This article introduces the laser for Ophthalmology treatment through the description of the laser penetration, light scattering, light absorption, and the three main pigments of the fundus.

The cornea, lens and vitreous of the eyeball are highly transparent, allowing light to pass through with little absorption. The transmittance of the laser with a wavelength of 450-1000 nm in the transparent refractive interstitium in the eye reaches 95%.

When light passes through the intraocular medium, the different refractive indices of the medium cause the light to scatter differently. Different degrees of light scattering exist in the cornea, aqueous humor, lens, and vitreous. The shorter the laser wavelength, the more scattering; with the increase of age, the intraocular medium becomes physiological or pathological opacity, and the light scattering is aggravated. Scattered light is separated from the straight beam, reducing the light intensity at the therapeutic target. Therefore, when the light scattering is heavy in the treatment, the high output power of the laser can be increased to achieve the effective light energy of the treatment target. However, the increased scattered light is absorbed by other tissues in the eye, increasing the light damage in the non-treatment area. A better way is to use lasers with longer wavelengths and lower scattering, such as red wavelength lasers, to increase light penetration.

In biological tissues, macromolecules such as water molecules, proteins and pigments absorb light of different wavelengths. The cornea and lens are mainly composed of water, which mainly absorbs infrared light, the protein mainly absorbs the ultraviolet light, and the fundus pigment mainly absorbs the visible light. In the laser treatment of fundus diseases, the choice of laser wavelength is related to the pigment of the lesion and its light absorption. The fundus tissue has a variety of pigments. Various pigments mainly absorb light of their corresponding specific wavelengths, and the biological effects produced are related to the laser energy and the absorption rate of the target tissue in this wavelength band.

1. Melanin: Melanocytes mainly present in the retinal pigment epithelium (RPE) and choroid, absorb visible light of any wavelength. In the laser treatment of fundus diseases, the photothermal effect is mainly produced by the absorption of laser energy by RPE and choroidal melanin. The absorption rate of RPE and choroid can reach 70% at wavelengths of 450-630 nm. However, from green to red, the absorbance decreases with increasing wavelength.
2. Hemoglobin: It has a high absorption rate for lasers with wavelengths of 400-600nm, while red and near-infrared light with wavelengths above 600nm are rarely absorbed by hemoglobin. Taking advantage of this feature, the red wavelength laser above 600 nm should be selected in the fundus laser treatment of vitreous hemorrhage and retinal hemorrhage, because it can penetrate the hemorrhage without being absorbed, and can act on the retinal target tissue; and vascular diseases such as retinal For hemangioma, it is not suitable to choose red laser, because it is not absorbed and cannot produce thermal effect.
3. Lutein: Lutein is the photoreceptor pigment of cone cells, mainly concentrated in the inner and outer plexiform layers of the retina, with the most in the macula, which has a higher absorption rate for light below 480nm. Therefore, the use of lasers with longer wavelengths in green for the treatment of macular diseases is safe for cone cells.

In the laser treatment of fundus diseases, the choice of laser wavelength is related to the depth of the lesion and its absorption of light. Fundus laser absorption sites are mainly in the RPE and choroid. However, different wavelengths of laser have different absorption rates at each level of the fundus tissue. At the same time, the longer the laser wavelength, the stronger the penetrating power, and the deeper the effect on the fundus tissue. The absorption rate of light from 450 to 630 nm can reach more than 70%. In this band, with the increase of wavelength, the absorption of RPE light decreases, and the absorption rate of choroid light increases. In the transparent state of the refractive interstitium, 57% of the green wavelength laser is absorbed by the RPE, 47% by the choroid; 50% of the yellow wavelength laser is absorbed by the RPE, 50% by the choroid, and luteol is not absorbed; 45% of the red wavelength laser is absorbed by RPE is absorbed, 55% is absorbed by the choroid, and the effect can reach the deep layer of the choroid (Figure 2-2). Therefore, green wavelength laser is the most commonly used laser spectrum for fundus photocoagulation; yellow wavelength laser can be used for retinal and subretinal diseases, especially diseases of the macula; and red and infrared wavelength lasers are the first choice for the treatment of choroidal diseases.

Phototoxicity refers to the damage to visual cells caused by low-energy light below the photocoagulation threshold. This phototoxic side effect mainly exists in the application of argon blue-green laser, in which the blue laser with a wavelength of 488 nm has phototoxic effect, while the laser with a wavelength above 514 nm rarely produces phototoxic effect, mainly due to thermal effect damage.