Rich,
Attempting to use a laptop outdoors recently, to
great frustration, I wonder about the phenomenon
of washout.
the ambient light is scattered and reflected off the
laptop screen and so produces an ambient light luminance
- say La - that adds to the light that is generated by
the display itself - Ld. It also adds to the luminance
Lb of the display when it displays black image content.
The full screen contrast of your laptop is normally
defined as the luminance for full white content Lw
divided by the aforementioned Lb (black content:
Cfs = Lw / Lb
Cfs may typically be in the order of around 700-1000
for a notebook display.
Values taken from a real measurements e.g. were
Lw = 310 cd/m²
Lb = 0,42 cd/m²
Cfs = 738(:1)
The same display produced a luminance Lad of about
5 cd/m² at an ambient light illuminance of 5000 Lux
under diffuse lighting conditions.
So this adds to both Lw and Lb produced by the display,
so the new full screen contrast at 5000lx ambient light
illuminance was:
Cfsa = (Lw + Lad)/(Lb + Lad) = (310 + 5)/(0,42 + 5)
= 315/5,42 = 58,12
So, the influence of the ambient light makes the
screen contrast drop from 738:1 to just about 58:1.
5000lx is not so untypical for daylight, it can be much
more on a sunny summer day.
Plus, the numbers above are for diffuse reflections
off the screen surface, which, from the numbers, seems
to have been a "glare display" with some antireflective
coating.
If you can see the ambient light source as a (dim)
mirror image on the screen surface, the contrast
may be significantly smaller, such as e.g. less
than 5:1 for the same display.
Or likewise, in a dark cinema, a brightly
lit door opens, and contrast diminishes.
In a front projection situation, the effect is much
more dominant because the projection screen is white
and so diffusely reflects much more light than an
LCD screen.
In the above example, the LCD screen produced only
like 5 cd/m² when diffusely lit with an ambient
illuminance of 5000lx. This is not very much and
probably that notebook screen has got some
antireflective coating to enhance it's ambient light
performance.
A projection screen with Gain=1 would produce like
1590 cd/m2 which is much more - but also inevitable
because the screen does reflect light on purpose.
Therefore, it is really difficult to have a front
projection system with a good contrast performance
under daylight conditions. You would need extremely
powerful projectors and when it really comes down
to it, the sun will probably win out all regardless.
All the best,
Dieter
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