Lab a* redness
The color channel dermatology research uses to quantify redness. Two letters and a number that beats 'my skin looks red today' for any actual measurement.
Definition
Lab a* (also written L*a*b*) is a color space defined by the International Commission on Illumination (CIE) in 1976. It separates a color into three channels: L* for lightness, a* for the red-green axis, and b* for the yellow-blue axis. The a* channel ranges from negative (green) to positive (red). In dermatology research, the a* value of a skin region is used as a quantitative measure of redness. Higher a* means more red.
Why dermatology uses Lab a*
Skin redness (erythema) is a continuous, color-driven phenomenon. RGB pixel values are device dependent and shift with lighting. Lab a* is designed to be perceptually meaningful and device independent, so the same a* value across two captures means the same redness. Research instruments like the Mexameter (MX 18) and Chromameter (CR-400) both report a redness index closely related to Lab a*. This is why clinical erythema endpoints almost always report a* or a normalized version of it.
Where you see Lab a* in skin research
- Rosacea trials: pre and post treatment Lab a* values on the cheek
- Sensitive skin and barrier studies: TEWL plus Lab a* together
- Post-procedure healing: laser, microneedling, peels track Lab a* over days to weeks
- Acne inflammation: peri-lesional Lab a* as an inflammation proxy
Standardizing the capture matters
The Lab a* value of a pixel changes meaningfully with lighting, distance, and camera. A bathroom selfie under warm light reads completely different from a bright daylight photo. For an a* measurement to be comparable across days you have to fix the capture conditions. This is the same standardization problem that affects acne counts, and the same fix applies: forced flash, fixed distance window, consistent camera.
How Trace measures Lab a*
Trace runs an on-device Lab color conversion on each daily scan, computes a* across multiple face regions (cheeks, forehead, perinasal), and surfaces a redness score that maps to the a* trend. The trend over thirty days is what tells you whether a tretinoin or an azelaic acid is lowering inflammation, or whether a new actives is raising it. All math is on the phone, no images sent to a server.
Trace's Lab a* measurement is engineered for trend-over-time, not for clinical diagnosis. Persistent or unexplained redness should be evaluated by a dermatologist.
Frequently asked
What is the difference between Lab a* and RGB redness?
RGB values are device dependent. The same scene captured by two different phones gives different RGB numbers, and the same phone gives different values under different lighting. Lab a* is designed to be device independent and perceptually meaningful, which is why dermatology research uses it instead of raw RGB.
Is Lab a* the same as the Mexameter erythema index?
Closely related, not identical. The Mexameter reports an erythema index calibrated to absorbed light at specific wavelengths. Both correlate strongly with redness. Lab a* is more accessible because it can be computed from any standardized color image.
Does Trace use Lab a* on device?
Yes. All color conversion and a* computation runs entirely on the iPhone. No image is uploaded for redness analysis.