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Red-green color blindness
Protan vs Deutan
Protan and deutan are the two red-green color blindness patterns. Protan affects the red (L) cone; deutan affects the green (M) cone. Both blur reds and greens, so they can look alike on a hidden-number plate. The giveaway is brightness: protans also see reds as darker and dimmer, while deutans keep roughly normal brightness. Each has a milder "anomaly" form and a stronger form where the cone is missing.
If a screening says you're red-green color blind, the next question is which kind. Protan and deutan create similar confusions but come from different cones, and separating them takes more than a plate test — usually an arrangement test like the Farnsworth D-15. Here is what sets them apart, the clue you may notice yourself, and why it matters for things like glasses.
Protan vs deutan at a glance
Both are red-green types and confuse similar colors. The brightness row is the clue you can sometimes notice yourself.
| Protan (red cone) | Deutan (green cone) | |
|---|---|---|
| Cone affected | L-cone ("red") | M-cone ("green") |
| Forms | Protanomaly (mild), protanopia (strong) | Deuteranomaly (mild), deuteranopia (strong) |
| How reds look | Darker and dimmer than normal | Normal brightness |
| Main confusions | Red–green, plus red with black/brown | Red–green, plus green with brown/grey |
| Brightness sense | Reduced for red light | Roughly normal |
| How common | ~1% of men each form | Deuteranomaly ~6% (most common of all) |
What is protan color blindness?
Protan types involve the L-cone, the cone most sensitive to long-wavelength ("red") light. In protanomaly the cone is shifted and the deficiency is mild-to-moderate; in protanopia it is missing and the deficiency is strong. The L-cone also carries much of our sense of red brightness, so losing or shifting it does more than mix up hues.
That extra effect is the protan signature: reds look noticeably darker. A red rose can read as deep brown or near-black, a red LED may look unlit, and a red traffic light can seem dimmer than the green beside it.
What is deutan color blindness?
Deutan types involve the M-cone, which is sensitive to medium-wavelength ("green") light. Deuteranomaly is the shifted, milder form and is the single most common type of color blindness; deuteranopia is the form where the cone is absent. The M-cone contributes little to overall brightness, so deutans usually keep normal brightness perception.
Without that brightness shift, deutan confusions are more purely about hue: greens drifting into brown and beige, reds and greens collapsing together, and some greys taking on a faint green or pink cast.
Protan vs deutan: the key difference
On a plate test, protan and deutan can look almost the same — both miss the same hidden numbers. Brightness is where they split. Because the L-cone drives our sense of red brightness and the M-cone does not, protans see reds as dimmer while deutans do not. That is the one clue you may spot in daily life without a clinic.
Precise sorting still needs an arrangement test. The Farnsworth D-15 reveals a confusion axis that points one way for protan and another for deutan, and an anomaloscope settles it in a clinic. A plate test screens for red-green deficiency; it does not reliably name the side.
Does it matter for color blind glasses?
It can. Filter glasses cut the band of the spectrum where red and green signals overlap, and lenses are often tuned slightly differently for protan and deutan. Knowing your side, and how strong the deficiency is, sets a realistic expectation: mild-to-moderate cases in bright light tend to notice the most, strong cases the least. Glasses enhance some colors for some people and do not cure color blindness.
Find out which type you are
A plate test screens for red-green deficiency but doesn't name the side. The color blind test estimates protan vs deutan and degree in about two minutes, and the Farnsworth D-15 shows the confusion axis that separates them.
Protan vs deutan — frequently asked questions
- What is the difference between protan and deutan?
- Protan affects the red (L) cone and also makes reds look darker and dimmer; deutan affects the green (M) cone and keeps brightness roughly normal. Both are red-green types and confuse similar colors, but only protan dims reds.
- What is the difference between protanopia and deuteranopia?
- Both are strong dichromat forms where a cone is missing. Protanopia means the red (L) cone is missing, so reds look very dark; deuteranopia means the green (M) cone is missing, with normal brightness. The milder shifted forms are protanomaly and deuteranomaly.
- Which is more common, protan or deutan?
- Deutan. Deuteranomaly alone affects about 6% of men, making it the most common form of color blindness. Each protan form affects roughly 1% of men.
- How do I know if I'm protan or deutan?
- A plate test like Ishihara screens for red-green deficiency but doesn't reliably name the side. An online color blind test can estimate it, and an arrangement test like the Farnsworth D-15 — or an anomaloscope in a clinic — confirms it. The everyday clue is whether reds look unusually dark, which points to protan.
Keep reading
Deutan color blindness
Deuteranomaly and deuteranopia in depth — the more common of the two.
Red-green color blindness
The whole family: all four forms, prevalence, and which colors go wrong.
Types of color blindness
Protan, deutan, tritan and monochromacy, with how each is diagnosed.
Farnsworth D-15
The arrangement test that shows which confusion axis is yours.
Sources
- National Eye Institute (NEI) — Types of Color Blindness
- American Academy of Ophthalmology — What Is Color Blindness?
- Colour Blind Awareness — Types of colour blindness
Last reviewed June 28, 2026. Informational only and regularly reviewed — not a substitute for advice from an eye-care professional.