Colour Blindness & Glasses: The Science of Lenses That "Unlock" Colours

The most common form of colour blindness is not the complete absence of colour, but difficulty telling red and green shades apart. It stems from an overlap in the sensitivity of two types of cone in the retina. In recent years, glasses with "notch" filters have promised more vivid colour perception for some people. Let's look at what actually happens optically, no marketing, just physics.

How we see colour: the three cones

In the retina we have three types of cone, sensitive to short (blue, S), medium (green, M) and long (red, L) wavelengths. Colour is not "read" by a single cone; the brain compares the relative signal ratios between the three. In the most common form of colour blindness, the sensitivity curves of the M and L cones shift and overlap more than they should. So a red and a green stimulus produce similar signal ratios, and the brain "confuses" them.

What a notch filter does

A notch filter is an optical coating that absorbs or reflects a very narrow "slice" of the visible spectrum. In lenses for colour blindness, the notch is placed roughly in the region where the sensitivity of the M and L cones overlaps most strongly, often around the green-yellow to orange wavelengths.

• By removing this "confusion zone", the light reaching the eye stimulates the two cones more unevenly.
• The signal difference between red and green grows, meaning contrast increases.
• The brain receives more "cleanly" separated data and the shades appear more saturated.

It is the same logic of spectral selectivity we meet in other optical lens technologies; if you're interested in the physics of coatings, see how anti-reflective coating works through interference and how polarization "kills" reflection.

Why it isn't a "cure"

Here lies the most misunderstood aspect. The notch filter doesn't change the cones nor add information that isn't there. It simply rearranges the incoming spectrum so the existing system can distinguish differences more easily. In practice this means:

The response varies greatly from person to person and depends on the exact type and severity of the colour blindness. Because the notch removes light, the lenses usually work best in bright light, which is why many solutions come in the form of sunglasses.

In practice: outdoors vs indoors

The same filter performs differently in the sun and indoors under artificial lighting. Outdoor light covers the full spectrum, so filtering boosts contrast more effectively. Indoor lighting (especially LED) may have "gaps" in its spectrum that limit the benefit. That's why manufacturers often offer different variants for each condition.

If you're considering lenses of this type in sunglasses form, it's worth first looking at the basic principles of lens choice. On the sunglasses page and in the guide to lens colour by use you'll understand how lens tint affects perception overall, which is relevant since notch filters work precisely on colour.

When it's worth trying

• You have diagnosed mild to moderate red-green colour blindness.
• Your activities depend on distinguishing colours (design, nature, sorting).
• You have realistic expectations: contrast enhancement, not full "acquisition" of colours.

For an accurate diagnosis of the type of colour blindness and whether these particular lenses suit your case, the most reliable route is an eye doctor. The science behind notch filters is elegant, but the response remains a personal matter.

If you want to explore lens and frame options that suit your needs, take a look at the sunglasses collection, and whenever you're in doubt about prescription or filter, consult the specialist first.