The Monk Skin Tone Scale, Explained for Patients

We mention the Monk Skin Tone scale fairly often when we talk about how our screening was tested. It is a fair guess that most readers have no idea what it is or why a health device would care about it. So here is the plain explanation, because the reason it matters is genuinely important for the people we serve.

What it is

The Monk Skin Tone scale is a 10-point scale of human skin tones, running from lighter to darker. It was developed to be more representative of the actual range of human skin than older scales used in technology and medicine. You can think of it as a shared, standardised vocabulary for skin tone — a way for researchers and device-makers to say precisely which tones a product was tested on, instead of vaguely.

For us, the relevant range is Monk 4 to 10. That covers the medium-to-dark tones common across our community in West Bengal, including the darkest tones at 8 to 10.

Why a health device cares about skin tone at all

Here is the part that surprises people. Some medical measurements work by passing light through the skin — pulse oximetry for oxygen levels, and non-invasive haemoglobin estimation, both do this. Skin pigment affects how light behaves. So the accuracy of these light-based measurements can genuinely depend on skin tone.

This is not theoretical. Standard pulse oximeters have been shown to read less accurately on darker skin, often overestimating oxygen levels, which is the dangerous direction. The root cause traces back to how these devices were calibrated and validated — frequently on populations that skewed lighter-skinned, leaving darker tones underrepresented in the very testing meant to prove the device works.

So when a device measures something through the skin, "which skin tones was this tested on?" is not a niggling detail. It is a core question about whether the device works for you.

Why we validate across the full range

This is where the scale becomes practical rather than academic. When we ran our validation study, we deliberately recruited across Monk Skin Tone 4 to 10, including the darkest tones, and we report our accuracy broken down by skin-tone group rather than as a single average. We did this because a single average can hide a failure on darker skin, and because the population we serve is mostly in exactly the tones that older device testing left out.

The result we care about most is that our oxygen-saturation accuracy held across every one of those groups, including the darkest. That is what lets us say the screen works for the people it is built for, not just on average.

Why this should matter to you

If you have medium or dark skin and you have ever had your oxygen measured by a clip on your finger, there is a reasonable chance the reading was less reliable than you were told — not because anyone meant to mislead you, but because the device was never properly tested on skin like yours. The Monk Skin Tone scale is part of how the field is finally fixing that, by making it possible to demand proof that a device works across the full range of real people.

We use it because "works for everyone" is a claim that should be backed by evidence for everyone, especially the people usually left out of the testing. The scale is how that evidence gets demanded, measured, and reported.

FAQ

What is the Monk Skin Tone scale? A 10-point scale of skin tones, from lighter to darker, designed to represent the real range of human skin more inclusively than older scales, and used to specify which tones a product was tested on.

Why does skin tone affect medical devices? Some measurements pass light through the skin, and skin pigment affects how light behaves. Devices calibrated mostly on lighter skin can read less accurately on darker skin.

Was this screening tested on darker skin tones? Yes. It was validated across Monk Skin Tone 4 to 10, including the darkest tones (8 to 10), with accuracy reported per skin-tone group.