Bench Degree·LASERSchapter

Chapter 15: Medicine, and the Ones Pointed at People

A wavelength that water absorbs in a fraction of a micrometre is what makes it possible to reshape a living cornea. The same reasoning, applied dishonestly, is what is being sold as a medical laser for ninety dollars online.

Chapter 10 said a material’s absorption at your wavelength decides everything. Nowhere is that more true than in tissue, because tissue is not one material. It is water, and haemoglobin, and melanin, and collagen, and each absorbs quite differently.

Which means a wavelength can be chosen to be absorbed by one component and not by its neighbours. That is the whole of laser medicine, and it has a name: selective photothermolysis. Heat the thing you mean to heat, using its own absorption as the address, and leave what is beside it alone.

Section 1: What Tissue Absorbs

Roughly, and enough to reason with:

Read the penetration column, not the wavelength column. That figure is what a surgeon is actually choosing. Anything that penetrates a millimetre will heat a millimetre, and if you need to remove a quarter of a micrometre of cornea, a millimetre is a catastrophe.

Section 2: Corneal Surgery, and Why 193 nm

Reshaping a cornea means removing tissue to a depth of a fraction of a micrometre, accurately, across a curved surface, on a living eye, without heating the tissue underneath.

The excimer laser at 193 nm is what makes it possible, and for two reasons at once.

It is absorbed within less than a micrometre. So a pulse cannot reach the tissue below what it is removing. The depth of the effect is set by physics rather than by the surgeon’s timing.

And at 193 nm the photon energy is high enough to break molecular bonds directly. The tissue is not boiled off; the bonds holding it together are severed and the fragments leave. This is photoablation rather than thermal ablation, and it means almost no heat is deposited in what remains.

Put those together and you get removal of about 0.25 µm (0.00001 in) per pulse, with the neighbouring tissue essentially untouched. A typical correction removes a few tens of micrometres in total, in a pattern computed from the eye’s own measured aberrations.

And Chapter 9’s clock is doing the other half of the work. Femtosecond lasers now cut the corneal flap as well, replacing the mechanical blade, because a pulse short enough deposits its energy and is gone before heat can spread. Two of this volume’s central ideas, absorption depth and pulse duration, are why a routine outpatient procedure exists at all.

IN PLAIN ENGLISH: They can reshape your eye because they found a colour of light that stops after a fraction of a micrometre and breaks bonds instead of boiling. Nothing about the surgeon’s hand is that precise. The wavelength is.

Section 3: The Rest of It, Briefly

Tattoo removal. Ink particles absorb far more strongly than surrounding tissue, so a very short pulse shatters them mechanically while the skin around them stays intact. Different ink colours need different wavelengths, which is why black is easy and green and yellow are difficult, and why a course of treatments uses more than one machine.

Retinal photocoagulation. Green light passes through the clear parts of the eye and is absorbed by the pigmented layer behind the retina, so it can weld a detached retina back down or seal leaking vessels through the intact front of the eye, without an incision. The first widely used laser surgery, and still among the most valuable.

Dermatology and vascular lesions. Haemoglobin’s absorption addresses the blood vessel and not the skin over it.

Dentistry, using erbium:YAG at 2940 nm, absorbed by the water in enamel and dentine within a micrometre, which is why it cuts a cavity with far less of the mechanical vibration that people find intolerable about a drill.

Cutting and coagulating in surgery, with CO₂, where the beam seals small vessels as it cuts because it is heating a controlled shallow layer.

Section 4: And Now the Dishonest Half

Everything above is a real clinical field, with dosimetry, regulatory approval, and trained operators.

There is also a large trade in devices sold online as medical or therapeutic lasers which are none of those things, and the physics of Chapter 2 says why they are dangerous.

The pattern is consistent. A high-power laser diode, often of the type recovered from optical drives or bought loose, in a plastic case, with a battery and a switch. Marketed for pain relief, hair removal, tattoo removal, acupuncture, “cold laser therapy”, or veterinary use. Sold for tens of dollars. Frequently with no interlock, no key, no wavelength marked, no power specification that means anything, and either no eyewear supplied or eyewear that does not match the wavelength, which Chapter 2 Section 6 explains is worse than none.

Independent teardowns by people who buy these in quantity and measure them find the same things repeatedly: output far above what is claimed, often in the multi-watt range, which puts them in Class 3B or Class 4; the wrong wavelength for the stated purpose; and, in devices sold as tattoo removers, pulsed outputs whose peak power is enormous and entirely unspecified.

The specific hazard is Chapter 2 Section 5. A device emitting several watts of 808 nm or 980 nm gives no visual cue at all. There is nothing to blink at. A user pointing it at their own arm has the beam scattering off skin, off jewellery, off a phone screen on the table, and their own eye focuses whatever reaches it. They have no way of knowing it happened until their vision has changed.

The honest position, stated as narrowly as it can be: these are not weak versions of clinical machines. They are uncontrolled lasers of unstated power and unstated wavelength, in a case, and the fact that a clinical field exists using similar wavelengths is what makes them sellable rather than what makes them safe.

And there is a separate point about efficacy that this book will not adjudicate. Low-level light therapy is a genuine research area with a genuine literature and genuinely contested results. Whether any of it works is a clinical question, not a physics one. What is not contested is that a device of unknown output cannot deliver a known dose, and every therapeutic claim in that literature depends on dose. A machine that cannot tell you what it emits cannot deliver a treatment even if the treatment is real.

SLOW DOWN. Check Your Understanding: A device is sold as a “5 mW therapeutic laser, 650 nm, Class 3R”. Measured, it turns out to emit 400 mW at 808 nm. Which of those two errors is more dangerous, and why is it not the power? Think before reading on.

The wavelength, by a wide margin, and the two compound. 400 mW is eighty times the claimed power and puts the device in Class 3B, where the beam and any specular reflection are immediately hazardous. But at the claimed 650 nm the user would at least see a bright red spot, and their aversion response would function, and any goggles they bought for a red laser would work. At 808 nm there is no visible cue whatsoever, no blink, and goggles chosen for 650 nm are transparent to it. The power error removes the safety margin. The wavelength error removes the warning, the reflex and the protection at the same time, and it does so silently. This is exactly why Chapter 2 puts the invisible band in its own section, and why the phone-camera test in that chapter is worth thirty seconds of anybody’s time.

Section 5: What This Chapter Established

Tissue is several materials, and a wavelength can address one of them. That is selective photothermolysis and it is the foundation of the whole clinical field.

Absorption depth is the number a surgeon chooses. 193 nm stops in under a micrometre, which is what makes corneal surgery possible; a wavelength that penetrated a millimetre could not do it at any power.

Pulse duration does the other half, per Chapter 9, by depositing energy faster than heat can leave.

And the same reasoning read backwards identifies the fraudulent devices, because a laser that will not tell you its wavelength cannot be delivering a dose to anything in particular.

Next: how you find out what a beam actually is, rather than what its label says, which is harder than it sounds and is the last practical skill in this volume.

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