What Is NB-UVB Phototherapy and How Does It Work?

Start Here: What Is Ultraviolet Light?

Sunlight contains a spectrum of light, most of which is invisible to the human eye. The ultraviolet (UV) portion of that spectrum sits just beyond what we can see. UV light is divided into three bands based on wavelength:

• UVA (315–400nm): Longer wavelengths, deeper skin penetration. Used in tanning beds. Associated with skin ageing and some skin cancers.

• UVB (280–315nm): Medium wavelengths. Responsible for sunburn. Also responsible for triggering Vitamin D production in the skin.

• UVC (100–280nm): Shortest wavelengths, mostly absorbed by the atmosphere. Used in germicidal lamps.

Phototherapy for skin conditions uses the UVB band — specifically a very narrow slice of it. That's where "NB-UVB" comes from. 

What Does "Narrowband" Mean?

Broadband UVB (BB-UVB) was the original form of UV phototherapy, delivering a wide range of UVB wavelengths (280–315nm). It was effective, but it also delivered wavelengths that caused unnecessary skin damage — particularly wavelengths below 300nm, which are strongly linked to burning and long-term skin harm.

In the 1980s and 1990s, researchers discovered something important: the therapeutic benefits of UVB phototherapy were concentrated in a very specific narrow range — around 311–313nm. Wavelengths outside this range contributed little therapeutic value but added to the skin damage burden.

Narrowband UVB (NB-UVB) devices filter out everything except this optimal range, delivering almost all of their output at 311nm. The result is a treatment that is:

• More effective per session than broadband UVB

• Safer because it eliminates the most damaging shorter wavelengths

• Better tolerated with a significantly lower risk of burning

NB-UVB at 311nm is now the global standard for UV phototherapy. It's recommended as a first-line treatment by dermatological guidelines in India, the UK, the US, and across Europe.

A related but distinct option is the 308nm excimer laser, which delivers an even more targeted wavelength using a xenon-chloride gas laser. While 311nm is delivered over a broad area (whole panels for widespread disease), 308nm is typically a handheld spot device used to treat small, well-defined patches with precision. Both wavelengths are clinically validated — the difference lies in how and where they're used, which we'll cover in detail below. 

How Does NB-UVB Actually Work on Skin?

The mechanism varies depending on the condition being treated, but the core principle is the same: ultraviolet light at 311nm interacts with cells and molecules in the skin, triggering biological responses that reduce disease activity. 

For Vitiligo

Vitiligo occurs when melanocytes — the cells that produce skin pigment (melanin) — are destroyed or become inactive in patches of skin. The result is depigmented white patches.

NB-UVB works by penetrating into the skin and stimulating dormant melanocytes that are still present, particularly those hiding in the roots of hair follicles. The UV light activates these cells, encouraging them to migrate outward, proliferate, and begin producing melanin again.

This is why repigmentation in vitiligo almost always begins as small dots of colour around hair follicles — called perifollicular repigmentation — before spreading outward and eventually coalescing into larger areas of restored pigment.

The 311nm wavelength also modulates the immune response in vitiligo-affected skin. Since vitiligo has an autoimmune component (the immune system mistakenly attacks melanocytes), suppressing this local immune activity is an important part of why NB-UVB works. 

For Psoriasis

Psoriasis is caused by an overactive immune response that dramatically speeds up the skin cell cycle. Normal skin cells take about 28 days to mature and shed. In psoriatic skin, this cycle happens in 3–5 days, leading to the characteristic thick, scaly plaques.

NB-UVB at 311nm penetrates the skin and damages the DNA of rapidly dividing skin cells in a highly targeted way, slowing their proliferation. It also suppresses the T-cells (a type of immune cell) that drive the inflammatory response in psoriasis. The result is a reduction in plaque thickness, scaling, and redness. 

For Eczema and Atopic Dermatitis

Eczema involves a dysregulated immune response in the skin, leading to chronic inflammation, barrier disruption, and the dry, itchy skin that characterises the condition.

NB-UVB phototherapy modulates this immune response — suppressing the specific inflammatory pathways (particularly Th2-mediated inflammation) that drive eczema. It also thickens the skin's barrier over time and reduces the populations of bacteria (particularly Staphylococcus aureus) on the skin surface, which are known to trigger eczema flares.

What Happens During a Session?

A standard NB-UVB session is simple:

1. You expose the affected skin area to the NB-UVB light source for a prescribed duration

2. The UV light is absorbed by the skin

3. You cover up and go about your day

Sessions typically last between 30 seconds and several minutes, depending on your skin type, your minimum erythema dose (MED — more on this below), and where you are in your treatment course. Treatment is usually prescribed 3–5 times per week.

There is no pain during a session. You may notice mild pinkness in the treated area afterward — similar to light sun exposure — which is expected and not a cause for concern. Burning, blistering, or significant redness is not expected and should prompt a conversation with your dermatologist. 

What Is MED and Why Does It Matter?

MED stands for Minimum Erythema Dose — the lowest dose of UV light that produces a faint but perceptible pinkening of your skin 24 hours after exposure.

MED is deeply individual. It varies based on your skin type, your natural UV sensitivity, your age, and the medications you take. Two people with the same skin tone can have significantly different MEDs.

Before starting a course of NB-UVB, your dermatologist will typically establish your MED by exposing small test patches of skin to increasing UV doses and observing the response. Your starting treatment dose is then set at a fixed percentage of your MED — usually 50–70%.

From there, your dose is gradually increased by 10–20% per session as your skin builds tolerance, with the goal of reaching a therapeutic dose that produces a mild, controlled response.

This is why dosimetry matters. If your device delivers a timer-based dose — "30 seconds" — rather than a measured dose in mJ/cm², there is no way to ensure you are actually receiving the amount of UV you were prescribed. As your UV tube ages and its output declines, a "30 second" session delivers progressively less therapy, with no indication that your dose has dropped.

Devices that measure actual UV output at each session and adjust accordingly ensure that every session delivers exactly the prescribed mJ/cm², regardless of lamp age. 

311nm vs. 308nm: What's the Difference?

Both wavelengths are used in UV phototherapy for the same conditions, and both are clinically validated with strong evidence. The difference between them is not about which one is "better" — it's about what you're treating, where on the body, and what setting you're in.

311nm NB-UVB — The Broad-Area Standard

311nm is delivered using fluorescent UV tubes (like the Philips TL/01) or LED arrays that illuminate a wide area. This makes it ideal for:

• Widespread disease — psoriasis or eczema covering large areas of the trunk, limbs, or back

• Multiple vitiligo patches — treating several areas simultaneously in one session

• Home therapy — practical for self-administered treatment three to five times a week over months

The 311nm wavelength has over 30 years of published evidence across hundreds of thousands of patients. It is the most extensively studied wavelength in phototherapy and the one recommended in virtually all international treatment guidelines as first-line therapy.

308nm Excimer — The Targeted Option

308nm is delivered using an excimer laser (xenon-chloride gas), which generates a highly concentrated beam of light. This makes it ideal for:

• Focal, well-defined patches — small areas of vitiligo or stubborn psoriatic plaques that haven't responded to other treatment

• High-energy spot treatment — the excimer laser can deliver a much higher dose to a small target area in a very short time

• Faster initial repigmentation in vitiligo — because the concentrated dose can stimulate melanocytes more aggressively in a specific spot, early repigmentation can appear sooner than with 311nm

The tradeoff is that 308nm excimer is a clinical device. The cost, size, and precision requirements of the laser mean it is only available in dermatology clinics — it is not practical for home therapy or for patients treating large or multiple areas.

How to Think About the Choice

For most patients managing vitiligo, psoriasis, or eczema at home — especially those with multiple patches or widespread disease — 311nm NB-UVB is the standard of care. For patients with a few stubborn spots who are already attending a dermatology clinic, 308nm excimer may be offered as an adjunct or alternative.

UVThera home devices use 311nm. The UVThera Clinical console, designed for clinic use, uses 308nm — giving dermatologists the option to offer precision spot treatment in-clinic alongside home-based 311nm therapy for their patients.

NB-UVB vs. Other Phototherapy Options

For most patients treating at home, 311nm NB-UVB is the standard of care. For patients already attending a dermatology clinic with focal disease, 308nm excimer may be offered as a complement — particularly for stubborn vitiligo spots that have not responded to 311nm alone.

Is NB-UVB Safe?

 Yes — with appropriate use, NB-UVB phototherapy has an excellent safety record built on over 30 years of clinical use.

The most commonly raised concerns are worth addressing directly:

Does NB-UVB increase skin cancer risk? The data from long-term studies is reassuring. Unlike PUVA (which combines UV with a photosensitising drug), NB-UVB has not been shown to significantly increase skin cancer risk even with prolonged treatment courses. The elimination of the most carcinogenic shorter UV wavelengths is one of the key advantages of narrowband over broadband UVB.

What about cumulative UV exposure? Total lifetime UV exposure is a relevant consideration in dermatology. This is one reason dermatologists recommend the minimum effective dose and do not prescribe unnecessary maintenance therapy once remission is achieved. It is also why precise dosimetry matters — overtreating is as undesirable as undertreating.

What about eye safety? UV light is harmful to the eyes. All NB-UVB therapy requires appropriate UV-blocking eyewear for the patient during treatment. Reputable home devices include UV-blocking apertures and eye protection in their instructions for use.

Can children use NB-UVB? Yes. NB-UVB is used in paediatric dermatology and has a well-established safety profile in children. Eczema and atopic dermatitis in children are common indications. Any home treatment in children should be supervised by a dermatologist.

What Results Can You Expect?

Results depend significantly on the condition being treated, the body area involved, and individual factors like how long you've had the condition and your skin type. That said, the clinical evidence gives us clear benchmarks:

Vitiligo: Initial perifollicular repigmentation typically appears at 6–12 weeks. Significant repigmentation in responsive areas by 3–6 months. Over 80% repigmentation is achievable in responsive patients after a full treatment course. Areas with fewer hair follicles (hands, feet, wrists) respond more slowly and less completely.

Psoriasis: Clearance often begins within 4–6 weeks. Many patients achieve 75%+ improvement (PASI 75) by 12 weeks of consistent treatment.

Eczema: Improvement in itch and inflammation typically within 4–8 weeks. NB-UVB is particularly useful for chronic, widespread eczema that doesn't respond adequately to topical therapies.

The Bottom Line

NB-UVB phototherapy — whether at 311nm or 308nm — is one of the most well-studied and effective treatments in dermatology. Both wavelengths work by using precisely calibrated ultraviolet light to directly modulate the biological processes driving vitiligo, psoriasis, and eczema: stimulating melanocytes, slowing cell proliferation, and suppressing inflammatory immune responses.

The choice between them comes down to your disease pattern and your setting. For most patients — particularly those managing widespread or multiple patches at home — 311nm NB-UVB is the right answer. For focal, stubborn lesions treated under clinical supervision, 308nm excimer offers an effective complement.

Neither is a quick fix. Both require consistency — typically 3–5 sessions per week over months — and patience through an early period where results may not yet be visible. But for the majority of patients, UV phototherapy delivers meaningful, lasting improvement that other treatments cannot match.

If you're considering NB-UVB phototherapy, the most important first step is a conversation with a dermatologist who can confirm it's appropriate for your condition, establish your MED, and set a protocol that gives you the best chance of a good outcome.