From: dmichel@prosound.de   
      
   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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)