IPL FIlters don't do what most people think.

Understanding IPL Filters

Why “that little piece of glass” matters more than most people realise

If you’ve spent any time around IPL, you’ve probably heard someone say something like:“Just chuck the 640 filter in and you’re only treating 640nm.”

It sounds tidy. It’s also not how IPL filters work, and that misunderstanding is one of the easiest ways to end up with inconsistent results, unnecessary inflammation, or straight-up avoidable burns.

Let’s clear it up properly.

The two biggest myths about IPL filters

Myth 1: “The filter only lets through the wavelength written on it”

When people see a filter marked 520, 590, 640, they often assume that number is the only wavelength coming out.

In reality, with the common cut-off filters used in IPL, that number is usually telling you the lowest wavelength allowed through, not the only wavelength.

So a “590” cut-off filter typically means:it blocks wavelengths below ~590nm and allows a broad band above it to pass (up to whatever the system’s lamp, optics, and water or IR filtering allow).

Myth 2: “That means I’m only passing one wavelength”

IPL is not a single-wavelength device. It’s a broad-spectrum light source (xenon flashlamp) producing a large spread of wavelengths.

The filter doesn’t magically turn IPL into a laser.It simply shapes the spectrum by removing parts of it.

So instead of “one wavelength”, you’re dealing with something more like:

• A band of wavelengths

• With peaks and valleys (not flat)

• That can shift depending on the lamp, pulse width, ageing, optics, and cooling

That matters a lot when you’re trying to predict what’s actually heating the skin.

How IPL filters actually work (the practical version)

Most IPL systems use long-pass (cut-off) filters.

Think of it like a gate, not a needle

A cut-off filter is basically saying:

• Below this number: mostly blocked

• Above this number: mostly allowed (but not perfectly)

So a “640” filter is not “640nm only”. It’s closer to:640nm and above (within the system’s usable output range).

And it’s not a perfect cliff edge

Real filters don’t have an instant on-off boundary. They have a transition zone where some energy is partially transmitted.

That means “590” might still leak some energy around the high 500s depending on the quality of the filter and the design of the optics.

The rest of the system still matters

Two IPL machines both using a “590” filter can deliver very different real-world output because of:

• Lamp type and age

• Reflectors and optics

• Internal heat and cooling design

• Water filters and IR management

• Pulse structure (single, double, triple pulses, pulse width, delays)

Same filter label, different spectrum, different risk.

Myth 3: “IPL doesn’t need calibration”

This one is very common, and it quietly causes more problems than most clinics realise.

The idea usually goes:“It’s just light, not a laser.”or“It still flashes, so it must be fine.”

Reality: IPL absolutely needs calibration

IPL output drifts over time. Not “maybe”. It will. Common reasons include:

• Lamp ageing: xenon lamps lose efficiency and their output characteristics change

• Optics degradation: reflectors dull, light guides cloud, filters get contamination or micro damage

• Cooling changes: temperature affects lamp performance and pulse consistency

• Capacitors and power delivery: electronics wear changes how energy is actually delivered

So even if the screen says 20 J/cm², the handpiece might be delivering less, more, or delivering it in a different pulse shape than you think.

Why that’s risky

• If the machine is under-delivering, operators often creep settings up to “get results”

• If the machine is over-delivering, you can get burns at settings that used to be safe

• If pulse shape has changed, you can get more epidermal heat even at the same displayed fluence

That’s when a clinic says:“We didn’t change anything… but suddenly clients are reacting.”

Calibration is how you catch that before it becomes an incident.

What “calibration” should actually mean

Not a quick clean. Not “we checked it flashes”.Proper calibration means verifying real-world output with the right instruments, typically including:

• Energy / fluence verification at the handpiece

• Pulse duration and pulse structure verification

• Consistency across spot sizes and repetition rates

• Safety checks (interlocks, cooling performance, contact sensors where applicable)

If you’re operating under a health licence, regular calibration is also part of staying defensible and compliant.

Why this misunderstanding is risky

1) Unexpected epidermal heating

Lower wavelengths (blue-green range) are absorbed more superficially and more aggressively by melanin and oxyhaemoglobin.

If someone assumes they’re “safe” because they’re using a higher numbered filter, but the system still leaks or delivers significant energy in the transition region, you can get:

• Excess redness

• Blistering

• Post-inflammatory hyperpigmentation (PIH)

• Hypopigmentation in higher Fitz patients

• 
  

2) The “wrong target” gets cooked

If you’re aiming for deeper targets (like deeper follicular structures), but you don’t understand that you’re still delivering a broad spread of wavelengths, you can end up heating superficial pigment harder than intended.

That’s when you see the classic:“Settings were conservative… so why did it bite?”

Because the spectrum did the damage, not just the fluence.

3) False confidence in filter labels

A filter marking is not a guarantee of safe output for all skin types.It’s one variable in a much bigger equation.

Treating a Fitz 4–5 patient with IPL safely is not “use a higher filter and send it”. It’s:

• correct spectrum management

• correct pulse structure

• correct epidermal protection

• correct patch testing and endpoint awareness

• correct device calibration and output consistency

• 
  

The simple way to think about it

Here’s the clean mental model:

• IPL produces a broad spectrum of light

• A cut-off filter removes the lower part of that spectrum

• You still get a wide band above the cut-off

• The cut-off isn’t razor sharp

• The rest of the machine changes the real output

• Misunderstanding this increases burn and pigment risk

• 
  

What you should be asking your IPL supplier (or yourself)

If you want predictable outcomes and safe treatment margins, ask:

• Is this a true cut-off filter system, and what’s the transition range?

• How is IR managed (water filtering, internal heat handling, optics)?

• Has the unit been calibrated recently so energy delivery is what the screen says?

• What is the actual spectral output with each filter (not just the label)?

• What training is provided on Fitz risk management and endpoints?

Because filters don’t create safety.Understanding does.