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Introduction. The SIMAC [1], [2] is constituted by the association of a Fabry-Perot Interferometer and a large aperture spectrograph [F/2]. This allows it to achieve a very high resolution (8.105 to 106) with a very short time of exposure. Its constitution leads to a semi-sequential recording process so that a linear dispersion is obtained on the photographic plate. Moreover it allows one to increase the signal to noise ratio without changing camera lens as necessary with a classical spectrograph. In this paper we describe the technical aspects of the instrument and we give some experimental results. A complete theoretical and experimental discussion has been given previously in [3]. - Theoretical discussion. In an other paper [4] we establish that the three high resolution spectrographs (grating, Fabry-Perot, SIMAC) accept, in the same time, the same amount of energy for the same resolution. It follow that the fondamental difference is the shortest value of the focal length that the instruments are able to receive because of the graininess of the photographic emulsion. Discussion about the signal to noise ratio indicates what is the most appropriated instrument suitable for a specific problem. - Technical issue. A) Interferometer: It is a classical Fabry-Perot interferometer. An important improvement in the linearity of the scanning has been obtained by use of a flow controller (constant differential type). B) Spectrograph: Grating is manufactured by Bauch and Lomb (250 X 125 mm, 300 Lines/mm, blazed at 63° 27). As the present time the camera lens is a catadioptric system at F/2 and the focal length is the shortest theoretically possible (250 mm). But recent works have shown that this camera has to be modified: because of the large aperture, the receiver is a photographic film and experimental works proved that it was not convenient for position measurements (displacement of gelatin, deformation of the support can reach 10 to 40 μ). Secondly, with a catadioptric system, the plate holder, which occult the incident beam, must be very small, so we lose the advantage described in part I. The new camera will have the shortest focus length consistent with a photographic plate (glass support). C) Optical mounting: The arrangement is classical. The highest signal to noise ratio is obtained by use of the whole diameter of the Fabry-Perot. A soustractive dispersion double monochromator is used to get absolute symetric images in view of the recording with a photoelectric spectrum plate comparator [3]. - Results. Three reproductions show the possibilities of the SIMAC. The first one is an isotope displacement of Samarium. The second and third ones are molecular spectra of Iodic absorption and Lanthane-Oxyd emission. In these last cases, the Fortrat-Deslandres parabols are clearly perceived owing to the linearity of the scanning. - Conclusion. If the SIMAC accept finally the same amount of energy by time unit that the others high resolution spectrographs, it presents the advantages of a great flexibility. Without any mounting modifications, it can be a high speed spectrograph or a good signal to noise ratio spectrograph; even, in this last case, it keeps the advantages of the great aperture instruments.

The precision of measurements on a photographic plate depends on the image area. But the dimensions of the images given by a grating spectrograph, a Fabry-Perot spectrograph and the SIMAC are very different. So it was interesting to develop a new comparison which takes into account not only classical illumination but also image size. Then, we shall first discuss, at a given resolution the optical arrangement which gives the greatest image area. Secondly, we shall evaluate the signal upon noise ratio dependence with the area and the optical density of the recording. - Greatest image obtention Grating spectrograph: The best illumination is obtained when, along the direction of dispersion, the image width is equal to the graininess « g » of the photographic emulsion. Its heigth is not definite, but, because of the curvature of the image, experimentalist use small part of the entrance slit. We have choosen following limitative criteria: the angular heigth must be such as the sagitta is equal to the limit of resolution. (This definition gives values in good agreement with practical ones.) Classical Fabry-Perot spectrograph: The width of rings must be equal to the « g » value: this condition determines the focal length of the objective located after the Fabry-Perot. Focal length of the grating spectrograph which assumes the separation of orders must be sufficient to assume the R/N resolution. (N coefficient of « finesse »). In fact we demonstrated that it was better to increase this length so that «etendue» (or acceptance) of the Fabry-Perot and the of the grating are adapted to each other. If optical magnification of the grating spectrograph is fixed to unity, slit must be enhanced. With this process, illumination and resolution are inchanged, but the useful part of the ring is more important. SIMAC: Because of the most important « etendue » delivered by the Fabry-Perot interferometer, it is possible to adapt a short focus grating spectrograph. But then, the total Fabry-Perot has to be illuminated to get the hightest possible image. Under these conditions, calculatins shows that, at a given resolutin, these three spectrographs deliver quite the same energy by unit time. - Signal to noise Ratio Clark Jone's and Felgett's works etablish the existence of a meximum of the detective quantum efficiency for the photographic receiver. For a given emulsion, there is an optimal density D0. So, we can separate the discussion into two parts. In the first case, we assume that we can reach in a reasonable time the optical density D0 with the slowest instrument. Under these conditions, if we use the Selwyn's law (density fluctuations are inversely proportionnal to the square root surface) and the reciprocity law, we see that the signal to noise ratio is varying as the square root time, exactly as in spectrometry. So, in this case, the three instrument are quite equivalent. The gain in rapidity is strictly compensated by a loss in signal to noise ratio. If we increase the image area of the Fabry-Perot spectrograph or of the SIMAC, we shall have the same exposure time and signal to noise ratio as with the grating spectrograph. Note, in this purpose that in the case of the SIMAC the change of focus length is not necessary because of the semi-sequential recording. In the second case (D0 is not accessible in a reasonable exposure time), the study of the variation of signal to noise ratio against density demonstrates that it is better to choose a short focus instrument. The decrease of the signal to noise ratio is compensated by the gain of detective quantum efficiency especially for very weak illuminations.

General expressions are obtained for the amplitude noise and frequency noise of steady-state oscillators. The nmr signal-to-noise ratios in the am and fm modes are then calculated. It is found that fm detection of nmr signals is insensitive to 1/f oscillator noise. Some measurements of noise and signal-to-noise ratio are reported and are shown to be in agreement with theory.

A description is given of a cathodoluminescence instrument and contol system. Study of signal to noise ratio. Improvement due to filter and use of a time averaging method. Luminescence spectra of CdS and HgS, illustrating the capabilities of the instument.

Quality of scintiscans taken with an Anger camera is dependent on the window settings. The authors derive an analytic expression to find the optimum window based on maximizing the signal to noise ratio. The signal is defined as the unscattered radiation both totally and partially absorbed in the NaI crystal, the noise is defined as scattered and background radiation. Signal and noise are characterized by an energy spectrum which are approximated by two Gaussians to simplify calculations. The effect of window settings on the line spread function obtained by an Anger camera with a 99Tcm line source in a water phantom were determined experimentally. The results suggest that the optimum window setting can be calculated using measured energy spectra of signal and noise.

The signal to noise ratio of a low-capacitance silicon junction detector may be increased by amplification with fast integrating and differentiating time constants. A simple circuit is described which enables the fast output pulse so produced to be shaped to allow correct operation of conventional pulse height analysing equipment.

After a consideration on the ultimate limit of three dimensional spatial resolution of Electron Microprobe Auger Spectroscopy (EMA), the practical limitation of spatial resolution and sensitivity are considered. The relation among the signal to noise ratio, the surface concentration of the detected element, primary beam current and the time required for an analysis is formularized in a simple equation which contains a coefficient representing the sensitivity of the element in given experimental condition. Preliminary results of an EMA with a field emission gun are briefly described in relation to the instrumental factors considered above.

Light emitted from a laser contains modulation due to power supply, intermode modulation, plasma noise, etc. A method for noise cancellation and therefore an improvement of signal-to-noise ratio in heterodyning signal is described.