Light, Color, and the Eye

Why do televisions use red, green, and blue? Why does white need a standard? Why do SDR and HDR use transfer functions instead of storing light in a simple straight line? And why can the same television look different in a bright room and a dark room?

The useful answer is that a television is built for a human observer.

A TV does not recreate the original spectrum of light from a scene. It produces a controlled pattern of light that can reproduce selected color and brightness relationships within the display's capabilities. Your eyes and brain turn that light into the picture you experience.

That distinction explains much of video engineering. Three classes of cone photoreceptors make three-primary color reproduction possible. Visual sensitivity changes with luminance and adaptation, which helps explain why transfer functions and viewing conditions matter. Chromatic adaptation means that white must be defined rather than assumed.

These ideas do not account for every choice in a television standard. Display technology, production practice, compatibility, bandwidth, and engineering history also matter. But human vision is the receiver at the end of the chain, so it shapes the entire system.

Light is physical, color is perceived

In physical terms, visible light is optical radiation capable of producing a visual sensation. Its wavelength boundaries are not exact. The International Commission on Illumination, or CIE, notes that the lower limit is generally taken somewhere between 360 and 400 nanometers, while the upper limit is generally taken between 760 and 830 nanometers. A shorter working range such as 380 to 780 nanometers is common in color measurement, but it should not be mistaken for a universal biological cutoff.^[1]

A light source can be described by its spectral distribution. In plain language, that means how much of a specified light-related quantity is present at each wavelength. A broad-spectrum source may produce energy across much of the visible range. A narrowband source may concentrate much more of its output into a smaller range.^[2]

That spectrum is a physical description of the stimulus. Color is the perception that follows.

CIE defines perceived color as a characteristic of visual perception described through attributes such as hue, brightness or lightness, and colorfulness. It also states that perceived color depends on the spectrum, the size and shape of the stimulus, its surroundings, the observer's state of adaptation, and previous visual experience.^[3]

This is why saying that a wavelength “is blue” is useful shorthand but not the whole story. Wavelength belongs to the radiation. Blue is the visual experience produced under particular conditions.

For television, the distinction matters because the signal is not a recording of every wavelength that entered a camera. It is a representation of chosen image values that a compatible display can turn back into light.

Why different spectra can look the same

Two lights can have different spectral distributions and still match in color for a specified observer under specified viewing conditions.

These matching stimuli are called metamers. CIE defines metameric color stimuli as spectrally different stimuli that have the same tristimulus values in a specified colorimetric system.^[4]

This is the foundation of display color reproduction.

A yellow object in the real world may reflect a broad and complicated spectrum. A television can create a similar color appearance by combining light from its display primaries. The screen does not need to reproduce the original spectrum wavelength by wavelength. It needs to create a suitable colorimetric match, within the limits of its gamut, luminance range, viewing conditions, and observer model.

The match is not guaranteed to be perfect for every person. Standard colorimetry uses defined standard observers so measurements can be repeated and compared. Real observers with normal color vision still differ, and a metameric match for one observer can shift for another. CIE refers to this as observer metamerism.^[5]

That limitation becomes more noticeable with some narrow-spectrum displays and light sources. It does not make colorimetry useless. It means that a measured match is always a match under stated conditions and for a defined observer model.

Three cone classes make three-primary color possible

The human retina contains three classes of cone photoreceptors used in normal photopic color vision. They are called L, M, and S cones, referring to their greater sensitivity to long, medium, and short wavelengths.^[6]

Their sensitivity ranges overlap. An ordinary visible stimulus usually affects more than one cone class, and the pattern of relative excitation matters more than a simple label such as red cone, green cone, or blue cone.

This trichromatic system makes three-primary color reproduction possible.

By adjusting three suitably chosen primary stimuli, a display can create a large range of combinations that match the color of other stimuli for a standard observer. CIE colorimetry formalizes color matching with three tristimulus values and standard color-matching functions.^[7]

Modern television systems also specify three color primaries. For example, ITU-R BT.709 defines red, green, and blue chromaticity coordinates for HDTV, along with D65 as the reference white. ITU-R BT.2100 uses the wider BT.2020 primary coordinates for HDR television and also specifies D65.^[8]

This does not mean that a television's red, green, and blue outputs are direct copies of the L, M, and S cone sensitivities. They are not.

It also does not mean that every panel contains only three identical types of subpixel. Some displays add a white component, use color conversion layers, or arrange subpixels differently. The important point is that the television signal and its colorimetry are based on three specified primary coordinates, while the panel must produce light that approaches those targets within its capabilities.

The eye does not send RGB pixels to the brain

It is tempting to picture the eye as a camera with three sensor channels. That analogy helps explain color matching, but it stops being accurate very quickly.

Cone excitations are only the beginning of visual processing. The retina contains neural circuits that compare signals across space and time, separate light and dark changes, sharpen boundaries, and send multiple processed pathways toward the brain. The conscious picture is not a direct dump of three cone values.^[9]

This distinction prevents a common misunderstanding.

A display can reproduce colorimetric values closely without reproducing the original spectrum, and without reproducing every aspect of the original visual experience. Depth, motion, glare, adaptation, field of view, surrounding light, and individual observer differences still affect what you see.

Color accuracy is therefore conditional. A display can approach the applicable colorimetric target under stated measurement conditions. That is meaningful, but it is not the same as recreating the original light field or guaranteeing identical perception for every viewer.

Rods, cones, and adaptation

Cones support daylight and color vision. Rods are more sensitive in low light and support much of scotopic, or very dim-light, vision. The human retina contains far more rods than cones overall, but the distribution is not uniform. The fovea at the center of vision is densely packed with cones, while rods are abundant farther from the center.^[9]

This is why a very faint star can sometimes be easier to detect when you look slightly beside it rather than directly at it. The example is useful, but television viewing usually occupies a mixed situation. Bright portions of the screen are mainly cone-mediated, while the dark room, peripheral field, and very low-luminance picture areas can involve rod-cone interactions.

The rod system by itself does not provide normal trichromatic color discrimination. Under twilight conditions, however, rod and cone signals can interact, so “rods see only black and white” is a useful simplification rather than a complete account of mesopic vision.

Your visual system also adapts to the amount and color of light around you.

After moving from a bright outdoor scene into a dim room, a television may initially appear subdued. After time in darkness, the same screen may appear much brighter. A picture that feels comfortable during the day may feel excessive at night because the observer and the room have changed, even when the television settings have not.

This is one reason professional evaluation environments control the display, ambient light, and surround. The goal is repeatability, not proof that every production room is built in exactly the same way. ITU-R BT.2035 explicitly defines a reference viewing method for HDTV evaluation while also noting that high-quality critical viewing facilities can be established in different ways.^[10]

Brightness perception and transfer functions

Human response to luminance is nonlinear and strongly affected by adaptation, contrast, spatial area, duration, and surrounding light. A doubling of physical luminance does not produce a simple, universal doubling of perceived brightness.

That does not mean the eye follows one perfect logarithmic curve. It means that a straight-line relationship between signal code and displayed light is not automatically the most useful representation for every part of a video system.

A transfer function defines a relationship between image values and light at a stage of the signal chain. The exact function and its purpose depend on the system.

For SDR television, ITU-R BT.709 defines a nonlinear opto-electronic transfer characteristic at the source. ITU-R BT.1886 defines a reference electro-optical transfer function for flat-panel displays used in HDTV studio production. BT.1886 states that its exponent-based reference function is based on measured CRT characteristics, preserving the established television appearance as flat panels replaced CRT reference displays.^[11]

That history matters. SDR “gamma” is not simply a modern perceptual code designed from scratch around human brightness sensitivity.

HDR uses different systems.

ITU-R BT.2100-3 specifies both Perceptual Quantization, or PQ, and Hybrid Log-Gamma, or HLG. The Recommendation states that PQ uses a nonlinear transfer function finely tuned to the human visual system so it can cover a very wide range of brightness levels efficiently for a given bit depth. HLG instead more closely follows previously established television transfer curves and provides a degree of compatibility with legacy displays and workflows.^[12]

So SDR gamma, PQ, and HLG should not be treated as interchangeable names for the same curve or as solutions with identical origins.

They are different transfer systems with different definitions, histories, and operating assumptions. Your television must identify the signal correctly and map it to the light output the panel can produce.

Why white needs a defined reference

The visual system partially adapts to the color of the prevailing illumination. This helps a familiar object remain recognizable across daylight, tungsten light, and other environments, although the perceived color does not remain perfectly unchanged.

Because adaptation and context affect what appears neutral, video cannot leave white as an undefined idea.

CIE standard illuminant D65 is a defined relative spectral distribution representing a phase of daylight with a nominal correlated color temperature near 6500 K. It is not a claim that one exact noon sky or one particular cloud condition is the only correct white.^[13]

For BT.709 and BT.2100 television colorimetry, D65 is the reference white. Both Recommendations give the CIE 1931 chromaticity coordinates as x = 0.3127 and y = 0.3290.^[8]

Those coordinates give production, display, and measurement systems a common target.

When a television's white point is displaced from that target, neutral grays and colors built from the primaries shift with it. The amount and visibility of that shift depend on the mode, the display, the observer, the room, and the content.

This is why a setting name alone cannot prove accuracy. A preset labeled Warm may be closer to D65 on one model and less accurate on another. To know how close a television is to the reference, use measurement or reliable model-specific testing performed in the relevant picture mode and signal format.

Viewing conditions change the picture you perceive

A television does not exist visually by itself. The wall behind it, lamps reflected in the screen, daylight entering the room, and the viewer's adaptation all affect the picture's appearance.

A dark surround can make the screen seem brighter and can change the way shadow detail and contrast are judged. Strong ambient light can lift the apparent black level through reflections and make dark detail harder to see. Colored light near the screen can influence adaptation and the appearance of neutral colors.

Professional recommendations control these variables because repeatable judgment requires more than a calibrated display.

ITU-R BT.2035 defines a reference environment for HDTV evaluation and states that the viewing environment can significantly change perceived image characteristics. ITU-R BT.2100-3 similarly specifies neutral D65 surroundings and controlled ambient light for critical HDR viewing, while making clear that viewing facilities need not be absolutely identical.^[10][12]

These recommendations apply to professional evaluation within their stated scopes. They are not rules that every living room must copy exactly.

For a home viewer, the practical lesson is simpler: control obvious reflections, avoid strong colored light near the screen, and choose a picture luminance that is comfortable for the room. A calibrated television can approach its reference targets, but the room still affects what reaches your eyes and how you interpret it.

What this means for your TV

The connection between vision and television can be reduced to four useful ideas.

Three cone classes make three-primary color practical

Television colorimetry uses three primaries because normal human color matching is trichromatic. The display uses those primaries to create colorimetric matches, not to reproduce every wavelength from the original scene.

Transfer functions are system definitions, not generic brightness controls

SDR, PQ, and HLG use different transfer relationships. Their design histories and purposes differ. The television must interpret the incoming format correctly before it can map the signal to the panel.

White is a measured target

For BT.709 and BT.2100 television systems, D65 provides the reference white. A picture-mode label does not establish conformity. Measurement determines how closely a particular television, mode, and signal condition approach the target.

The room is part of the viewing system

Ambient light, reflections, and adaptation affect perceived contrast, black level, brightness, and color. Professional reference conditions are designed for repeatability. Home conditions can be less formal, but they still matter.

Where this leaves us

Television does not reproduce reality wavelength by wavelength. It produces light that is organized to create intended color and brightness relationships for a human observer.

That system works because normal color matching is trichromatic, because different spectra can be metameric, because visual sensitivity changes with luminance and adaptation, and because a shared reference white gives production and display systems a common neutral point.

The system also has limits. A display has a finite gamut and luminance range. Real observers differ. Rooms differ. Transfer functions have defined scopes. A consumer television may approach a reference target in selected measurements without becoming identical to a professional reference monitor or recreating the original scene's complete light field.

Once you understand the receiver, the rest of video engineering becomes easier to follow. Color spaces, white balance, SDR gamma, PQ, HLG, tone mapping, and room setup stop looking like unrelated technical choices. They become connected parts of a system designed to turn signal values into a controlled visual experience.