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Today’s concept of time is based on Einstein’s theories of special (SR) and general relativity (GR). Many physicists anticipate that GR has an issue since it is not compatible with quantum mechanics. Here we show: SR and GR work well for each observer describing his unique reality, but “Einstein time” (Einstein’s concept of time) has an issue. It arranges all events in the universe in a 1D line on my watch, yet neither cosmology nor quantum mechanics care about my watch. Einstein time hides the big picture! In Euclidean relativity (ER), we replace egocentric Einstein time (proper time of one observer) with universal Euclidean time (proper time of all objects/observers). It originates from an absolute point O (Big Bang). In Euclidean spacetime (ES), all energy is moving radially away from O at the speed c. For each object, time flows in a unique 4D direction related to its position. Einstein time makes us believe that time would flow in one direction for all objects in the universe. Unlike other ER models, we claim that an observer’s reality is only created by projecting ES orthogonally to his proper 3D space and to his proper flow of time. ER gives us the same Lorentz factor as in SR and the same gravitational time dilation as in GR, but now we learn that they stem from a projection. ER outperforms SR in explaining time’s arrow and mc2. ER outperforms a GR-based cosmology in solving competing Hubble constants and declaring cosmic inflation, expansion of space, and dark energy redundant. Most important, ER is compatible with quantum mechanics: It solves the wave–particle duality and quantum entanglement while declaring non-locality redundant.

Today's concept of time traces back to Albert Einstein's theories of special (SR) and general relativity (GR). In SR, uniformly moving clocks are slow with respect to my clocks. In GR, clocks in a more curved spacetime are slow with respect to my clocks. Many physicists anticipate that GR has an issue as it is not compatible with quantum mechanics. Here we show: Einstein's concept of time has an issue because the proper time of some observer is taken as the fourth coordinate of all objects in the universe. We choose "Euclidean time" (proper time of an object), which is the absolute value of a 4D vector "flow of time" divided by the speed of light. This vector is pointing from the Big Bang in Euclidean spacetime (ES) to the object. In Euclidean relativity (ER), each clock has a unique flow of time related to its position in ES and is thus slow with respect to an observer's clock. It is not slow in its, but in his proper flow of time. Unlike other ER models, we claim that an observer's reality is formed by projecting ES to his proper 3D space and to his proper flow of time. GR misinterprets this projection as a curved spacetime. We derive the same Lorentz factor as in SR and the same gravitational time dilation as in GR. Predictions made by SR are correct because the Lorentz transformation is equivalent to one 4D rotation of an object's flow of time. A cosmology that is based on GR needs additional concepts, such as dark energy, to compensate for the ignored 4D vectors "flow of time". ER is superior to SR and GR as it solves 13 fundamental mysteries, such as time, time's arrow, mc2, two competing Hubble constants, the wave–particle duality, and quantum entanglement.

Today’s concept of time traces back to Einstein’s theory of special relativity (SR). In SR, he shows how inertial systems relate to each other. In general relativity (GR), he considers gravitation a property of curved spacetime. Here we prove: Einstein makes two mistakes in his concept of time. (1) He claims that clocks in a system K’ could synchronize at any instant with clocks in K, where K’ moves relative to K. If they did, they would have no clockwork. (2) He assigns variables of time to K’ (or else K) instead of assigning them to the measuring observer. Mislead by SR, Einstein makes a third mistake: In GR, he selects again an indefinite metric. Our “Euclidean relativity” (ER) is based on a Euclidean metric. We postulate: (1) In Euclidean spacetime (ES), all energy is moving radially away from an origin at the speed of light. (2) The laws of physics have the same form in each reality (projection of ES to an observer’s 3D space). (3) All energy is “wavematter” (electromagnetic wave packet and matter in one). Previous ER models run into paradoxes as they conceive of ES as reality. We show: The Lorentz transformation in SR is recovered in ER; gravitation relates to a rotation; ER is compatible with quantum mechanics. Einstein’s mistakes in SR have no measurable consequence, but GR is only an approximation for individual observers. We solve 12 fundamental mysteries, like today’s Hubble constant, dark energy, wave–particle duality, and quantum entanglement.

Today’s concepts of space and time trace back to Albert Einstein’s theories of relativity. In special relativity, he derives relations of how a “moving observer” experiences space and time with respect to an “observer at rest”. In general relativity, he derives relations of how mass and energy are affecting space and time. Both theories have been very successful, but fail to solve fundamental mysteries such as competing values of the Hubble constant, dark energy, the wave–particle duality, and quantum entanglement. Here we show that this failure is due to prioritizing a primary observer: Einstein’s relations are valid only for a “system at rest” or a momentarily comoving reference frame. There is no superordinate reference frame in which all observers (“at rest” and moving) are treated alike. In what we call “Euclidean relativity”, we replace Minkowski spacetime (MS) with Euclidean spacetime (ES). We claim that an observer’s reality is formed by projecting ES to 3D space. The major benefit is: ES is a superordinate frame which is not limited to individual observers. It even gives us a Theory of Everything. Matching the symmetry simplifies physics! Alternative models of Euclidean relativity run into paradoxes as they claim reality to be in ES. Our theory profits from two concepts: “distance” (space and time in one) and “wavematter” (electromagnetic wave packet and matter in one). Time is a subordinate quantity: covered distance divided by the speed of light. Wavematter is a generalized concept of energy: Waves and particles are the same thing (energy), but seen from two perspectives. Length contraction, time dilation, acceleration, and gravitation are geometric effects.

Today’s concepts of space and time trace back to Albert Einstein’s theories of relativity. In special relativity, he derives relations of how a “moving observer” experiences space and time with respect to an “observer at rest”. In general relativity, he derives relations of how mass and energy are affecting space and time. Both theories have been very successful, but fail to solve fundamental mysteries such as competing values of the Hubble constant, dark energy, the wave–particle duality, and quantum entanglement. Here we show that this failure is due to prioritizing a “system at rest”: In Einstein’s relativity, there is no superordinate reference frame that would treat objects at rest and moving objects alike. We replace Minkowski spacetime with Euclidean spacetime (ES) and claim that reality is formed by projecting ES to 3D space. Alternative models of Euclidean relativity run into geometric paradoxes as they claim reality to be in ES. Our theory profits from two new concepts: “distance” (space and time in one) and “wavematter” (electromagnetic wave packet and matter in one). In Cartesian ES coordinates, it is not predefined for all objects alike which axial distance relates to time. Time is a subordinate quantity: covered distance divided by the speed of light. Wavematter is a generalized concept of energy: Each object is wave from an external view, but matter from its internal view. Length contraction, time dilation, acceleration, and gravitation are geometric effects. We even derive a Theory of Everything in ES. Matching the symmetry simplifies physics.

We introduce two symmetric concepts to physics: “distance” (space and time in one) and “wavematter” (electromagnetic wave packet and matter in one). We claim that physics has chosen the wrong concept of time: It is not aware that the time of a moving object flows in a direction other than my time. We provide 15 proofs for our claim by solving 15 mysteries of physics graphically! For example, we prove that length contraction and time dilation are actually geometrical effects in a 4D manifold that we call “Euclidean spacetime” (ES). We prove that the discrepancy in calculating the Hubble constant stems from a systematic error in the redshift measurement. We prove that what I deem wave, deems itself matter, which solves wave–particle duality. We even untangle quantum entanglement without the issue of non-locality. We finally understand why alternative theories of Euclidean relativity have failed: They are all based on four dimensions of space rather than distance. A huge amount of energy was injected into ES at a point that we take as origin. Ever since has this energy been moving radially away at the speed of light. We live in the 3D hypersurface of an expanding 4D hypersphere. Hyperspherical coordinates have the great advantage that they reduce all that is ever happening to just one formula. So, it is the Theory of Everything in these coordinates: “Energy is covering radial distance which, divided by Euclidean time, is equal to the speed of light.” Acceleration and force like gravitation emerge from a rotation of objects in ES and a subsequent projection to 3D space. Matching the symmetry simplifies physics.

We present a theory of relativity based on four dimensions of space. In terms of impact, 4D space compares to 4D spacetime as the heliocentric compares to the geocentric model! We claim that 4D space is neither expanding nor curved by energy or mass. We also claim that time, although a useful concept in everyday life, is not a dimension of the universe. Time is just a misinterpretation of a suppressed dimension of space. The suppression is due to length contraction when moving at the speed of light. We derive this effect from 4D space geometry and prove that time dilation is equivalent to length contraction in one dimension of 4D space. We claim that the universe is a 3D projection of an expanding 4D hypersurface. Our situation is similar to that of an ant: Since it doesn't observe a third dimension, it can't understand 3D effects. Since we don't observe a fourth dimension, we can't understand 4D effects unless we discuss them in 4D space! We solve ten mysteries at one single blow: (1) the mystery of time, (2) of time's arrow, (3) of E_0=mc^2, (4) of why the background radiation is isotropic, (5) of Hubble's law, (6) of why the universe is flat, (7) of discrepancies in the Hubble constant, (8) of expanding space, (9) of dark energy, and (10) of spontaneity in particle physics. The secret to 4D space is its full symmetry in all dimensions. It thus shines with simplicity and beauty.

The measurement of evoked potentials has become a standard tool to test new hardware and software for electroencephalography (EEG). In this study, we investigate where to detect and how to improve visually, auditory and somatosensory evoked potentials with a reduced number of electrodes. We measured a total of 50 evoked potentials in healthy subjects, and we were able to detect visually, auditory and somatosensory evoked potentials with just three electrodes. We also investigated where to measure a combination of visually, auditory and somatosensory evoked potentials and found the best positions to be Oz, O1, O2, TP9 and TP10. In the second part of this study, we analyzed how the evoked potentials depend on the segmentation frequency selected to superpose EEG responses. We found that the detection of visually evoked potentials requires the segmentation frequency to match the stimulus frequency with an accuracy of at least 99.92 percent. The detection of auditory evoked potentials and somatosensory evoked potentials requires a matching of at least 99.95 percent. Therefore, a correct matching of the segmentation frequency with the stimulation frequency is the primary key to improving the quality of evoked potentials.

In the previous chapter we learned that matter is acting on light in manifold ways. Now we are going to address the huge variety of how light is acting on matter. Specific tissue parameters as well as laser parameters contribute to this variety.

In this and the following chapter, we will discuss basic phenomena occurring when matter is exposed to light.

The application of multiple fibres for the conformal irradiation of tumours by laser-induced interstitial thermotherapy (LITT) has been investigated. A study was performed to evaluate the coagulated zones produced in porcine muscle tissue in vitro. For delivering specified powers into the tissue, a multifibre system was developed which allows the simultaneous use of up to four fibres. A new quantitative method of magnetic resonance imaging (MRI) has been applied for real-time thermometry. It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetisation. The MR results were compared with the measurements of fibreoptic thermometers. Since the acquisition time of the selected MR sequence takes only 3 s per slice and the calculation of the temperature measurement could be realised within a few seconds, the temperature mapping works closely to real time. The accuracy of the temperature measurements in muscle tissue was 1.5°C. Whereas single-fibre applications induced convex-shaped isotherms, concave structures were generated by a multifibre LITT.

Threshold values of laser-induced optical breakdown (LIOB) on the surface of human corneal tissues, human enamel, and bovine brain tissues are presented. The data are obtained by using a regeneratively amplified Nd:YLF laser and a multistage dye laser system, respectively. The measured decrease in threshold fluence at shorter pulse durations is in good agreement with the authors' theoretical model.

Caries – the most frequent cause for dental surgery – still is mainly treated with conventional mechanical drills, although lasers have meanwhile been successfully applied to various clinical disciplines. Since ultrashort laser pulses with sufficient pulse energies have only been available at low repetition rates (< 1 kHz) in recent decades, solely continuous wave radiation or pulse durations longer than thermal diffusion processes were applied with the result of severe thermal damage and pain. In this report we present results on dental tissue ablation obtained with a novel thin disk Yb:KYW regenerative amplifier system that does not require chirped pulse amplification (CPA). We show that femtosecond laser pulses provide us with today’s optimal tool to treat dental decay in an acceptable time, in an excellent quality, and with unsurpassed caries selectivity. The superior quality is a result of the non-thermal laser-tooth interaction. All our results are based on environmental scanning electron microscopy.

Several laser systems for the removal of hard dental substances are currently under investigation. However, in most cases, such systems have been demonstrated to be inefficient or have led to undesirable thermal side-effects. This paper reports, for the first time, the removal of enamel and dentin by a picosecond laser system, a solid-state Nd:YLF laser. Very precise cavities can be obtained in the enamel and dentin of extracted human molars when laser pulses are distributed onto well-defined areas of the teeth. Scanning electron microscopy shows that the quality of the cavities is superior to that achieved by other laser systems. The cavity walls are very steep, and their surfaces are characterized by a sealed structure. In contrast to laser systems with longer-duration pulses, picosecond laser pulses ablate with less thermal damage to the surrounding substance. The results of dye penetration tests and polarized microscopy show that even mechanical shock-wave effects are negligible. When the Nd:YLF laser is applied to carious enamel, the ablation rate is found to be about ten times higher than for sound molars, thus making the Nd:YLF laser a caries-selective laser system.

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm2. Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.

Der Laser stellt heute bereits eines der wichtigsten Werkzeuge in der Augenheilkunde dar. Ob man mit ihm auch Kurz-und Weitsichtigkeit ohne Komplikationen beheben kann, wird sich innerhalb der nachsten Jahre zeigen.

Background and Objective Diode pumped, all-solid-state ultrafast lasers are now widely used to perform minimally invasive refractive surgery and keratoplasty procedures. Despite such use, a systematic study concerning ultrafast laser–tissue interactions is lacking. We determined the corneal ablation threshold as a function of the laser pulse width and stromal depth by simultaneous monitoring of the intensity of the laser-induced plasma and the second harmonic generation signals (SHG) from the collagen. Study Design/Materials and Methods Ablation thresholds in porcine cornea samples were determined using three diode pumped all-solid-state ultrafast lasers (a Nd:glass femtosecond laser, a Yb:KYW femtosecond laser, and a Nd: YAG picosecond laser) over a range from 800 femtoseconds to 20 picoseconds. Results Corneal ablation threshold remained nearly constant within the first 200 µm of stroma and was consistent with previous findings with the threshold proportional to the square root of the laser pulse width. Conclusion Corneal ablation thresholds can be precisely determined by simultaneous monitoring of the intensity of the laser-induced plasma and the SHG from the cornea. Lasers Surg. Med. 39:654–658, 2007. © 2007 Wiley-Liss, Inc.

Although lasers are nowadays widely accepted as a popular scalpel of minimally invasive surgery (MIS), one of the most common orthopedic surgeries-the replacement of the knee joint-is still performed using an ordinary oscillating saw. Since ultra-short laser pulses are usually considered to be inefficient regardless of their high precision, the newest development of femtosecond laser systems has not yet been clinically applied to any mass ablation situation. However, thin disk Yb:KYW lasers meanwhile provide sufficient output power to ablate bone tissue within a reasonable time frame. Our results mainly focus on ablation rates obtained at different spot distances, repetition rates and pulse energies. It is shown that femtosecond laser pulses at high repetition rates are a promising tool for orthopedic surgery.

Der Laser stellt heute bereits eines der wichtigsten Werkzeuge in der Augenheilkunde dar. Ob man mit ihm auch Kurz-und Weitsichtigkeit ohne Komplikationen beheben kann, wird sich innerhalb der nachsten Jahre zeigen.

In this and the following chapter, we will discuss basic phenomena occurring when matter is exposed to light. While here we will be concerned with various actions of matter on light, the opposite effect will be discussed in Chap. 3. Matter can act on electromagnetic radiation in manifold ways. In Fig. 2.1, a typical situation is shown, where a light beam is incident on a slice of matter. In principle, three effects exist which may interfere with its undisturbed propagation: reflection and refraction, absorption, scattering.

Several laser systems are currently under investigation for the purpose of removing hard dental tissues. However, either undesired thermal side effects or the lack of efficiency have already been demonstrated in most cases. In this paper, advantages and limitations of using ultrashort laser pulses with either picosecond or femtosecond durations are discussed. The major advantages associated with these pulse durations is the ability to produce very precise cavities without significant thermal side effects. Even disruptive effects due to shock wave generation seem to be negligible at moderate pulse energies close to the ablation threshold. The quality of these cavities is found to be superior to the quality achievable with other laser systems. Moreover, a spectroscopical analysis of the laser-induced plasma sparks enables an on-line health diagnosis of the irradiated volume. Limitations arise from the development of a suitable delivery system and from the cost of generating ultrashort laser pulses.

Threshold measurements for laser-induced breakdown of different organic tissue (cornea, brain, enamel) and inorganic matter (fused silica) have been extended to the sub-lOOfs time regime. A Ti:Sapphire laser system delivers 450 microjoules at repetition rates up to 1kHz. Single shot experiments are performed with pulse durations from 35fs to 240fs. To ensure a consistent determination of the threshold fluence, damage is detected by the alteration in amplitude of scattered light and/or by the occurrence of a plasma spark. In our single shot experiment with human enamel, no significant threshold dependence on pulse duration was observed. For soft tissue (cornea, brain), however, a slight decrease of threshold fluence is found when shortening the pulse duration. Fused silica is chosen as an additional target to compare the obtained data with those of recent experiments.

We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dependence of the fluence at the threshold of laser- induced optical breakdown on the laser pulse duration has been investigated experimentally and theoretically for human cornea, human enamel, and bovine brain tissue. For the experiments in the range from 100 fs to 200 ps, we used a femtosecond dye laser system and a picosecond Nd:YLF laser system. We observed a significant decrease of the fluence at the threshold when reducing the pulse duration. The measured dependence on the pulse duration is in good agrement with our model.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this chapter, we will discuss principal applications of lasers in modern medicine. Due to the present boom in developing new laser techniques and due to the limitations given by the dimensions of this book, not all disciplines and procedures can be taken into account. The main intention is thus to focus on the most significant applications and to evoke a basic feeling for using certain techniques. The examples are chosen to emphasize substantial ideas and to assist the reader in grasping some technical solutions. Potential difficulties and complications arising from either method are addressed, as well. However, we should always keep in mind that any kind of laser therapy will not be indicated if alternative methods are available which offer a better rate of success, are less dangerous to the patient, and/or easier to perform.

The potential of all-in-one techniques in modern femtosecond solid state lasers is being discussed. For first experiments we designed an electrooptically switched Ti:Sapphire laser. Q-Switching and dumping was performed at half-wave voltages. The generation of ultrashort pulses with energies at the μJ-level in one single cavity seems to be within the realms of possibility.

The physical parameters of the plasma-induced ablation mechanism were investigated usinga picosecond Nd:YLF laser system. The laser consists of a diode-pumped oscillator and a lamp-pumped regenerative amplifier. It operates at a wavelength of 1.053 1um with pulse durations of 30 p5 and pulse energies up to 1 mJ. The laser beam was expanded to a diameter of 4 mm and focussed to spot sizes of about 30 ,um. At these high power densities a localized plasma was induced at the focal spot. Surfaces of extracted human teeth were used as target material. In order to study the effect of accompanying shock waves, dye penetration tests, hardness tests andpolarized microscopy were performed. At moderate pulse energies no significant impact of shockwaves was observed. Because of this result, the terms "plasma-induced ablation" or "plasma-mediated ablation" are more appropriate for ablations solely induced by plasma ionization, and should be distinguished from photodisruptive ablations. In another series of experiments thegenerated plasma sparks were spectroscopically analyzed. From the measured spectra, mean

A picosecond Nd:YLF laser system was used to remove sound and carious enamel by the mechanism of plasma-induced ablation. The plasma spark was optically imaged onto the entrance pupil of a spectrometer. The spectra were scanned between 400 - 700 nm with a typical resolution of 0.2 nm. Calcium in neutral and singly ionized states and the sodium doublet at 589 nm were observed. The second harmonic of the laser wavelength was generated in an external BBO crystal, thereby converting about 10 (mu) J of the pulse energy to radiation at 527 nm. The amplitude of the diffuse reflected second harmonic was used as a reference signal for normalization of the spectra. Several sound and artificial caries regions of different teeth were investigated. The spectra obtained from caries always showed a strong decrease in amplitude of all mineral lines, if compared to sound enamel. These results can be explained by the demineralization process of dental decay. Thus, caries infected teeth are easily distinguished from sound probes, enabling a computer controlled caries removal in the near future. The possible setup of such an automated system is discussed.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The effect of different wavelengths on corneal tissue ablation has been studied using the 1st, 2nd, and 4th harmonics of a picosecond Nd:YLF laser system. This laser system consists of an actively mode locked Nd:YLF oscillator generating 30 ps pulses and a regenerative amplifier. The laser beam was focussed onto fresh human donor or calf corneas. The corneas could be moved in 3D patterns using computer controlled stepping motors. The amount of the ablated tissue and the quality of the resulting surfaces were judged by light and scanning electron microscopy. The histology showed that the ablation depths of the IR and green laser pulses are very similar at a given energy density. SEMs revealed that UV light causes a denser packaging of the underlying collagen fibers, similar to the pseudo Bowman's membrane generated with excimer lasers. Further studies are planned concerning the investigation of the 5th harmonic at 211 nm.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Three different laser systems have been investigated according to their possible application in dentistry: a free running and a Q-switched microsecond Ho:YAG laser, a free running microsecond Er:YAG laser and picosecond Nd:YLF laser system consisting of an actively mode locked oscillator and a regenerative amplifier. The experiments focused on the question if lasers can support or maybe replace ordinary drilling machines. For this purpose several cavities were generated with the lasers mentioned above. Their depth and quality were judged by light and electron microscopy. The results of the experiments showed that the picosecond Nd:YLF laser system has advantages compared to other lasers regarding their application in dentistry.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A laser system is described that offers manifold applications in ophthalmic surgery. The Nd:YLF solid-state laser emits at a wavelength of 1.053 micrometers . With pulse durations of 40 psec less energy is required to obtain optical breakdown compared to nanosecond lasers. The lower pulse energy allows a substantial confinement of the laser-tissue interaction. With a highly sophisticated delivery system eye movements can be tracked during surgery and a precise placement of the laser focus can be achieved. The high transmission coefficient of ocular structures for wavelengths around 1 micrometers enables the Nd:YLF laser to perform several internal eye surgeries such as cataract fragmentation, intrastromal refractive surgery, internal sclerostomy, or iridotomy. First clinical results of some of these procedures are summarized.

Degeneration of retinal pigment epithelial (RPE) cells severely impairs the visual function of retina photoreceptors. However, little is known about the events that trigger the death of RPE cells at the subcellular level. Two-photon excited autofluorescence (TPEF) imaging of RPE cells proves to be well suited to investigate both the morphological and the spectral characteristics of the human RPE cells. The dominant fluorophores of autofluorescence derive from lipofuscin (LF) granules that accumulate in the cytoplasm of the RPE cells with increasing age. Spectral TPEF imaging reveals the existence of abnormal LF granules with blue shifted autofluorescence in RPE cells of aging patients and brings new insights into the complicated composition of the LF granules. Based on a proposed two-photon laser scanning ophthalmoscope, TPEF imaging of the living retina may be valuable for diagnostic and pathological studies of age related eye diseases.