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The dichoptic true‐red tetrachromacy glasses enable a transformative shift in color perception by remapping hues onto a two‐dimensional plane—contrasting sharply with the one‐dimensional hue progression characteristic of standard trichromacy. In this engineered system, each hue is defined not solely by its chromatic value on the two‐dimensional plane but also by its independent saturation and luminance levels, thereby expanding the perceptual framework into a four‐dimensional color space that incorporates the novel L-/L+ stereoscopic opponency axis.
For instance, consider a dichromatic gradient spanning blue, cyan, and green. When this gradient is positioned orthogonal to the L+ channel, it gives rise to a distinct trichromatic hue subspace that encompasses a full range of luminosity variations, ultimately integrating with achromatic states. In effect, the addition of the L+ channel orthogonal to the anomalous trichromatic (protanopic) (S, M, and L-) input creates a richer, more nuanced palette than is available to conventional trichromats.
This phenomenon is exemplified in the accompanying program, Trichromatic Hue Gradients of True-Red Tetrachromacy. In this setup, one eye receives a robust trichromatic signal—predominantly mediated by the S, M, and L- cones—while the contralateral eye, equipped with the L+ filter, is limited to deep (i.e. true) red spectral input. Although the absence of the L- contribution in the above example would ordinarily yield dichromatic vision, the complementary L+ channel ensures that the overall percept remains trichromatic. The resulting visual output, while exhibiting slight anomalies due to the engineered spectral partitioning (e.g. imperfect binocular fusion), is sufficiently similar to standard trichromacy to function effectively in everyday perceptual contexts.
In summary, the locally trichromatic subspace—comprising dichromatic gradients of S, M, and L- channels juxtaposed against the orthogonal L+ input—constitutes a vital subset of the higher-dimensional tetrachromatic color space enabled by true-red non-retinal tetrachromacy. This integration of dichoptic inputs not only validates the concept of a two-dimensional hue plane but also underscores the profound implications of engineered spectral filtering in expanding human color perception.
Figure 10/4: This is image shows the 2-dimensional true-red tetrachromatic main hue categories on the hexagonally depicted surface of an unfolded sphere. This hexagonal-circular depiction removes any redundancies of the previous depictions 10/1-2.
Here's a simple visulization of the TRNT hue space. In this case, it's the surface of an unfolded tetrahedron.
Figure 10/1: TRNRT 2D hue subspace only showing the main hue categories. If not unfolded, tetrite (red-white) lies in the center of the volume of this tetrachromatic hue-saturation subspace.
Figure 10/1b: The same hue subspace as in Figure 10/1, but with the duplicate main hue categories omitted. Without tetrite, there are about 14 unique and saturated hues.
Figure 10/1c: The same hue space as in Figure 10/1, but with additional intermediate hues. All of these colors are unique hues.
Figure 10/2: The same hue subspace as in Figure 10/1. The red line approximates the hues (and colors) of TRNRT that my RGB screen can display .
Instructions: Cone Responsivity Curves of Multiple Color Visions
Left–click on the graph to add a spectral line which represents a light with this single (very narrow) wavelength.
Drag its colored top handle vertically to adjust its intensity.
Drag its black bottom handle horizontally to change its wavelength.
Right–click on either handle to remove that line.
In Yellow Tetrachromacy mode, you may click near a cone–curve’s peak to select that cone. The selected cone is highlighted and the next line you add will be locked to that cone. Wehn locked, a small white square indicator appears on its top handle, and the color of the top handle will be locked to the selected cone's color.
Notes:
In protanopia the M cone is sending a green signal. In the program, however, yellow is used to better simulate the opponency of S vs. M. In reality, for a protanopa cyan will not look white and green not yellow, but conversely white will look cyan and yellow will look green. In protanopia, cyan and white are the same hue.
The curves are simplified and more complex in reality. This is only a simulation.
In the Yellow Tetrachromacy cone model it's more difficult to showcase the new opponencies that the M'/L' cone type creates compared to True-Red Tetrachromacy. That's why in the Yellow Tetrachromacy cone model you have to click on the peak of a cone type to bind the next line and its color to this cone type. The new "primary" color of Tetrachromacy is simulated with a second type of trichromatic yellow that's orthogonal to the S, M and L color mixes due to the binocular (interocular) mix.
Below are the same cone models, but slightly more accurate. However, they're still simplified.
You can clearly see that and how the three spectral sensitivity curve representations (Fig. 11/1-3) differ from each other. While they seem relatively similar and they all cover the standard visible range of human vision, they differ greatly in the red-yellow-green region of the spectrum. When comparing wavelengths and a selection of their combinations from normal trichromacy to true-red tetrachromacy and other cone models, you can see the increase in colors, hue categories and color concepts from mono- to di-, tri- and tetrachromacy.
Figure 11/1: Spectral responsivity curves of normal trichromacy. Original Image Source.
In normal trichromacy each cone class sensitivity can be represented by a normalized bell curve. In Fig. 11/1 you can sees these curves and how they allow you to separate singular wavelengths or wavelength combinations on the continuous spectrum.
Figure 11/2: Approximate spectral responsivity curves of true-red tetrachromacy.
Here in Fig. 11/2 you can see an approximation of each cone class sensitivity of the virtual cone classes in true-red tetrachromacy. The L+ cone class is seen separately by the one eye, while the S, M & L- cone classes are seen by the second eye. The monochromatic L+ eye functions as a separate color channel that can be put orthogonal to and mix indepentedly (i.e. binoculary) with the S, M & L- color channels.
3D True-Red Tetrachromacy 2D Hue Ordering Test
3D True-Red Tetrachromacy 2D Hue Ordering Test additional instructions:
left click + drag: rotate the the hue sphere around the center point
right click + drag on color dot: exchange a specific color dot with another by dragging it onto another
mouse wheel up/down: zoom in and out
The "Trichromatic View" shows how the true-red tetrachromatic hues look like to a normal trichromat. Once the true-red tetrachromatic colors have been jumbled up, you can try to order them correctly.
Dichoptic True-Red Tetrachromacy Planar Hue Ordering Test
Program 13.3/1: Dichoptic true-red tetrachromacy planar hue ordering test.
Until now, I have presented evidence suggesting that the enhanced color vision generated by the dichoptic true-red tetrachromacy glasses constitutes a functional form of tetrachromacy. In fact, on paper and presumably also in practice, the functionality of dichoptic true-red tetrachromacy surpasses the most common retinal tetrachromacy found in humans by at least two orders of magnitude, as we will see later.
This section is currently a WIP!
How, then, does dichoptic true-red tetrachromacy compare to a functional retinal tetrachromacy in humans? Directly answering this question is challenging because, although we can quantitatively compare normal trichromacy to a putative retinal tetrachromacy, it currently is impossible to directly measure subjective color qualia. Thus, researchers typically rely on an subject's behavioral evidence, assessing whether color-discrimination tasks align with what one would expect from a genuinely strongly functional tetrachromatic visual system.
A parallel can be drawn from the transition between dichromacy (e.g., deuteranopia) and typical trichromacy to illustrate what a new cone might add to color experience. Deuteranopes have only two functioning cone classes: S and L. Deuteranopes lack an entire dimension of color discrimination compared to normal trichromats. For example, within the approximate range from 500-700 nanometers, a deuteranope perceives only one hue that varies by saturation and brightness. Similarly, the range from 380-485 nanometers collapses to a second unique hue that varies by saturation and brightness according to the overlap of the cone types' sensitivities. Put simply, a deuteranope’s entire color palette reduces to S (blue), L (red), S+L (white), K (black), or mixtures thereof.
When a third cone class (M) is added, one obtains trichromacy. Crucially, the new “green” axis allows for color experiences that are orthogonal to any S/L combination, introducing not just new hues but also a fundamentally different approach to saturation, in addition to a higher dimensional color space. In dichromacy, combining S and L might yield only a magenta/fuchsia/pink which can be indistinguishable from white; in trichromacy, introducing M results in a more nuanced continuum of hues that eventually converges to a trichromatic white when all three cones are equally stimulated. Deuteranopes cannot separate green from red purely by hue because they do not possess enough cone classes—a direct consequence of the principle of univariance. By contrast, trichromats do so effortlessly, navigating colors within a fully three-dimensional color space and a circular continuum of hues.
Extending this logic to tetrachromacy, one would expect an idealized “yellow” retinal tetrachromat—who has a fourth M' or L' cone peaking around greenish-yellowish (approximately 545 nm ±5–7 nm)—to distinguish a pure spectral yellow from a red-green mixture solely by hue. In normal trichromacy, a perfect mix of red and green light appears indistinguishable from a monochromatic yellow; they form a metameric match. A trichromat simply cannot separate red-green from yellow in the absence of contextual clues. Conversely, a retinal “yellow” tetrachromat would unambiguously label red-green (e.g. Agre) as one hue and yellow (e.g. Ellow) as another, akin to how trichromats can separate magenta from white or red from green regardless of brightness or contrast.
So, where does true-red tetrachromacy (TRNRT) stand in comparison? Examining the red-yellow-green range provides a telling illustration. A trained trichromatic observer with TRNRT can consistently and behaviorally differentiate a pure yellow from a red-green mixture by hue alone. A purely yellow stimulus appears black to one eye and yellow to the other (i.e. Ellow), while a red-green mixture appears red to one eye and green to the other (i.e. Agre)—producing categorically different combined hues. Under normal viewing conditions (unaided eyes), these two would look identical (both “yellow”) and may only differ in saturation, luminance and contextual clues, but with the true-red glasses they appear as distinct hues regardless of saturation, luminance and contextual clues. Consequently, the color-discrimination behavior of observers with TRNRT in the red-yellow-green region effectively becomes trichromatic, not dichromatic. This gain in discriminability extends to other tetrachromatic hues documented throughout prior examples, reinforcing that TRNRT generates a genuine and moderately functional form of tetrachromacy with a 4-dimensional color space.
In conclusion, while true-red non-retinal tetrachromacy (TRNRT) produces hues that may lack the vividness of strongly functional retinal tetrachromacy, it remains sufficiently robust to deliver moderately functional and behavioral tetrachromatic vision across relevant spectral ranges. Observers can distinguish colors along previously conflated and/or inaccessible axes, demonstrating that even “impossible” binocular color blends can powerfully extend the dimensionality of human color space.
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