Why Spot Size Matters in Targeted 308nm Phototherapy: Precision, Coverage, and Treatment Time

A patient has one small vitiligo patch near the eye.

Another has twelve scattered lesions across the hands and forearms.

Both are prescribed targeted 308nm phototherapy.

Should they be treated with the same spot size?

At first glance, the answer may seem obvious. A smaller spot appears more precise. A larger spot appears faster.

But real treatment is rarely that simple.

A small treatment field can follow a lesion boundary closely, yet it may require repeated repositioning when several lesions must be covered. A larger field can reduce the number of placements, but it may be harder to match around a narrow, curved, or irregular lesion.

The most useful spot size is therefore not automatically the smallest or the largest.

It is the one that fits the lesion.

That depends on:

• lesion size

• lesion shape

• number of lesions

• anatomical location

• surrounding healthy skin

• treatment-head stability

• prescribed dose

• irradiance

• repositioning time

• treatment documentation

This is why two devices that both deliver 308nm light can create very different clinical workflows.

The wavelength may be the same. The treatment experience may not be.

One Wavelength, Two Very Different Treatment Workflows

Targeted phototherapy delivers ultraviolet radiation directly to selected skin lesions rather than exposing a large area of uninvolved skin.

That is its central advantage.

In localized vitiligo, psoriasis, and other selected conditions, the treatment head can be directed toward the affected area while surrounding skin is shielded or avoided.

However, the phrase “targeted treatment” does not describe one universal treatment field.

Different systems may use:

• a fixed round spot

• a rectangular treatment window

• interchangeable masks

• adjustable apertures

• reduction tips

• templates

• a larger fixed irradiation area

The practical difference becomes obvious during treatment.

Imagine two patients:

Patient A: One 1 cm facial lesion

The main concern is following the lesion margin without repeatedly exposing the surrounding pigmented skin.

A smaller or adjustable treatment field may make sense.

Patient B: Several larger patches across the forearms

The challenge is no longer only boundary precision. It is also the number of placements, total session time, overlap control, and operator workload.

A very small spot may still be precise, but it may not be efficient.

This is why spot size should be viewed as a workflow parameter rather than a simple performance ranking.

What Spot Size Really Changes

Spot size describes the area illuminated during one placement of the treatment head.

It does not describe:

• wavelength

• irradiance

• dose

• fluence

• treatment frequency

• clinical efficacy

• total treatment time by itself

These specifications are related, but they are not interchangeable.

A larger treatment field covers more skin per placement.

A smaller treatment field covers less skin per placement.

That sounds basic, but the downstream effects are important.

Spot size can influence:

• how closely the treatment field follows lesion borders

• how many times the applicator must be moved

• how easy it is to treat irregular shapes

• how much normal skin may be included

• how likely gaps or overlaps are

• how long the operator spends repositioning

• how easy the session is to document

• how comfortable the patient remains during treatment

The spot size does not determine the dose delivered to each square centimetre. That depends on the device output and prescribed settings.

A larger treatment field is not automatically a higher dose.

A smaller field is not automatically safer.

The clinical value comes from matching field size to lesion geometry and treatment workflow.

When a Smaller Spot Helps

A smaller treatment spot can be useful when the lesion itself is small, narrow, isolated, or surrounded by a substantial amount of unaffected skin.

Typical examples may include:

• a small facial vitiligo patch

• a lesion close to the eye or hairline

• a narrow border around the lips

• a small lesion on a finger

• an irregular residual patch after partial repigmentation

• an isolated psoriasis plaque

• a treatment area close to sensitive or unaffected skin

In these situations, a smaller field may help the operator follow the visible lesion more closely.

But  'smaller' should not be confused with 'automatically more accurate.'

Accuracy still depends on:

• whether the treatment edge is clearly visible

• whether the handpiece remains stable

• whether the operator can reproduce the position

• whether shielding or templates are used

• whether the lesion border has been marked

• whether the patient moves during treatment

• whether the same area is accidentally exposed twice

A small spot used without a consistent movement pattern can still create uneven coverage.

For example, when treating a narrow curved lesion, the operator may need to rotate or reposition the treatment head several times. Each movement creates another opportunity for overlap or a missed area.

The smaller field improves control only when the workflow also supports control.

When a Larger Spot Can Save More Than Time

A larger treatment field is often discussed only as a way to shorten treatment time.

That is part of the story, but not the whole story.

A larger spot can also reduce:

• the number of handpiece placements

• the number of alignment decisions

• the chance of leaving untreated gaps

• the amount of time the patient must hold still

• the operator’s repetitive movement

• the complexity of documenting treated areas

This may be useful for:

• larger localized vitiligo patches

• several adjacent lesions

• medium-sized psoriasis plaques

• broad but still localized treatment regions

• busy outpatient workflows

• patients who find prolonged positioning difficult

Consider a rectangular lesion measuring 5 cm by 6 cm.

A device with a small treatment window may require many separate placements to cover the area.

A larger treatment field may cover it in one or a few placements.

The light delivery time per placement may not change dramatically, but the total session can still become shorter because less time is spent moving, aligning, checking, and documenting the handpiece.

This distinction matters.

Total treatment time includes more than the time the light is on.

It may include:

• lesion inspection

• positioning

• shielding

• alignment

• exposure

• repositioning

• repeated exposure

• treatment logging

• patient movement between sites

A larger treatment area may reduce the non-exposure part of the session.

That can improve workflow even when the prescribed dose remains unchanged.

The Hidden Problem: Overlap, Gaps, and Repositioning

Spot size becomes most clinically interesting when the lesion cannot be covered in one placement.

At that point, the operator must tile the treatment field across the lesion.

This creates two common risks.

Overlap

If one treatment placement partially covers an area that was already treated, that region may receive repeated exposure.

The degree of extra exposure depends on:

• how much the fields overlap

• whether the same dose is delivered at each placement

• how accurately the operator tracks the treated area

• whether the lesion border is visible

• whether a template or guide is used

Overlap does not automatically cause an adverse reaction, but uncontrolled overlap can make dose distribution less predictable.

Gaps

If each placement is spaced too far apart, small strips of the lesion may receive little or no treatment.

The final pattern can resemble tiles with narrow untreated lines between them.

This may be more likely when:

• the spot is small

• the lesion is large

• the border is irregular

• the treatment area is curved

• the patient moves

• many placements are required

• the operator does not use a fixed sequence

Why a repeatable movement pattern matters

A practical approach is to treat the lesion in a consistent order.

For example:

• left to right

• top to bottom

• centre outward

• according to a marked grid

• using numbered treatment zones

This sounds simple, but it makes the workflow more reproducible.

For larger or irregular lesions, clinics may also use:

• lesion photographs

• transparent templates

• skin-safe border markings

• treatment maps

• standardized documentation

• aperture masks

The goal is not to turn treatment into a complex geometric exercise.

It is to reduce uncertainty.

Irregular Lesions Change the Meaning of “Treatment Area”

A device specification may state that its treatment area is 3 cm², 16 cm², or 30 cm².

That number describes the maximum field generated by the equipment.

It does not mean every lesion of that size can be covered perfectly in one placement.

A 20 cm² lesion may be:

• long and narrow

• round

• crescent-shaped

• divided into separate islands

• wrapped around a finger

• positioned over a joint

• partly repigmented

• crossed by normal skin

The real match between spot size and lesion depends on shape as much as area.

A large rectangular field may suit a broad flat patch.

It may be less suitable for a narrow curved lesion unless masks or reduction tips are available.

Likewise, a smaller spot may fit a narrow lesion well but become inefficient when many adjacent areas must be treated.

This is why product specifications should be interpreted with lesion geometry in mind.

Anatomical Location Matters

A spot size that works well on the trunk may be awkward on the hands, face, scalp, elbows, or knees.

Face

Facial lesions may have narrow margins and lie close to the eyes, lips, nostrils, or hairline.

Precise positioning and shielding can be more important than maximum coverage.

Hands and feet

Acral areas contain multiple small surfaces, curves, fingers, toes, and bony contours.

A large flat field may not contact or align evenly across every surface.

A smaller or shaped aperture may be more practical.

Elbows and knees

These areas are curved and may move during treatment.

A stable handpiece and repeatable angle can matter as much as nominal spot size.

Scalp

Hair can block light and make lesion borders difficult to see.

A smaller treatment field may help navigate localized areas, but repositioning and hair separation add time.

Trunk and limbs

Broader, flatter lesions may benefit more from a larger field, particularly when several adjacent placements would otherwise be required.

The correct question is therefore not:

What is the largest spot size available?

It is:

Can the treatment field be applied consistently to the body areas this clinic treats most often?

Why Spot Size Alone Cannot Predict Session Time

It is tempting to assume:

larger spot = faster treatment

Often, that may be directionally true.

But spot size alone cannot predict the total session duration.

Treatment time also depends on:

• total lesion area

• number of lesions

• prescribed dose

• device irradiance

• exposure time per placement

• movement between lesions

• shielding requirements

• patient positioning

• operator experience

• treatment records

• equipment setup

Consider two examples.

Example 1: One larger lesion

A broad treatment field may cover the lesion in one placement and reduce repositioning.

Example 2: Ten small scattered lesions

A large field may still require ten separate placements because the lesions are not adjacent.

The field is larger, but the number of treatment locations does not change.

In this case, treatment-head mobility, alignment speed, patient repositioning, and documentation may matter more than the maximum spot size.

This is why clinics should avoid predicting throughput from spot size alone.

A device with a larger field may improve efficiency for certain lesion patterns, but not every patient benefits equally.

Spot Size, Irradiance, and Dose Are Different Questions

Three device specifications are often discussed together:

• spot size

• irradiance

• dose

They describe different parts of the treatment.

Spot size

How much skin is covered during one placement?

Irradiance

How quickly is energy delivered per unit area?

Dose

How much energy is delivered per unit area during the treatment?

A device can have:

• a small spot with high irradiance

• a large spot with lower irradiance

• a large spot with high irradiance

• an adjustable spot with constant or variable output

The treatment workflow depends on how these characteristics interact.

For example, a large spot may reduce the number of placements, while higher irradiance may reduce the exposure time per placement.

But neither characteristic should be interpreted outside the treatment protocol.

The prescribed dose still depends on:

• disease

• body site

• skin response

• previous treatment

• device settings

• clinical judgment

For patients, this explains why two treatments that both use 308nm light can take different amounts of time.

For distributors, it explains why “large spot” and “high power” are not complete sales arguments.

For clinicians, it reinforces the need to compare the full workflow rather than one specification.

Matching Lesion Pattern to Treatment Workflow

The table below is not a treatment prescription. It is a practical framework for equipment evaluation.

The key word is balance.

Too small a field may increase treatment complexity.

Too large a field may reduce boundary control.

The ideal field is the one that covers the intended lesion accurately without making the session unnecessarily difficult.

What Patients Should Understand About Spot Size

Patients may see “large treatment area” or “precision spot” in product descriptions and assume one is automatically superior.

A more useful interpretation is:

A small spot may be useful when:

• the lesion is small

• the border is narrow

• surrounding skin should be avoided

• the body area is difficult to access

A larger spot may be useful when:

• the lesion is broader

• several lesions are close together

• many small placements would otherwise be needed

• shorter session workflow is important

However, spot size does not determine whether treatment will work.

Clinical response also depends on:

• diagnosis

• disease stability

• anatomical location

• treatment frequency

• dose progression

• adherence

• skin response

• combination therapy

• individual biology

A larger spot does not guarantee faster repigmentation.

A smaller spot does not guarantee a better response.

Spot size mainly changes how the treatment is delivered.

What Clinics Should Ask Before Comparing 308nm Devices

A useful equipment comparison should begin with the clinic’s patients.

1. What lesion sizes are treated most often?

If most patients have small isolated lesions, an adjustable or smaller field may be valuable.

If larger localized patches are common, wider coverage may improve workflow.

2. Are lesions usually isolated, adjacent, or scattered?

Adjacent lesions may be covered efficiently with a larger field.

Scattered lesions still require separate positioning regardless of maximum spot size.

3. Is the treatment field fixed or adjustable?

Clinics should ask whether the device offers:

• multiple apertures

• reduction tips

• treatment masks

• adjustable spot size

• interchangeable templates

• shielding accessories

4. Is the field edge clearly defined?

A clear treatment boundary can help reduce accidental exposure outside the intended area.

5. How easily can the applicator be repositioned?

Handpiece weight, balance, cable movement, articulation, and treatment-head visibility affect repeated placements.

6. How is overlap controlled?

Ask whether the manufacturer provides guidance, templates, treatment maps, or other support for larger lesions.

7. Can treatment records identify lesion location?

For patients with multiple lesions, documentation may be as important as treatment speed.

8. Is the device practical for difficult anatomical areas?

A large spot may be impressive on paper but awkward on fingers, toes, ears, or facial contours.

9. What irradiance and dose-control modes are available?

Spot size should be evaluated together with output stability, dose mode, time mode, and MED workflow where applicable.

10. Does the system match the clinic’s patient volume?

A busy clinic needs efficient positioning, treatment delivery, cleaning, and record keeping—not only a large treatment window.

What Distributors Should Explain to Buyers

Distributors often lead with simple claims:

• larger spot

• faster treatment

• high power

• precise targeting

• short treatment time

These claims may be partly true, but they are incomplete.

A more credible conversation begins with the customer’s workflow.

Ask:

• What types of lesions do you treat most often?

• Are lesions usually small, large, scattered, or adjacent?

• Which body areas are most common?

• How many patients are treated each day?

• Does the clinic need adjustable masks?

• Is treatment documentation important?

• Does the clinic prioritize compact design or maximum coverage?

• Will one operator treat many lesions in a single session?

A distributor should avoid claiming:

• the largest spot is always best

• a small spot is always more precise

• larger coverage guarantees better results

• spot size alone determines treatment time

• all 308nm devices with similar treatment areas perform the same

The stronger message is:

Spot size should be matched to lesion pattern, anatomical location, dose workflow, and clinic volume.

That explanation is more useful than simply repeating the number of square centimetres.

How Spot Size Fits into 308nm Laser and LED Workflows

Both excimer laser systems and 308nm LED phototherapy devices can be used for targeted treatment, but their treatment fields, delivery systems, and operating logic may differ.

Some laser systems offer:

• smaller adjustable spots

• aiming beams

• interchangeable templates

• flexible apertures

Some 308nm LED systems offer:

• broader fixed treatment areas

• long-life light sources

• compact clinical formats

• dose and time modes

The practical comparison should not become:

laser equals precision, LED equals coverage

That is too simplistic.

Actual devices vary.

Some lasers offer larger treatment fields. Some LED systems use reduction masks. Some fixed-field devices may be highly practical for medium-sized lesions, while some adjustable laser systems may fit narrow borders well.

Technology matters.

But lesion-to-field matching still matters within each technology category.

What Spot Size Cannot Tell You About Clinical Response

Spot size can shape the workflow.

It cannot predict the patient’s outcome by itself.

Clinical response to 308nm treatment may vary according to:

• body site

• disease duration

• lesion stability

• hair-follicle density

• acral involvement

• treatment frequency

• total number of sessions

• dose tolerance

• topical combination therapy

• patient adherence

Facial and neck vitiligo often responds differently from acral lesions on the fingers and toes.

A device specification cannot remove those biological differences.

This is important for both patients and distributors.

The spot size should be presented as an operational feature—not as a guarantee of repigmentation or plaque clearance.

Conclusion: The Best Spot Size Is the One That Fits the Lesion

A smaller treatment spot may improve control around a small or irregular lesion.

A larger spot may reduce repositioning and make broader localized treatment more efficient.

Neither is universally better.

The right spot size depends on:

• lesion size

• lesion shape

• lesion number

• anatomical location

• normal-skin protection

• treatment-head stability

• dose workflow

• clinic volume

The most useful way to think about spot size is not: How large is the treatment window?

It is: How well does this treatment field match the lesions we actually need to treat?

In targeted phototherapy, precision does not simply mean using the smallest possible spot.

Precision means covering the intended lesion accurately, consistently, and without making the treatment unnecessarily difficult.

FAQ

What is spot size in 308nm phototherapy?

Spot size is the area of skin exposed during one placement of the treatment head. It may be fixed, adjustable, or modified using masks, apertures, or reduction tips.

Is a smaller spot always more precise?

No. A smaller spot may help follow narrow lesion boundaries, but treatment accuracy also depends on positioning, field visibility, operator technique, shielding, and overlap control.

Does a larger spot make treatment faster?

It can reduce the number of placements for larger or adjacent lesions. However, total session time also depends on dose, irradiance, lesion number, repositioning, patient positioning, and documentation.

Does spot size affect the UV dose?

Spot size describes the treatment area. Dose describes the energy delivered per unit area. A larger spot does not automatically mean a higher dose.

Why are overlap and gaps important?

When several placements are needed, overlapping areas may receive repeated exposure, while gaps may receive insufficient coverage. A consistent treatment sequence helps improve uniformity.

Is a large treatment area better for vitiligo?

It may be useful for larger localized or adjacent patches. Small, narrow, facial, or irregular lesions may benefit from a smaller or adjustable field.

Can the same spot size be used on every body area?

Not always. Flat areas, curved joints, fingers, toes, scalp margins, and facial lesions may require different positioning or apertures.

Does spot size determine treatment results?

No. Clinical response also depends on disease type, body site, lesion stability, dose, treatment frequency, adherence, and individual patient factors.

What should clinics check besides spot size?

Clinics should also evaluate irradiance, dose control, treatment-head handling, field-edge definition, masks, shielding, MED functions, treatment records, and service support.

What should distributors avoid claiming?

Distributors should not claim that the largest spot is always faster, that the smallest spot is always more precise, or that spot size alone determines clinical outcomes.

References

[1] Schatloff DH, et al. The Role of Excimer Light in Dermatology: A Review. 2024.

[2] DermNet New Zealand. Targeted Phototherapy.

[3] DermNet New Zealand. Excimer 308-nm Light Treatment.

[4] Yu Y, et al. A Prospective Randomized Half-Body Study Comparing 308nm LED Light and 308nm Excimer Laser for Vitiligo. 2023.

[5] Passeron T, et al. Use of the 308-nm Excimer Laser for Psoriasis and Vitiligo. 2006.

[6] Shi Q, et al. Comparison of the 308-nm Excimer Laser with the 308-nm Excimer Lamp in the Treatment of Vitiligo. Photodermatology, Photoimmunology & Photomedicine. 2013.

[7] Sun Y, et al. Treatment of Vitiligo with the 308-nm Excimer Laser: A Systematic Review. 2015.

[8] Majid I, Imran S. Efficacy of Targeted Narrowband UVB Therapy in Vitiligo. 2014.

[9] Feldman SR, et al. Efficacy of the 308-nm Excimer Laser for Treatment of Psoriasis: Results of a Multicenter Study. Journal of the American Academy of Dermatology. 2002.

[10] Mehraban S, Feily A. 308nm Excimer Laser in Dermatology. Journal of Lasers in Medical Sciences. 2014.