Notes by Dr. Optoglass: Color Bit Depth, Middle Grey and White Balance of the Human Eye

Topics Covered:

  • Color Bit Depth
  • Middle Grey
  • Whiteness and White Balance

Colors, like features, follow the changes of the emotions – Pablo Picasso

We have already seen that the color space of the eye, as defined by the CIE 1931 XYZ color space, estimates about 10 million colors for our vision. The word size required to represent 10 million colors is 24-bits. If each color is divided into its RGB components, the color bit depth required per channel is 24/3 = 8-bits.

The industry standard for color bit-depth for consumer electronics is 8-bits per channel. Professional monitors are designed to ‘see’ and ‘show’ 10-bit images. Why do we need 10-bit, 16-bit or 32-bit images?

The color space of consumer displays is limited to a standard called sRGB (similar to Rec. 709 for HDTV).

Even though a display can show 10 million colors in 8-bit mode under the sRGB color space, it still misses out on many colors within the CIE 1931 XYZ gamut. For this reason a higher bit depth (10-bit, 16-bit, etc) is chosen to allow for more colors under wide gamut color spaces. A wide gamut color space is a bigger balloon than sRGB, and one hopes to ‘reign in’ more colors that way – by casting a wider net, so to speak.

In either case, comparing why a display or television panel needs more bits to how many bits per channel the eye really needs, is a waste of time.

Therefore, as long as the color space is CIE 1931 XYZ, the color bit depth per channel per pixel for the human eye is 8-bits.

Middle Grey

Middle grey is a tone that is perceptually about half way between black and white on a lightness scale. On a linear scale, it is supposed to be exactly on the 50% mark. In practice, though, this might not always be the case. E.g., the middle grey of the sRGB color space is at 46.6%.
Middle Grey Values

To get the middle grey value use this formula

Middle Grey % = 100 / Maximum Dynamic Range

Traditionally, middle grey is represented on the log scale. In the olden days, film stock had only about 5.5 stops of dynamic range, and for this reason middle grey fell on 18% from the white level. 18% grey cards are still used today, even though modern image sensors have at least 8 stops of dynamic range. This is why modern camera meters normally choose 12% as middle-grey.

Today, there are cameras quite capable of 13+ stops of dynamic range, and even 12% middle grey needn’t be accurate.

The eye has a dynamic range of 10 stops, for which the middle grey will fall at about 10%. If we assumed 20 stops, the middle grey will fall at about 5%. If we assume 30 stops, the middle grey is about 3.3%.

Whiteness

How white is white? Is there such a thing as perfect white?

Not really. In order to measure white a theoretical light source is needed. According to CIE, this source is D65, also called a standard illuminant, which is intended to represent average daylight and has a correlated colour temperature of approximately 6500 K.


Usually, in the video and film world daylight is said to be about 5600K, which roughly corresponds to D55. According to CIE D55 represents morning and afternoon daylight – not high noon. But since most productions shoot in morning or afternoon light, and not at high noon, 5600K is a great standard to follow. However, human perception of white is based on high noon.

The sRGB color space also uses D65 as its white point. We can safely assume the eye’s white point is between 6000K and 6500K.

White Balance

The eye is balanced to the sun’s color temperature, and when colored lights are used (as with red light in a photographic darkroom), the eye is unable to balance – which means change – its white point to keep a white object white. We perceive a white object to still be white, mostly based on our experience of the object, but it will not appear white. An important example of this fact is the different color temperatures of paper used in printing or painting art. Slight variations in the white point of the paper can change the perception of the colors being printed.

In the video world, indoor lighting is frequently artificial, or at least from various sources with different color temperatures. If sources all have a similar color temperature (like incandescent bulbs for example), the white balance point can be shifted accordingly and the scene will be recorded as if daylight balanced lights were used. When lights of different color temperatures are used together, the white balance becomes a matter of subjective creativity.

The process of shifting the white point in camera is called white balancing.

Takeaways:

  • The color bit depth required by the eye is only 8-bits per channel.
  • The eye has a middle grey point of roughly 10%, corresponding to a 20-stop dynamic range.
  • The standard white point for sRGB and HDTV is D65, which corresponds to 6500K.
  • The process of shifting the white point in camera is called white balancing.

Links for further study:

Next: Motion and the Frame Rate of the Human Eye
Previous: Dynamic Range of the Human Eye

2 replies on “Notes by Dr. Optoglass: Color Bit Depth, Middle Grey and White Balance of the Human Eye”

  1. Misleading information about the human eye’s DR. You wrote that human’s eye DR is roughly 10 stops which resembles a 10% middle grey (as per the formula), then in your tips you say: “The eye has a middle grey point of roughly 10%, corresponding to a 20-stop dynamic range”!??
    Could you explain please or it is just a typo?

  2. The fact is, high bit-depth video is more efficient and just because 8bits can represent enough color, transitions between colors  will be unpleasant without dithering and adding noise. Higher-bitdepth video allows for smoother transitions with less of that noise.

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