Published LLLT studies use both 650nm and 670nm wavelengths, and current clinical evidence does not strongly differentiate between these two ranges for hair regrowth outcomes. Both wavelengths fall within the therapeutic window for photobiomodulation, so the real question is whether your specific device delivers enough total energy per session to stimulate follicular activity.
Understanding the LLLT Therapeutic Window
Low-Level Laser Therapy (LLLT) works through photobiomodulation: light energy absorbed by cytochrome c oxidase in mitochondria stimulates cellular metabolism and, in follicle cells, prolongs the anagen (growth) phase.
The therapeutic window for this effect sits between 630nm and 670nm for red light, with near-infrared at 780-830nm as a secondary range. FDA-cleared LLLT devices for hair loss use wavelengths within this range.
| Wavelength | Penetration Depth | Common Devices | FDA-Cleared |
|---|---|---|---|
| 650nm | 2-3mm into scalp | iRestore, HairMax LaserBand | Yes |
| 655nm | 2-3mm into scalp | Capillus | Yes |
| 670nm | 3-4mm into scalp | Theradome | Yes |
| 808nm (near-IR) | 5-8mm into scalp | Some combination devices | Varies |
The 20nm difference between 650nm and 670nm translates to roughly 1mm of additional penetration depth. Whether this matters depends on where your follicle bulbs sit, which varies by scalp region and individual anatomy.
Step 1: Establish Your Pre-LLLT Density Baseline
Before starting any LLLT device, capture your baseline density in myhairline.ai. This reading becomes the reference point for all future comparisons.
Photograph your primary areas of concern: the hairline, crown, and midscalp. Use consistent overhead lighting and position your phone 15cm from the scalp. Record the specific LLLT device you plan to use, including its wavelength and diode count.
Without a baseline, you cannot determine whether any later density change is from the laser or from natural fluctuation, seasonal shedding, or concurrent treatments.
Step 2: Track Your First Device for a Full Treatment Cycle
LLLT requires 16-24 weeks of consistent use before density changes become statistically meaningful. Track density every 4 weeks during this period.
| Week | Expected Observation | Action |
|---|---|---|
| 0-4 | No visible change | Continue protocol, track baseline variance |
| 4-8 | Possible initial shedding | Normal, do not discontinue |
| 8-12 | Early density stabilization | Compare to baseline |
| 12-16 | First measurable density gain (if responsive) | Document percentage change |
| 16-24 | Full treatment response visible | Assess total efficacy |
Log every session in myhairline.ai with your device name and wavelength tag. Consistency matters: missing more than 2 sessions per week reduces efficacy because the photobiomodulation effect requires regular stimulation to maintain mitochondrial upregulation.
Step 3: Analyze Whether Wavelength Is the Variable That Matters
If you are considering switching from a 650nm device to a 670nm device (or vice versa), isolate the wavelength as the variable by controlling for everything else.
The key confounders to control:
Diode count. A device with 272 diodes at 650nm delivers more total energy than a device with 80 diodes at 670nm, regardless of wavelength. Total energy dose (measured in joules per cm2) matters more than the specific nanometer setting.
Session duration. Different manufacturers recommend different session lengths. A 30-minute session at 650nm delivers roughly twice the energy of a 15-minute session at 670nm, making any wavelength comparison invalid.
Concurrent treatments. If you are using minoxidil (40-60% regrowth efficacy) or finasteride (80-90% halt further loss, 65% experience regrowth) alongside LLLT, the drug effects will dominate the density signal. Isolating the LLLT wavelength effect requires using LLLT as a standalone treatment.
Step 4: Run a Split Comparison (If You Have Two Devices)
The most rigorous personal test uses two different wavelength devices on different scalp zones simultaneously.
Assign your 650nm device to one area (for example, the left side of your crown) and your 670nm device to the right side. Track both zones independently in myhairline.ai over 24 weeks.
This within-subject comparison eliminates seasonal variation, dietary changes, and stress effects because both zones experience the same systemic conditions. The only variable is the wavelength applied to each zone.
At the 24-week mark, compare the density percentage change in each zone. A difference of less than 5% between zones suggests the wavelength is not a meaningful factor for your hair type.
Step 5: Evaluate the Dose-Response Relationship
LLLT follows a biphasic dose response curve, sometimes called the Arndt-Schulz curve. Too little energy produces no effect. The right amount stimulates growth. Too much energy inhibits cellular activity.
| Energy Dose | Expected Effect |
|---|---|
| Below 1 J/cm2 | Insufficient stimulation |
| 3-5 J/cm2 | Optimal therapeutic range |
| Above 8 J/cm2 | Possible inhibitory effect |
Most FDA-cleared devices are calibrated to deliver 3-5 J/cm2 per session at the recommended usage time. Doubling your session length does not double the benefit and may push you into the inhibitory zone.
If your myhairline.ai data shows density declining after increasing LLLT session duration, you may be overdosing. Return to the manufacturer's recommended protocol and track the recovery.
What the Research Actually Shows
A 2024 meta-analysis of 22 randomized controlled trials found that LLLT devices across the 630-670nm range produced statistically significant hair density increases compared to sham devices. The pooled effect size did not significantly differ between studies using 650nm versus 670nm devices.
The factors that did correlate with better outcomes:
- Higher total energy dose per session (within the 3-5 J/cm2 range)
- Greater number of diodes (more uniform scalp coverage)
- Longer treatment duration (24 weeks outperformed 16 weeks)
- Patient compliance (3+ sessions per week)
When to Consider Switching Devices
Switch devices based on your tracking data, not marketing claims. If your myhairline.ai density timeline shows no improvement after 24 weeks of consistent use with one device, switching to a device with more diodes or a different wavelength is a reasonable next step.
Track your new device for another 24-week cycle before drawing conclusions. The total investment is nearly a year, but this is the minimum timeframe needed to generate reliable density data from LLLT.
Start tracking your LLLT response with objective density data at myhairline.ai/analyze to see whether your device and wavelength combination is producing results.
This article is for informational purposes only and does not constitute medical advice. Consult a board-certified dermatologist for personalized guidance on laser therapy for hair loss.