Laser terms explained simply: from chromophore to thermal relaxation time
Anyone who reads data sheets or attends laser training comes across technical terms such as chromophore, selective photothermolysis, thermal relaxation time, Q-switched or fractional. This glossary explains the most important laser terms in simple words, each as a question with a short, direct answer.
You will find out what pulse durations in milliseconds, nanoseconds and picoseconds mean, how joules and watts, ablation and coagulation, spot and scan field differ and what separates laser light from LED and IPL. Each term is illustrated using a SIACSKIN device, for example the DEWEON 4LD, the AlexDUAL UFO, the PICOLO XL, the FRACO CO2 XL or the TULIX.
What is a chromophore, and what target structures does a laser have in the skin?
A chromophore is a substance in the skin that absorbs light of a particular wavelength and converts it into heat. The three most important are melanin in hair and pigment spots, haemoglobin in the blood and water in the tissue. In tattoo removal, the ink in the skin takes on this role. Which chromophore is hit is decided by the wavelength: each is absorbed strongly by one of these substances and more weakly by the others. This makes it possible to heat a target while sparing the surroundings.
The DEWEON 4LD targets the melanin in the hair with 755 to 1064 nm. The FRACO CO2 XL uses the water in the tissue at 10,600 nm, and the DERMAFORCE BBL PRO reaches pigment and blood vessels with its filters.
What does selective photothermolysis mean in simple terms?
Selective photothermolysis means that light destroys a structure selectively through heat, without damaging the tissue next to it. Photo stands for light, thermo for heat, lysis for dissolution. This works when three things fit together. The wavelength must be absorbed more strongly by the target than by the surroundings. The pulse must be short enough for the heat to stay in the target. And the energy must be enough to really damage the target. Hair removal, pigment treatment and vascular treatment are based on this principle.
The AlexDUAL UFO puts it into practice like this: 755 nm for melanin-rich hair on fair skin, 1064 nm for dark skin types, plus pulses of 0.3 to 100 ms that can be adapted to hair thickness.
What is thermal relaxation time, and why does pulse duration depend on it?
Thermal relaxation time is the time in which a heated structure gives off about half of its heat to the surroundings. It depends on size: a coarse hair needs many milliseconds for this, the epidermis a few milliseconds, and a pigment granule or tattoo particle only a fraction of a microsecond. If the pulse is shorter than this time or about as long, the heat stays in the target. If it is markedly longer, the heat flows away and warms the neighbouring tissue.
This is why the DEWEON 4LD works with 1 to 200 ms for hair, while the PICOLO XL delivers pulses of 280 picoseconds for tiny pigment particles. That fine hairs need shorter pulses than coarse ones is the same rule on a small scale.
Milliseconds, nanoseconds, picoseconds: what do the units of pulse duration tell you?
The unit of pulse duration reveals what a laser is built for and how it acts. A millisecond (ms) is one thousandth of a second, a nanosecond (ns) one billionth, a picosecond (ps) one trillionth. Pulses in the millisecond range heat larger structures such as hair follicles or vessels, which is referred to as long-pulsed. Nanosecond and picosecond pulses are so short that they shatter the smallest particles such as tattoo ink and pigment rather than heating them. Pulse duration, pulse width and pulse length mean the same thing.
At SIACSKIN you will find all three ranges: the AlexDUAL UFO works with 0.3 to 100 ms, the PICOLO L in the nanosecond range and the PICOLO XL with 280 ps.
What is the difference between joules and watts for a laser?
Joules measure the energy of a pulse, watts the power, that is, energy per second. One joule delivered in one second is one watt. The same energy in a shorter time gives a higher peak power: 1 J in a millisecond corresponds to 1000 W, and in a nanosecond to a billion watts. This is why pulsed lasers for tattoo and pigment are described in millijoules per pulse, while the wattage on the data sheet usually means the average or electrical power. Read both values together and always with the pulse duration.
The TULIX is specified at 5 to 30 W and 0.5 to 300 mJ. With the PICOLO L, it is up to 1800 mJ per pulse at 1064 nm, delivered in the nanosecond range.
What does fractional mean for a laser, and what are microthermal zones?
Fractional means that the laser does not treat the skin over the whole area but in a grid of many tiny points. Each point creates a narrow column of heated or ablated tissue, the microthermal zone. Healthy skin remains intact in between, and healing starts from there. In this way the skin tolerates a deep effect with a markedly shorter healing time than after a full-surface treatment. The density indicates what proportion of the area is hit and, alongside the energy, determines the intensity.
The FRACO CO2 XL works fractionally and ablatively on a field of up to 20 × 20 mm. The TULIX places non-ablative points of 50 to 200 µm in an equally large scan field.
What does ablative mean for a laser, and what does it mean for the skin surface?
Ablative means tissue-removing: the laser vaporises tissue rather than only heating it, and in doing so opens the skin surface. Small wounds are created, which weep, crust and have to heal. This is exactly what is intended: damaged tissue is removed, and healing stimulates the skin strongly to renew itself. This is why 1 to 3 sessions are often enough, but the skin needs time, careful wound care and consistent sun protection. With non-ablative lasers, the surface stays closed and the downtime is shorter, but more sessions are needed.
The FRACO CO2 XL is an ablative fractional CO2 laser with 10,600 nm. In CoolPeel mode it ablates more gently, and according to SIACSKIN the skin then heals in 3 to 4 instead of 10 to 14 days. The TULIX, by contrast, works non-ablatively.
Coagulation means that proteins in the tissue congeal under heat, without tissue being ablated. From about 60 °C, collagen changes: the fibres contract, and in the following weeks the body replaces the damaged tissue with new tissue. This is the core of all non-ablative skin rejuvenation and tightening. Blood vessels are also closed by coagulation. With an ablative laser, a coagulation zone forms as a rim around the ablated channel.
At 1550 nm the TULIX creates coagulation columns in the dermis, and at 1927 nm in the epidermis, without ablating tissue. The EUPHEUS M8 achieves a comparable effect at depth with radiofrequency through fine needles.
The photoacoustic effect is a mechanical action: an extremely short laser pulse heats a pigment particle so quickly that it expands abruptly, a pressure wave forms and the particle shatters. The heat has no time to flow into the surroundings. The shorter the pulse, the greater the mechanical component and the finer the fragments that the immune system then carries away. In treatment, the effect shows as an audible crackle and as an immediate whitish discolouration of the area hit. Longer pulses, by contrast, act predominantly through heat, that is, photothermally.
The PICOLO XL uses this effect with pulses of 280 picoseconds for tattoo removal and pigment. The PICOLO L and the PICOLO S work in the nanosecond range, where the thermal component is somewhat higher.
What does Q-switch (quality switching) mean for a laser?
Q-switch, short for quality switching, is an optical switch in the laser that first stores energy and then releases it abruptly. While the switch is closed, the laser crystal charges up without emitting light. When it opens, the stored energy discharges in a single pulse of a few nanoseconds with very high peak power. This is how the short, powerful pulses arise that break up tattoo ink and pigment. The Q stands for the quality factor of the resonator in which the light travels back and forth.
The PICOLO L and the PICOLO S are Q-switched Nd:YAG lasers in the nanosecond range. Even shorter pulses are delivered by the PICOLO XL, a picosecond laser with 280 ps.
What does the abbreviation Nd:YAG stand for, and how do you read the name of a laser?
Nd:YAG stands for neodymium-doped yttrium aluminium garnet, a crystal in which laser light of 1064 nm is produced. Most lasers are named after their laser medium: alexandrite, CO2, diode, thulium. The medium, however, only reveals the wavelength. What a device treats with it is indicated only by the added reference to the operating mode. Long-pulsed means pulses in the millisecond range, as hair removal requires. Q-switched means pulses in the nanosecond range for tattoo ink and pigment. If 532 nm is also stated, the frequency of the Nd:YAG is doubled with a second crystal.
The AlexDUAL UFO uses long-pulsed Nd:YAG for skin types V to VI. The PICOLO L is a Q-switched Nd:YAG with 1064 and 532 nm. The DEWEON 4LD, by contrast, generates 1064 nm with diodes, so it is not an Nd:YAG laser.
What is the difference between spot size and scan field in a laser?
The spot size is the area of a single laser pulse on the skin, and the scan field is the area that a scanner scans with many small spots in succession. For lasers used in hair removal, it is the spot that counts: the larger it is, the faster an area is treated and the deeper the light reaches, because less of it scatters sideways. Fractional lasers, by contrast, place microscopically small spots and distribute them with a scanner as a pattern over a field.
The AlexDUAL UFO has square spots of 10 to 40 mm. With the TULIX, the single spot is only 50 to 200 µm in size, and the scan field measures up to 20 × 20 mm. The FRACO CO2 XL also covers up to 20 × 20 mm.
What do monochromatic, coherent and collimated mean for laser light?
The three words describe what distinguishes laser light from every other light source. Monochromatic means single-coloured: the laser emits exactly one wavelength. Coherent means that all the light waves oscillate in step. Collimated means that the beam hardly diverges and can be concentrated onto small areas. A flashlamp such as in IPL, by contrast, emits a broad spectrum in all directions, which is narrowed only by filters. An LED lies in between: a narrow band, but not coherent.
At SIACSKIN, the DEWEON 4LD is a laser with four fixed wavelengths, the DERMAFORCE BBL PRO a broadband source with 400 to 800 nm and 9 filters, and the VCL DERMALIGHT an LED system with 750 to 890 nm.
What do pass, overlap and stacking mean in a laser treatment?
A pass is a single sweep over the whole treatment area. Overlap describes how far neighbouring spots cover each other, and stacking means delivering several pulses in succession to the same site. All three determine how much energy the skin receives in total. In hair removal using the stamping technique, you place the spots without gaps and with little overlap, since too much of it leads to heat build-up. With fractional lasers, several passes increase the density, and stacking increases depth and heat per point.
The rectangular spots of the DEWEON 4LD, for example 15 × 40 mm, can be placed neatly next to one another. With a fractional laser such as the TULIX, the number of passes also helps determine the intensity of a session.
A clinical endpoint is the visible skin reaction by which you recognise that the setting is working and is not too strong. It looks different for every treatment. In hair removal, it is slight redness and small swellings around the hair follicles, which appear after a few minutes. In tattoo removal, the area hit turns whitish immediately. With the non-ablative fractional laser, it is an even redness with slight swelling. Grey discolouration, blisters or severe pain are warning signs of too much energy.
The SIA AI in the AlexDUAL UFO and the AI in the PICOLO XL set parameters automatically. Only the endpoint at the test patch shows you whether they suit the skin.
