What is a bias light?

A bias light is a light source used to illuminate the area directly behind the display it's attached to. 

 

Why is a bias light important?

• Bias lights help to reduce eye fatigue while working (especially with High Dynamic Range images).
• Constrains the colorists adaptation state to ensure it stays phototopic, rather than mesopic where color perception becomes compromised.
• Constrains the viewers perception of black to compensate for non-zero black levels.

SMPTE specifies a 5 nit ambient surround for reference viewing environments (not working in a perfectly dark or lights out grading room) see SMPTE ST 2080-3.

What type of bias light should I use?

It's imperative that the chromaticity of bias lights used in professional content creation or color critical scenarios match the display chromaticity of the reference white in use. For Dolby Vision, this is D65 (6500K). Likewise, you should use a bias light with adjustable luminance (usually in the form of a manual dial or dimmer) and avoid options that have variable color temperatures. The MediaLight Pro2 is a good example that meets these criteria. Dolby also recommends using a neutral gray wall color for the bias light to bounce off of.

 

 

Note: For more information on properly setting up your grading or mastering suite, refer to our dedicated article Color Grading & Mastering Suite Setup.

How do I ensure my bias light is set to 5 nits?

Accurately measuring the surround luminance value created by the bias light is complex:

• Need specialized measurement device
• Confusion on where to measure from (viewing angle and distance, direct from light source vs. background reflectance, etc.) 

To combat this, Dolby has developed a set of 5 nit PQ test patches that you can use to perceptually set your bias light correctly. You can download these patches here: DOLBY BIAS LIGHT TIFFS.

 

1. Load the TIFF image into your color corrector that corresponds to your bias light setup. If your bias light is attached to 3 sides (inverted 'U') of your display (sitting on a desktop), use Set5NitsSurround_ThreeEdges.tif. If your bias light setup is 4 sided (mounted on a wall), use Set5NitsSurround_FourEdges.
2. Ensure that your color management settings inside the color corrector are configured to output a PQ (ST2084) signal to your display.
3. Confirm that your display is properly configured to display an HDR PQ signal (color space P3 or Rec.2020).
4. With the patch displayed fullscreen, perceptually match the ambient luminance created by your bias light to the 5 nit perimeter of the test patch. This can be done by manually adjusting the intensity control or dimmer of your bias light until a close match is achieved. Dolby recommends making the judgements from the position the operator or colorist will be sitting (ideally 3 picture heights per SMPTE ST 2080-3).