannot be held
low because it is augmented by reflected ambient light, then saturation is
lost.
That is why, in extreme shop environments, plasma displays may look washed
out. The colours are less vibrant because saturation is lost. The contrast
is reduced, leading to a flatter image. And you don't have those inky deep
shadows.
But again, in the correct environment a plasma display can produce
exceptionally vivid, vibrant images of a stunning naturalness. I do want to
emphasize that, because shop viewing can be completely misleading. If anyone
is reading this and has only seen a plasma display in a brightly lit shop, I
would urge seeking out a proper demo.