The radiation from a laser source is constituted of light rays, which can be considered as quasi-parallels. The eye, due to its function, can be assimilated to a converging lens. When a high-power laser-beam travels through the eye, its power gets focused on a smaller spot, localized on the retina. This power concentrated on a small diameter spot creates irreversible damages to the eye. However, the power is in itself not the only danger for the eye. Indeed, some factors are as relevant as the power concerning the potential damages : wavelength, exposure duration and continuous/pulsed nature of the exposition.
As can be seen on Fig. 2, the eye is a complex organ constituted of many different biological and optical elements, with different refractive indexes. Thus, while propagating through the eye, the light ray encounters mediums with different optical index and transparencies. Depending on the medium and on the wavelength of the beam, the effect will be very different.
Cornea has a refractive index of 1.377. Its absorption spectrum is presented on Fig. 3. Cornea does not contain blood vessels, is about 1 mm thick, and has a diameter of about 12 mm. It is isolated from ambient air by a lachrymal film.
The plot in Fig. 3 shows that the most absorbed wavelengths are located in the far infrared domain (800 to 2400 nm) and in the ultraviolet domain (less than 300 – 400 nm). These wavelengths will thus provoke the most severe damages on this complex optical element.
Moreover, these injuries will have different aspects depending on the frequency of the absorbed light. Weak ultraviolet rays UV-B, UV-C provoke conjunctivitis, epithelium photokeratisis and latencies. These lesions come with red blotches and lacrimation, and are not irreversible. They disappear after a maximum of 48 hours due to the natural recovery process of the eye.
Strong ultraviolet UV-B, UV-C mostly provoke damages on the Bowman membrane and on the cornea's stroma. The Bowman layer never recovers from any lesion. The thickness of the Cornea is mostly due to the stroma. It is constituted of collagen fibers with a precise diameter (35 nm) and precise spacing (59 nm). These fibers are grouped in layers parallels to the surface of the cornea.
Previously mentionned UV rays provoke neo-vascularization of the cornea, characterized by the appearance of blood capillaries. This process can lead to a worsening of the damages and finally to an oedema associated with the production of lactic acid. The accumulation of this acid is responsible for a milky aspect of the cornea leading to a loss of transparency (see Fig. 4). These lesions are irreversible, and the cornea is lost. One can proceed with surgery, but it leads to an opaque scar. In order to get the eye functional again, the only solution is to transplant a new cornea.
Weakly energetic infrared rays slightly burn the epithelium and can create astigmatism. The lesions lead to an opacity having the same diameter as the beam. When the energy delivered by the beam is higher than a certain level (typically 30J/cm²), infrared rays can damage the stroma in the same manner as UV rays do (loss of transparency of the cornea). At such powers, this type of radiation is absorbed and converted into heat, thus creating a hole in the cornea and leading to a flow of aqueous humor. These damages are irreversible and require surgery, generally leading to ether an opaque scar, or a transplantation.
As shown on fig 2, Iris is at the interface of the anterior and posterior chambers. It is constituted of colored pigments, and is thus responsible for the eye color. The pupil is located at its center. Iris is a muscle, dilating or contracting the pupil in order to regulate the light flow through the eye, thus playing the role of a diaphragm (its diameter varies from 1.5 to 9 mm). Laser radiation does not create irreversible lesions, but only pigmented areas. They appear after a laser impact, leading afterwards to an oedema and to the apparition of a miosis. However, lesions of this kind gradually disappear within 2 to 3 weeks.
Nevertheless, in case of repeated impacts, the pigments may migrate towards the anterior chamber, and the iris may atrophy or even tear.
At high powers, the Iris loses its color on the impact site, and in the worst case gets paralyzed and finally necrosed.
As previously written, the Iris plays the role of a diaphragm. However, its aperture is dynamically varying and depends mainly on the radiation wavelength. For example, the diameter of the pupil exposed to UV rays is of the order of 1 mm, while it is around 7 mm if exposed to visible and near-IR radiations. In the case of still higher wavelengths, its aperture can reach 11 mm. Thus, the iris does not play its protecting role any more against visible and near-IR radiations for the deepest eye structures, bringing upon them major hazard.
Situated behind the pupil, the crystalline lens separates aqueous humor from vitreous humor. This optical element plays the role of a transparent biconvex lens. Using its accommodation ability, it focuses the light rays from any object on the retina. In order to obtain a neat image on the retina of an object placed at any distance from the eye, the ciliary muscle can distort the crystalline lens and consequently modify its curvature. As any transparent medium, its absorption depends on the light wavelength. (See Fig. 5)
The crystalline lens, as any converging lens, focuses all the light rays parallels to the optical axis (presently the line of sight) on the focal point situated on the fovea (area located at the centre of the retina) – as long as the eye does not present optical defects such as myopia, astigmatism... Light rays parallels one to the other but tilted with respect to the optical axis will be focused on another point of the retina.
The wavelengths presenting the biggest hazard for the crystalline lens are near-ultraviolet and far-infrared. The damages consist either in ovoid opaque grey/white areas situated along the trajectory of the incident laser beam, or in permanent lesions where thermal effects lead to cataract.
The retina is a 0.5 mm thick surface situated at the back of the eye. It is the screen on which the crystalline lens focuses the light rays coming from the observed objects. One understands then quickly that retina is the sensitive part of the eye.
The retina contains the neuroreceptors responsible for the sight. It plays the role of a photographic film. It transforms the inverted image of the observed object into an analogical signal for the neurons. This transformation is done as follows : the light focused on the retina travels through the retinal layers and through the pigmented epithelium adherent to the choroid. The role of the epithelium is to protect the retinal receptors. The filtered light then activates photoreceptors, which can be either rods or cones. The latter then transmit a signal through synapses to the bipolar cells. This signal then propagates by chemical diffusion inside the bipolar cells and through synapses towards the ganglion cells, which constitute the optical nerve. All these cells thus transport the visual information from the eye to the brain. These different complex steps of sight allow us to see a detailed image of the observed object.
Fig. 6 presents the absorbed and transmitted wavelengths along their trajectories towards the back of the eye. One can notice that visible and infrared rays are the most transmitted wavelengths to the retina. These frequencies correspond to a maximal aperture of the pupil, which indeed partially explains this maximal transmission to the back of the eye. One then quickly understands that visible and infrared (IR-A) rays will cause maximal damages on the retina. However, the seriousness of the lesions can vary, depending on their localization and diameter on the surface of the retina.
The most commonly observed lesions are burns with coagulation and destruction of the tissues. They are mostly located on the pigmented epithelium, being very absorptive. The diagnosis is the apparition of a central unpigmented area encircled by a pigmented ring, whose diameter depends on the image size. In many cases, the epithelium then separates from the choroid.
An injury of the retina is accompanied with a physiological bedazzlement leading to a non-negligible decrease of visual perception. It can also be joint to a significant decrease of the retinal sensibility. This loss is characterized by a difficulty to adapt to obscurity and by a loss of chromatic perception.
The localization of these lesions on the retina depends on the angle between the incident ray and the line of sight (see Fig. 2). Indeed, the radiation can be focused on a spot on the macula and destroy the area. Then appears a scotoma - namely an area deprived of sight. The macula lutea (yellow spot) is mainly constituted of cones on a ~2 mm wide area. A small 0.2 mm diameter depression is located at its center – perfectly on the optical axis. Its name is fovea, and it contains a high density of cones. This area thus corresponds to the maximal visual acuteness used for diurnal vision.
Laser rays propagating along the line of sight will be perfectly focused on the macula. If the spot is small enough, the fovea alone will be damaged. In this situation, the visual acuteness will be reduced by a factor of two. If the spot size is larger, the radiation can destroy the whole macula, leading to the loss of ¾ of the visual acuteness. Fine details won't be perceptible any more and images will become blurred.
Light rays tilted with respect to the line of sight provoke lesions on the retinal periphery, thus only altering the peripheral vision.
The fovea does not contain blue-light-sensitive cones and is thus particularly insensitive to blue radiation.
As shown in Fig.7, the epidermis is made of different layers. The penetration length of a laser radiation will strongly depend on its wavelength.
The effects of laser radiation are less important on skin than on the eye. The power is generally not concentrated (in the absence of any optical element), and the pain perception is quicker.
The skin mostly undergoes thermal damages, as the epidermis cannot stand thermal powers higher than a few 0.1 W/cm² continuously or a few W/cm² during short pulses (peak power). As a reference, a clear weather sunlight exposes skin to 0.14 W/cm².
Of course, the thermal effect not only depends on the beam power, but also on the skin pigmentation type. Indeed, the skin pigmentation protection efficiency depends on its coloration (see Fig. 5). For example, a 5 to 10 Joules pulse has no effect on a (reflective) white skin, while it burns a pigmented skin – this stresses the role of melanin and haemoglobin in the radiation absorption process. According to Fig. 7, visible and near infrared (<1.4 µm) are mostly reflected by the skin, while the other frequencies are mostly absorbed. The latter frequencies will thus be responsible for most of the skin injuries.
The skin constitutive layers have different resistances to radiation. The thick hyperkeratotic layers are resistant while the thinner layers in the dermis, closer to the surface, are more sensitive.
The observed lesions will then depend on the radiation frequency and are described in the following lines:
UV rays, depending on their type (UV-A or UV-B), penetrate at different depths inside skin layers. UV-B rays are absorbed in the external layer - the epidermis – and are responsible for red blotches similar to sun burns. UV-A rays penetrate more deeply than the previous ones, and are responsible for many skin diseases. Both types are responsible for skin affections including ageing, erythema (red blotches), pigmentation increase, light-sensitization or even cancer.
Visible and infrared radiations act on deeper layers through thermal effects. These radiation types can then induce vessels dilatation and red blotches, leading to skin burns from the surface to deeper layers.