Laser pointer | EPFL Graph Search

A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode emitting a very narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light. The small width of the beam and low power of typical laser pointers make the beam itself invisible in a clean atmosphere, only showing a point of light when striking an opaque surface. Laser pointers can project a visible beam via scattering from dust particles or water droplets along the beam path. Higher-power and higher-frequency green or blue lasers may produce a beam visible even in clean air because of Rayleigh scattering from air molecules, especially when viewed in moderately-to-dimly lit conditions. The intensity of such scattering increases when these beams are viewed from angles near the beam axis. Such pointers, particularly in the green-light output range, are used as as

Defect formation and mitigation during laser powder bed fusion of copper

Carl Tore Viktor Lindström

Additive manufacturing (AM) is a group of processing technologies which has the potential to revolutionize manufacturing by allowing easy manufacturing of complex shapes and small series. One AM-method which is of high interest for processing of metals is laser powder bed fusion (LPBF), where a powerful laser is used to melt powder in a layer-by-layer fashion into a consolidated part.Many defects can form during LPBF processing, but copper, copper alloys, and similar metals are particularly prone to the formation of pores.The reason why these metals tend to have these defects is the high reflectivity decreasing the effective energy input, and the high thermal conductivity transporting the absorbed energy away. Several ways of decreasing the pore formation tendency are known: increasing the power, increasing the absorptivity, and alloying to decrease thermal conductivity.The relative effectiveness of these strategies are however not known, making it difficult to choose the correct strategy for the different industrial applications of these metals.In this thesis a simple analytical model for predicting the occurrence of these defects is proposed, allowing a lowest possible processing power given the material properties to be estimated. The model agrees well with experiments, and can be used to predict the transition from so called balling mode processing to conduction mode processing, and for qualitative analysis of different processing strategies. Two strategies of decreasing the porosity are proposed and tested: coating of the powder with an absorbing layer and processing with a green laser, which is mor e strongly absorbed by copper than the conventional IR-lasers.The copper powder was coated with thin layers of tin and nickel using a simple and cheap immersion deposition method, and it is shown that the amount of porosity is decreased more than can be expected of the alloying alone, showing both that the method is working and that laser interaction with the solid is important for the heat balance of the system.The interaction with the solid is shown through computational fluid dynamics simulations to arise from periodic fluctuations of the molten metal which cause laser light to be reflected forward, pre-heating the powder bed.The preheating is larger the higher the solid absorptivity is, explaining the porosity decrease when using coated powders, and shows that processing using a green laser increase the laser coupling more than the absorptivity of the liquid would.Single line tracks made from fine 5 µm pure copper with a green laser source show that the desired conduction mode melting can be achieved at a power below 70 W, an order of magnitude lower than what is needed for processing of the conventionally used 45 µm powder with an IR-laser.

EPFL2021

Spectral cola or how to turn green laser light red

Csaba Forro, Thibaud Humair, Xavier François Mettan

A green laser pointer shone through a glass of cola will appear red when looking from above the glass. Lowering the laser so that it is deeper in the cola, the colour will vary from orange to deep red. Fluorescence and absorption spectra were recorded consistent with the reddening of the laser. The depth dependence of the colour of the immersed laser is attributed to a more significant absorption of the short wavelengths by the cola, making the laser look redder as it is lowered deeper into the cola. To confirm this hypothesis, we simulated the fluorescence spectrum we would get at various depths by applying the Lambert law on our measured fluorescence spectrum. These spectra were then converted into red, green and blue (RGB) values. These values were compared to those measured on images of the immersed laser beam at corresponding depths.

Iop Publishing Ltd2014

Eye-guided laser surgery

Klaus Benedikt Schönenberger

The application of lasers in microsurgery and in particular in neurosurgery offers many advantages such as, e.g., hemostasis, no-touch (i.e. aseptic) procedures, and pinpoint accuracy. However, these advantages can often not be fully exploited since adequate laser beam delivery techniques are not available. Traditionally micromanipulators attached to the surgical microscope and also laser-handpieces are used. Such techniques require that one hand of the surgeon is used to direct the laser beam, leaving only one hand free for the adjustments of the microscope, suctioning, and dissection. Moreover, precise and continuous aiming along complex targets in deep confined anatomical spaces is extremely difficult with such systems. In this work, we present a novel technique for laser surgery which allows to overcome the basic pitfalls of the traditional delivery techniques. The idea is to slave the laser position to the surgeons line of sight. Such an eye guided laser surgery allows to free completely the surgeon's hands and enables him to easily guide the laser along any target line in the object field of the microscope by simply looking at it. The goal of this thesis work consisted in demonstrating the technical feasibility of this idea by developing an experimental prototype. In a first step the problems of eye tracking on an operating microscope are addressed. Among the different possibilities to detect the position of gaze, we have chosen a video based technique. A special eyepiece allowing to obtain the infrared image of the surgeon's eye and an optimized infrared illumination source allowing to enhance the contrast of the eye image are described. The second step consisted in critically reviewing the different solutions to couple a laser beam to the operating microscope and in discussing all aspects concerning the system soft- and hardware. The technical performances of this first eye-guided laser aiming system are then analyzed. The results confirm that the speed of response of the system is sufficient for microsurgery. The resolution of the laser aiming in the operating field depends on the magnification of the microscope. A value of 62 micrometers is achieved at the highest magnification. A second part of the thesis is concerned with the study of some aspects of human vision relevant to the ability of the surgeon to control the laser with his regard. The most important issues in this context are saccadic eye movements, the fixation characteristics and ocular dominance. They have been investigated by measuring the corresponding characteristics of test persons on the eye guided laser system. The eye movements during laser aiming consists of a succession of rapid saccades and longer fixations of the eye. The dynamics of these jerks constitutes a problem with respect to laser dosimetry. We demonstrate that adequate filtering allows to obtain a smooth laser aiming with a constant laser energy deposition. The stability of fixation is analyzed with respect to ocular dominance and it is shown that there is no disturbing influence on the ability to maintain stable laser aiming. The system is slightly sensitive to head motions of the surgeon. A 3-D mathematical model has been developed to describe this sensitivity. The model has been verified experimentally. It constitutes to our knowledge the first description of the sensitivity of the eye tracking technique to head motions. It allows to optimize the parameters of the prototype in order to limit the effect of head movements on the accuracy of the eye tracking. The experiments on eye guided laser aiming with test persons also revealed that untrained persons are able to perform complex laser aiming and point addressing tasks with this first prototype with a minimum of training. The use of the eye guided laser system in real operating conditions is described in the last part of this work. Different surgeons were able to perform the first eye guided laser assisted microvascular anastomoses in vivo on the rat. One of them used the system for the first time and his successful interventions clearly demonstrated the important ergonomic advantages of this highly evolved man-machine interface.