What is Bias Lighting?

Bias lighting is the lighting that is placed behind the screen you are viewing such that it raises the ambient light levels in the viewing area without directly shining light toward the viewer nor shining light past the viewer toward the screen (where it could create reflections and other viewing problems). Because the light originates outside of the sightline of the viewer and is not in a direct path to reflect onto the screen, you get all the benefits of increased luminance in the room without the problems of glare or light shining directly from the source into your eyes.

In addition to warding off eye fatigue there are two great benefits. First, the additional indirect lighting provided by the bias lighting increases the luminance ratio of the on-screen image.

Refer to the optical illusion image above to see the effect made apparent. The bar that stretches across the center of the image is one constant shade of gray (RGB: 142, 142, 142) but it appears to be lighter in on the dark side of the gradient and darker on the light side of the gradient. This illusion, known as the simultaneous contrast illusion, readily illustrates how our eyes perceive gray to be darker and richer when seen against a lighter background and more washed out when seen against a dark background. Illuminate the wall behind your screen and the same contrast illusion takes effect: the grays and blacks on your screen will appear richer and the contrast will seem stronger between them and the surrounding area.

Second, by increasing the luminance ratio, the white level or brightness can be decreased which will result in extending the life of the backlight and the display saving you money.

For interpretation of digital radiographic images a bias light source CCT (color corrected temperature) of 5,500 - 6500K (CIE D65 white point) is recommended. Most backlight display systems whether CCFL or LED are calibrated to a 6500K white reference point.

The RightLight Bias Lighting System is designed to work with the RightLight Monitor to provide the appropriate amount and quality of ambient light for the optimal viewing and interpretation.

References:

SMPTE Recommended Practices document: RP166-1995: ‘Critical Viewing Conditions For Evaluation Of Color Television Pictures’