In a 2015 study: “A Prospective, Randomized, Double-Blind Comparison of an Ablative Fractional 2940-nm Erbium-Doped Yttrium Aluminum Garnet Laser With a Nonablative Fractional 1550-nm Erbium-Doped Glass Laser for the Treatment of Photoaged Asian Skin”. Found the researchers that:

“Reductions in pigmentation and uneven tone/erythema scores were significantly greater after Er:YAG, while wrinkle score reduction was significantly greater after Er:glass. Physician and patient assessments for the overall features showed greater improvement in the Er:glass. Treatment-related pain or adverse events were less in the Er:YAG.”

Let’s start by looking at the difference between the two wavelengths. Er:Glass at 1550 nm and Er:YAG at 2940 nm.

At 1550 nm the water absorption coefficient is 1/2000 of that of 2940 nm. We also find some melanin absorption at 1550 nm which is no longer present at 2940 nm and there is still some haemoglobin absorption at 1550 nm what also is not present at 2940 nm.

What does this mean in practice? At 1550 nm you still have a photothermal reaction, where you have a photoablative reaction at 2940 nm.

What happens with Er:Glass fractional, is that you create small ‘wells’/’tunnels’ in the skin where you coagulate and evaporate the tissue. These ‘wells’ can range from 400 μm (micrometer) to 1,4 mm (depending on various factors). The average thickness of the epidermis is 0.1 mm or 100 μm.

Er:YAG has very little penetration depth compared to Er:Glass, about 50 µm (depending on various factors) per pulse. But unlike with Er:Glass where you are more limited in the amount of passes you can go over an area during a treatment, with Er:YAG you can increase the passes. This makes Er:YAG more suitable for problems in the epidermis, like pigmentation. Whereas Er:Glass is more suitable for deeper laying problems, like rhytids.

Keep in mind that you can ablate with an Er:YAG as deep as you can treat with an Er:Glass. But with a fractionated machine this is not very practical to do. With a full ablative Er:YAG machine you will have no limitation on ablation depth, since you just keep doing passes until you reach the required depth.

A combination fractional treatment of Er:Glass and Er:YAG is actually the ideal for impressive anti-ageing and skin rejuvenation results without extended recovery time. Although, each of the wavelengths on its own will also not disappoint.