Last updated: September 9, 2026 | 12-minute read
UVB phototherapy dosage is the ultraviolet-B energy delivered per unit area of skin during a treatment, usually measured in mJ/cm². The prescribed dose depends on the skin condition, individual sensitivity, and response to previous sessions. Exposure time depends on the device's UVB irradiance at the treatment position.
This guide focuses on narrowband UVB (NB-UVB), commonly around 311–313 nm. It explains starting-dose tables, Minimal Erythemal Dose (MED), treatment timing, dose adjustments, and equipment checks. The numerical examples are educational; a dermatologist should prescribe the individual treatment plan.
What happened when a first-time user got the dose wrong — and what it reveals about the system
Patient reviewing UVB phototherapy dosage session log beside clinical light panel
Consider a hypothetical home-treatment mistake: a person with psoriasis copies a session time from another UVB device and develops painful redness after several treatments.
The copied time does not establish how much UVB reached the skin. If the home unit delivers higher irradiance at the treatment position, the same number of seconds delivers a larger dose. An unsuitable starting dose, medication-related sensitivity, or rapid increases could also contribute.
The lesson is practical: check the prescribed dose, device output, treatment position, and previous skin response together. Skin phototype helps guide the prescription; irradiance and exposure time determine the incident energy. The National Psoriasis Foundation recommends prescribed home treatment with medical follow-up.
Understanding the units: what MED and mJ/cm² actually mean
Diagram showing ultraviolet radiation striking skin surface with UVB phototherapy dosage measurement overlay
The Minimal Erythemal Dose (MED) is the lowest test exposure that produces just-perceptible redness under a defined assessment procedure, commonly about 24 hours later. Its value varies with the person, body site, and UV spectrum. MED itself can be expressed in mJ/cm². See the BAD/BPG measurement guideline.
| Term | Meaning | Role in treatment |
|---|---|---|
| Dose, in mJ/cm² | UV energy delivered per unit area | Records the prescribed and delivered exposure |
| Irradiance, in mW/cm² | UV power reaching each unit area | Determines how quickly a dose is delivered |
| Exposure time, in seconds | Duration of treatment | Connects irradiance to dose |
| MED | A measured skin-redness threshold | Helps assess sensitivity and select a starting dose |
For constant irradiance:
Dose (mJ/cm²) = Irradiance (mW/cm²) × Time (seconds)
Time (seconds) = Dose (mJ/cm²) ÷ Irradiance (mW/cm²)
One mW equals one mJ per second, and 1 J/cm² = 1,000 mJ/cm². Check units before entering a value. If output varies, dosimetry must account for the variation. The BPG dosimetry report explains the measurement framework.
How skin phototype sets your starting dose — and why the table is not the whole story
Fitzpatrick skin type scale chart showing Types I through VI with a published NB-UVB starting dose example
Fitzpatrick skin type describes typical burning and tanning responses. It can guide UVB phototherapy dosage within a clinical protocol, but it does not directly measure an individual's sensitivity.
The table below comes from the office-based psoriasis protocol in the LITE trial, Supplement 1, Appendix A. It illustrates one published approach, rather than a universal starting-dose chart.
| Fitzpatrick Type | Typical Sun Response | Starting Dose in This Protocol |
|---|---|---|
| I–II | Burns readily; tans poorly or with difficulty | 300 mJ/cm² |
| III–IV | Tans more readily; burning response varies | 500 mJ/cm² |
| V–VI | Usually tans deeply and rarely burns | 800 mJ/cm² |
These values are not individual treatment recommendations. Psoriasis, vitiligo, and eczema can require different protocols. The BAD/BPG 2022 guideline recommends MED testing or a small-area test before treatment.
A clinician also reviews treatment area, medicines, recent UV exposure, and previous reactions. Report new creams and medicines before the next session. The AAD advises against applying salicylic acid before UVB treatment, because it can reduce effectiveness. Follow product-specific instructions for other topical treatments.
How irradiance and distance determine your actual dose — a worked example
Illustrated cross-section of UVB phototherapy dosage calculation with light panel distance and irradiance labels
Suppose a prescribed target is 300 mJ/cm² and the relevant UVB irradiance is 3 mW/cm²:
Exposure time = 300 ÷ 3 = 100 seconds, or 1 minute 40 seconds.
The following assumed readings show how a change in irradiance affects the calculation. They are not measurements from a real device or recommended treatment distances.
| Measurement Position | Hypothetical UVB Irradiance | Time for 300 mJ/cm² |
|---|---|---|
| 15 cm | 3 mW/cm² | 100 seconds |
| 30 cm | 2 mW/cm² | 150 seconds |
Use irradiance established at the prescribed treatment position. Nearby extended lamp arrays do not necessarily follow a simple inverse-square relationship. Doubling distance therefore does not automatically reduce irradiance to one quarter. The Light Measurement Handbook explains the importance of source geometry.
Why dosage is device-specific: the role of irradiance settings and modes
Dose increases depend on the previous skin reaction and the time since the last treatment. A plan should specify when to increase, repeat, reduce, or pause the dose.
In the 2024 LITE randomized trial, prescribed home NB-UVB with guided dosimetry was noninferior to office treatment for the study's psoriasis outcomes. Persistent redness occurred more often at home. The findings support supervised home treatment with appropriate equipment, training, and follow-up.
There is no single percentage increase for every patient or device. One published office protocol in the trial allowed a 20% increase after minimal redness lasting less than 24 hours, but repeated the previous dose when redness lasted 24–48 hours. These examples show why both severity and duration matter; follow the clinician's own protocol.
After missed sessions, check the restart instructions. The length of the gap determines whether the previous dose can be repeated, reduced, or restarted at a lower level. Avoid adding extra exposure to catch up.
A simple treatment log helps the clinical team review UVB phototherapy dosage:
| Record | Include |
|---|---|
| Session | Date, time, treatment area, and interval since the last exposure |
| Delivery | Dose in mJ/cm², device settings, and exposure time where applicable |
| Position | Prescribed distance, positioning, and shielding |
| Skin response | Redness, tenderness, itching, blistering, and how long symptoms lasted |
| Changes | New medicines, creams, extra sunlight, or equipment changes |
Most psoriasis schedules involve two or three treatments weekly, according to the AAD. Improvement takes a course of treatment; a few sessions without visible change do not justify faster increases. Knowing what not to do after phototherapy, including avoiding unplanned UV exposure, also helps keep the plan consistent.
Does your device deliver what it claims? Why manufacturing standards matter for dosage accuracy
Quality inspection of light therapy panel irradiance output on test station
Output needs verification throughout the device's service life. Lamp aging, temperature, dirty optical surfaces, and lamp replacement can affect irradiance. Dose-controlled equipment also needs its monitoring system checked. The BPG dosimetry report describes these measurement and maintenance issues.
For clinics and buyers, a useful test report identifies:
- The exact model, serial number, lamp type, and wavelength spectrum.
- Treatment distance and position, including output variation across the treatment area.
- The measuring instrument, calibration date, and test conditions.
- Timer or dose-control accuracy, output stability, and maintenance requirements.
Quality and safety documents answer different questions:
| Document | What It Supports | What to Verify Separately |
|---|---|---|
| ISO 13485 / MDSAP | Medical-device quality management and regulatory auditing | Model-specific UVB output and calibration results |
| ETL Listed | Compliance with applicable safety standards | The evidence behind each irradiance claim |
| IEC 62471 | Photobiological hazard assessment | Clinical dosing instructions for the intended treatment |
| FDA registration and listing | Establishment and device-listing information | Applicable marketing authorization; registration alone is not approval |
Match every document to the manufacturer, model, and intended use. A certificate for a red/infrared device does not establish suitability for UVB treatment. For UVB phototherapy dosage, the practical evidence is a documented dose supported by verified output at the prescribed position.
Key Takeaways
- Dose and time are different: mJ/cm² records exposure; seconds depend on UVB irradiance.
- Starting doses are individual: use the prescribed protocol, with skin-sensitivity assessment.
- Position and configuration matter: keep them consistent and use the correct output data.
- Adjustments follow skin response: report delayed reactions and treatment gaps.
- Documentation supports accuracy: combine calibration records with clinical follow-up.
FAQ
What is a typical starting dose for NB-UVB phototherapy by skin type?
One published psoriasis protocol uses 300 mJ/cm² for Types I–II, 500 for III–IV, and 800 for V–VI. These are illustrative values from the LITE trial protocol, not universal starting doses. Your clinician selects the dose for your condition and sensitivity.
How do I calculate my exposure time from an irradiance figure in mW/cm²?
Divide the prescribed dose in mJ/cm² by the relevant UVB irradiance in mW/cm². For example, 300 ÷ 3 = 100 seconds. The irradiance must match the device's spectrum, configuration, and treatment position.
What is the difference between MED and mJ/cm² in phototherapy dosage?
MED is a measured threshold for skin redness. The unit mJ/cm² measures radiant exposure and can express both MED and treatment doses. A prescription based on a fraction of MED therefore requires the measured MED value.
How often should UVB phototherapy sessions be scheduled per week?
Two or three sessions weekly are common. Frequency and spacing depend on the condition and prescribed regimen. Twice-weekly treatment can be an active treatment schedule. Follow the assigned appointments instead of adding sessions to compensate for a gap. See the AAD patient guide.
By how much should I increase my UVB dose each session?
Use the increase set by your treatment team. Redness, discomfort, or a treatment interruption can require repeating, reducing, or withholding the next dose. The BAD/BPG guideline recommends defined treatment and adverse-reaction protocols.
What happens if I receive too high a UVB dose in a single session?
Excess exposure can cause delayed redness, tenderness, pain, or blistering. If you suspect a burn, pause further exposure and contact your treatment team. Severe pain, blistering, or extensive redness needs prompt medical assessment. See BAD's NB-UVB patient information.
Can I use the same dosage protocol from a clinic device on a home device?
A clinician may adapt a clinical protocol for a suitable home unit. Exposure time and controller settings must match that unit's calibration and instructions. Have the home plan confirmed before transferring any settings.
What not to do after phototherapy to avoid disrupting the dose plan?
Avoid tanning beds, extra UV sessions, and unplanned sun exposure. Use the skincare recommended by your treatment team and report delayed reactions. Products applied afterward cannot change the energy already delivered, but some may irritate skin or affect later UV sensitivity. Follow BAD's treatment-day advice.
How does treatment distance affect the dose I actually receive?
Distance changes irradiance and its distribution across the skin. The relationship depends on the source and optics, especially close to a large lamp array. Use the prescribed position and verified output data; avoid estimating dose from a universal distance rule.
At what cumulative dose should UVB phototherapy be reassessed?
There is no universal lifetime J/cm² cutoff. Review progress and adverse effects during treatment. BAD recommends skin-cancer surveillance at appropriate intervals after more than 500 whole-body NB-UVB treatments, particularly with other risk factors. This is a surveillance recommendation, not an automatic stopping point. See recommendation R6.
Long-term evidence varies: a large vitiligo cohort found no increased skin-cancer risk, while a Finnish registry study observed increased incidence. These observational findings do not establish a universal safe-dose ceiling.
References
- British Association of Dermatologists and British Photodermatology Group. NB-UVB phototherapy guideline, 2022.
- Moseley H, et al. Measurement of ultraviolet radiation levels in ultraviolet phototherapy, 2015.
- Taylor DK, et al. Dosimetry and calibration in ultraviolet radiation therapy, 2002. Measurement background; read with the later guidelines above.
- Gelfand JM, et al. The LITE randomized clinical trial, 2024. Starting-dose table and adjustment example: Supplement 1, Appendix A, office-based psoriasis protocol.
- British Association of Dermatologists. Phototherapy – NB-UVB: patient information.
- American Academy of Dermatology. Psoriasis treatment: phototherapy.
- National Psoriasis Foundation. Phototherapy for psoriasis.
- Ryer A. Light Measurement Handbook: measurement geometries.
- Bae JM, et al. Skin cancer and precancer in patients with vitiligo receiving long-term NB-UVB, 2020.
- Åkerla P, et al. Skin cancer risk of NB-UVB phototherapy: registry study of 4,815 patients, 2024.
Official quality, safety, and regulatory sources are linked in the manufacturing-standards table. Manufacturer specifications support product identification, not clinical dosing recommendations.