1,721,032 research outputs found
Will direct exposure to the Sun damage all digital cameras equally?
No full textConsiderations of optical damage induced by thermal effects were initiated by an apparent paradox: small and large diameter lenses with the same f-number produce an image of the Sun with the same irradiance. Thus, if one assumes that a damage threshold is given by irradiance (or intensity, as it is specified by most laser optics companies, in relation to continuous-wave lasers), all digital cameras should have the same probability of being damaged when pointed at the Sun. However, smartphones and in general small cameras are unaffected, while optical sensor arrays of larger cameras are often overheated and irreversibly damaged by the Sun. Therefore, the temperature rise responsible for the damage is not simply due to the irradiance of the Sun's image, but is actually related to lens size in a more subtle way. A simple thermal model shows that such a temperature rise is proportional to the linear power density (power/illuminated spot diameter) at focus. A web search shows that this important result is generally overlooked, although it is used by two renowned laser optics companies. This short article discusses the heating of an optical sensor as a practical example of applications of geometrical optics and heat transport laws at an undergraduate level
Charge density in non-homogeneous media
No full textExpressions for charge density in non-homogeneous dielectric and ohmic conductors are very simple to derive, but are generally omitted even in relatively sophisticated textbooks. A deceptive situation for which basic laws seem to be violated in an ohmic conductor is highlighted
Gauss Theorem and Pointlike Charges: When Infinitesimals Make the Difference
No full textAgnesi, University of Pavia, Pavia, Italy
A classic application of the Gauss theorem is examined, showing how the choice of the Gaussian surface and the pointlike nature of the charge can determine results somehow unexpected for the student. Particular care must be paid not only to points where the field becomes in- finite, but also to infinitesimal displacements of the charge and their direction. Pedagogical implications are outlined
Femtosecond optical parametric oscillator with 3D-printed polymeric parts
No full textA synchronously-pumped optical parametric oscillator (SPOPO) based on a periodically-poled, MgO-doped lithium niobate (MgO:PPLN) crystal was realized exploiting additive manufacturing technology for the pump and focusing section of the cavity, where careful predesign of the tightly-packed mounts and optics can be very helpful in view of a more traditional mechanical realization. Surprisingly, the 3D-printed plastic parts ensured long-term stability of the (quite sensitive, in principle) femtosecond SPOPO
Jitter investigation of narrow-bandwidth passively Q-switched Nd:YAG unidirectional ring laser
No full textA passively Q-switched Nd:YAG undirectional ring laser with external feedback is reported. The laser generates 50 ns single-axial-mode pulses up to 6 kHz, with energy 34 mu J, M2 < 1.05, and pulse jitter <50 ns rms, which is quite remarkable for this class of devices. Jitter was effectively minimized by using relatively high-peak-power pump pulses of 10 W, in agreement with a model considering both pump fluctuations and spontaneous emission noise. This represents an improvement by a factor of 8 with respect to a similar laser device we recently reported
Self-Mixing Displacement Measured by a Two-Color Laser in 66-nm Steps
No full textWe demonstrate a new self-mixing interferometer (SMI) simultaneously working on two wavelengths, the fundamental at 1064 nm and the second harmonic at 532 nm delivered in the same beam. The beam is sent to the remote target in the usual way, and in the return path, two photodiodes detect the two self-mixing signals without crosstalk. As the source, we were able to use a diode-pumped neodymium-doped yttrium vanadate (Nd:YVO4 laser crystal) with potassium titanyl phosphate (KTP nonlinear crystal) two-color laser of a commercial pointer. With simple processing of the two self-mixing signals, we are able to obtain the interferometric signals cos (2ks) and sin (2ks) at 532 nm, from which the unambiguous measurement of arbitrary target displacement follows with counts in units of λ/8 of the second-harmonic wavelength or 66 nm
Multi-GHz tunable-repetition-rate mode-locked Nd : GdVO4 laser
No full textWe report on a simple design for a multi-GHz tunable- repetition-rate diode-pumped picosecond laser. Using a plano-Brewster Nd:GdVO4 crystal in a V-folded cavity employing only readily available commercial components, we achieved passive mode-locking with 4.4-ps pulses tunable in the range 2.5-2.7 GHz. This laser is meant to be employed in the MIR experiment that aims at the detection of the Schwinger radiation (dynamical Casimir effect)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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