The first laser variable to consider is the wavelength. Since ophthalmic lasers range from ultraviolet (UV) to infrared (IR) wavelengths, an appropriate wavelength must be selected to interact with a specific tissue. For example, the cornea strongly absorbs UV-B, UV-C, and some IR wavelengths while allowing transmission of visible and near IR.² An UV emitting laser, such as the argon-fluoride excimer (193 nm), would be excellent for the cornea, but that wavelength could not reach the iris or the retina. A visible or near-IR wavelength can easily reach the retina, and therefore those wavelengths would be most appropriate for retinal- or iris-related laser procedures (Fig. 4.1).

As described in Chapter 3, spot size, pulse duration, and the energy level all factor into the power density that a laser can deliver. Of the three factors listed, the pulse duration is critical. Ophthalmic lasers are generally classified as either continuous wave (CW) or pulsed. Lasers that are considered CW generally will have pulse durations in the hundreds of milliseconds to second range, and the output is generally listed in milli-Watts (mW). Increasing or lengthening the pulse duration for the CW lasers will result in more tissue response in the form of a deeper or bigger laser burn. Pulsed ophthalmic lasers range from femtoseconds to nanoseconds, and they are programmed to emit pulses in the micro- or milli-joule range.
These pulse durations allow for massive differences in power density. The pulsed lasers generally interact with ophthalmic tissues with a photodisruptive or photomechanical mechanism. CW lasers will generally result in photothermal or photochemical outcomes (Fig. 4.2).

If pulse duration and spot size are held constant, increasing the energy setting will result in a higher-power density. However, changing the pulse duration or spot size will have a much more profound effect on power density. Most ophthalmic lasers will have a fixed spot size. Some examples include the neodymium yttrium aluminum garnet (Nd:YAG) (several microns spot size), selective laser trabeculoplasty (SLT) (400 mcm spot size), excimer, and femtosecond lasers. Adjustable spot sizes can be used with argon/green lasers for retinal applications as well as for treating other areas of the eye. In these circumstances, if the laser spot diameter is increased, then the power density will be decreased provided all other variables remained constant. If desiring to keep the power density constant after the spot size was increased, then the power must also be increased. If the laser spot diameter is decreased, then the power must also be decreased if desiring to maintain a constant power density. Spot size can also be affected by a magnifying laser lens. Assuming all laser parameters are held constant, a magnifying lens will decrease the spot size and increase power density.

The number of laser applications must be recorded to report the total cumulative energy dose during a procedure. The risk of complications from the laser procedure increases in proportion to the total cumulative dose of energy delivered into the eye. Therefore, the best rule-of-thumb is Use the lowest energy setting, the least number of shots, and the lowest duration possible to accomplish the desired effect. This does not mean to always use the lowest energy possible. Consider a Nd:YAG capsulotomy: if the tissue is not responding well to 1.5 mJ of energy and it takes 100 shots to clear the posterior capsule, the total cumulative energy is 150 mJ. If instead the energy were increased until the tissue responded better, perhaps 2.5 mJ, and it only took 35 shots to clear the capsule, the total energy would then be 87.5 mJ. By increasing the energy of the individual shot slightly to achieve a better tissue response, the total cumulative energy is less, and the patient has a smaller chance of complications than if the physician had used many more shots with a lower energy.

There are some lasers that are considered pigment independent and some that are considered pigment dependent. A pigment-independent laser does not need pigment to affect the targeted tissue. Examples of nonpigmented tissue would be the cornea, lens, and vitreous. Lasers that are effective on nonpigmented tissues need to be focused at the plane of the target tissue. A femtosecond laser focuses its energy at
specific preprogrammed corneal depths. An Nd:YAG uses an alignment system to ensure that the laser is focused at a specific plane. A pigment-independent laser can be used on pigmented tissues, but whether the tissue is transparent or pigmented does not dramatically change how the laser interacts with the tissue⁴,⁵.

With pigment-dependent lasers, a pigment is required to transfer the energy from the laser to the tissue. Absorption of laser light by a pigment is converted to heat, meaning pigment-dependent lasers have a thermal effect on the tissue. The primary ocular pigments are melanin, hemoglobin, and the macula’s xanthophylls (Fig. 4.3). Melanin is heavily present in the iris and retinal pigment epithelium (RPE). Melanin absorbs across the entire visible spectrum, which gives it a brown to black color. It absorbs IR less effectively.⁴,⁵ The less efficient the absorption of a wavelength, the more that wavelength is transmitted through, and therefore the deeper its penetration into the tissue. Therefore, a red or IR laser would be ideal for coagulating structures deep to the RPE, while the RPE would almost fully absorb a green laser.