The most important thing for laser treatment of fundus diseases is to reasonably select and match four basic elements: laser wavelength, spot diameter (determining the irradiation area A of a single laser spot), exposure time (T) and output power (P).
Spot diameter (D), exposure time (T) and output power (P) are often referred to as laser parameters.

In the laser treatment of fundus diseases, the choice of laser wavelength should firstly be based on the nature, location and level of the lesion, and the characteristics of the pigments it contains, and secondly consider whether the refractive interstitium is cloudy. Selecting the ideal wavelength can make the laser effectively penetrate the refractive interstitium to the treatment site, with high absorption rate in the target tissue, and low absorption and scattering rates in the eye refractive interstitium that the light path passes through. While producing the best curative effect, it does not produce side damage. The basis for selecting the laser wavelength is as follows:

1. The nature of the lesions: For retinal hemorrhage and edema photocoagulation, a wavelength laser that is not easily absorbed by hemoglobin and has strong penetrating power, such as red or infrared wavelength lasers, should be used. For retinal choroidal hemangioma lesions, or lesions rich in blood vessels, a yellow laser with high hemoglobin absorption rate is preferred. It is recommended that the yellow wavelength laser is the first choice, followed by the green wavelength laser. When treating retinal choroidal tumors, yellow or red wavelength lasers can be used. Combined with limited vitreoretinal traction, use red or infrared laser. Red light and infrared laser can pass through the vitreous hemorrhage, and the effect is deep, and will not aggravate the shrinkage of the vitreoretinal tissue.
2. The location and level of the lesion: Green laser is the first choice for retinal photocoagulation treatment from the posterior pole to the periphery, because the green laser has the highest absorption rate in the RPE layer, is not easy to penetrate the vitreous membrane (Bruch membrane), and has good safety. The macula is rich in lutein, and the yellow laser is the first choice for macular diseases, followed by red and green lasers to avoid thermal damage to visual cells caused by lutein absorbing light energy. For subretinal diseases, such as choroidal neovascularization, choroidal hemangioma, etc., red and infrared wavelength lasers with high choroidal absorption rate should be selected.
3. Pigment contained in lesions: There are mainly three types of pigments that absorb laser light in the fundus: melanin, hemoglobin and lutein. Melanin: mainly exists in the retinal pigment epithelium (RPE) layer and choroid. It absorbs all wavelengths in the visible light range. The absorption rates of different wavelengths are green, yellow, and red lasers from high to low. Hemoglobin: The order of absorption of different wavelengths of laser light is the same as that of melanin. Lutein: The order of absorption for different wavelengths is blue, green, and red lasers. The more pigments in the fundus, the more light absorption, and the stronger the laser reaction intensity. During the treatment, attention should be paid to adjusting the laser power according to the type and content of the pigment of the laser target.
4. Refractive interstitium: The opacity of the refractive interstitium directly affects the ocular biological characteristics of the laser and the retinal photocoagulation response. When the refractive interstitium is transparent, the green laser should be the first choice for retinal photocoagulation. When the refractive interstitium is opacified (such as lens opacity), if a short-wavelength laser such as a green laser is used, scattering will occur, so an easily penetrating yellow or even red wavelength laser should be selected. When performing retinal photocoagulation on eyes with vitreous hemorrhage, a red wavelength laser should be used, because it is not absorbed by vitreous hemorrhage, it can efficiently penetrate the vitreous hemorrhage and reach the retina.

After determining the laser wavelength that should be selected for treatment, or when there is only a single-wavelength laser, it is necessary to flexibly apply laser treatment parameters, which can expand the treatment indications, improve the cure rate, and reduce the occurrence of laser complications. In laser treatment, the intensity of the spot reaction is determined by the laser energy density (D, unit J) of the irradiation site, and the laser energy density is determined by three parameters of the laser: laser power (P, unit mW), irradiation area (A, Unit cm2) and exposure time (T, unit second s) are jointly determined. When the incident angle is equal to zero, that is, when the laser beam is completely perpendicular to the irradiation plane, the irradiation laser energy density D=P×T/A. It can be seen from the above formula that the laser energy density D is proportional to the laser power P and the irradiation time T, and is inversely proportional to the irradiated area A. The three variables of laser irradiation area (A), laser power (P) and exposure time (T) are related to each other, and can all affect the intensity of the spot response of the target tissue. For example, when the spot size is determined, to achieve the same spot response, the laser power and exposure time can be adjusted complementarily; and increasing the power or exposure time can increase the laser energy density, resulting in a stronger spot response. Under the given optical power and irradiation time, reducing the irradiation area can also increase the laser energy density, that is, enhance the intensity of the spot response. Therefore, it should be noted that regardless of the laser, with a larger laser dose (energy density D), even an argon laser can reach deep into the choroid.

In the clinical application of laser, the spot size (irradiation area A) is usually adjusted by changing the spot diameter. During laser treatment, different sizes of light spots should be used according to the laser treatment site, treatment purpose, lesion shape and size. In retinal photocoagulation therapy, a spot with a diameter of 200-300 μm is mostly used for retinal photocoagulation in the posterior pole; the diameter of the spot can be gradually increased from 200-500 μm to the peripheral part. The spot size in the macular area is generally set at a diameter of 100-200 μm, and a spot with a diameter of 50 μm can be used close to the fovea. It should be noted that if the light spot is too small (such as 50 μm in diameter), the energy density is too concentrated, which may damage Bruch’s membrane, leading to bleeding or inducing choroidal neovascularization and other complications; when the light spot is too large (such as 1000 μm in diameter), the center of the light spot will shrink without adhesion. effect. Therefore, for the purpose of photocoagulation therapy to eliminate the retinal non-perfusion area, prevent the formation of new blood vessels or make them degenerative (such as diabetic retinopathy, central vein occlusion and retinal vasculitis), it is better to use a larger spot. For the purpose of photocoagulation therapy is to enhance the scar adhesion reaction of the retina and choroid (such as closing retinal holes), an excessively large spot cannot be used. In PDT and TTT treatment, the diameter of a single spot can be greater than 1000 μm according to the size of the lesion. For focal lesions, such as retinal choroidal tumors and choroidal neovascularization, a single spot or multiple fusion spots should be used to cover the entire lesion according to its location and size

The exposure time is related to both the limitations and depth of the spot response. The shorter the exposure time, the more limited the effect of the light spot, and the sharper and clearer the edge. However, small spot, short exposure time, and high power can easily cause Bruch’s membrane and retinal perforation, and induce hemorrhage or choroidal neovascularization. Therefore, exposure time less than 0.1 seconds should be avoided as much as possible during photocoagulation treatment of fundus diseases. If a small spot and short exposure time are used, the set initial laser power must be reduced, and then the laser power should be adjusted from low to high until an ideal spot response occurs. The longer the exposure time, the more severe the retinal edema in the spot, the more blurred the edge of the spot, and the more the laser damage extends to the depth of the retina and choroid. In the laser treatment of retinal outer layer and choroid lesions, especially in the treatment of choroidal tumors, in addition to selecting an appropriate laser wavelength, prolonging the exposure time helps to deepen the depth of laser action to a certain extent. During retinal photocoagulation treatment, the exposure time of the posterior pole and the periphery is usually 0.2 seconds, which can be extended to 0.4 seconds according to needs; the exposure time of macular degeneration is usually 0.05 to 0.1 seconds. During photodynamic therapy laser treatment, the exposure time is mostly 83 seconds, and choroidal tumors can be extended to 124 to 166 seconds. The exposure time of transpupillary hyperthermia is usually 1 to 3 minutes (min).

Retinal choriopathies of different nature and different parts require different levels of speckle response. Selecting the appropriate laser power is an important part of achieving an effective spot. In laser treatment, the laser wavelength is usually determined first, then the spot size and exposure time are initially set, and the laser output power is finally adjusted. The laser power is determined by various factors such as the opacity of the refractive interstitium, the location of retinal choroid lesions, the content of pigment, the amount of blood vessels, whether there is bleeding, edema, and exudation. During treatment, the initial laser power should be placed at a lower position and gradually increased according to the spot response until the expected spot response is achieved