In photography, reducing amount of light is a standard problem. A standard solution is to use a neutral gray filter. Photographic gray filter of this size would cost thousands of $, and clearly would be an overkill (spectrographic superbness is not required here).
Cheaper solution (I think):
- polarizing foil should eat half the light. Two pieces of a polarizing foil allow to adjust the light transmittance, by adjusting the angle between polarising axes. After you are sure how much light is enough you can cut it so that it was more usable.
Some thinking is required to make the foil solution that convenient (air bubbles, light leakage around the screen etc.).