Measured spectrum of a tungsten halogen lamp. 

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... Figure 2 shows the measured spectrum of a usual customary tungsten halogen lamp. For learning purposes, this graph requires special considerations. Students may identify this spectrum as blackbody radiation, on the one hand due to the form of the curve, and on the other hand based on their knowledge that thermic radiators like tungsten lamps may be regarded as blackbody radiators. Actually, a tungsten lamp radiates only a small part (about 4%) of its radiation in the visual part of the spectrum. The important point to keep in mind is that the spectral sensitivity of the detector system is extremely dependent on the irradiated wavelength. Figure 4 shows the measured spectral sensitivity with a logarithmic scaling. Attention should be paid to the fact that there is a factor 1000 in the spectral sensitivity between 500 nm and 1000 nm. Without accurate calibration, neither two measured “intensities” at different wavelengths may be compared nor can any statements be made about the spectral power distribution of the radiation examined. For the calibration of the radiometric system used, the distributer of the spectrometers’ manufacturer kindly provided us with a radiometric calibration standard. In this case, it is a calibrated tungsten halogen light source with a well-known spectral power distribution. This calibration standard provides absolute spectral irradiance in μW/cm2/nm at the fiber port. The source can be calibrated specifically for a bare fiber or a fiber with attached cosine corrector. Irradiance is the power of electromagnetic radiation per unit area incident on a surface, such as the fiber's cross-section at its tip. Nevertheless, with a bare fiber it is not possible to measure the true irradiance because the coupling of light into the fiber is highly dependent on the incidence angel. Instead, the irradiance should be proportional to the cosine of the incident angel. Furthermore, a probe is needed with a 180-degree field of view. The spectrometers’ manufacturer provides a so- called cosine corrector: a window made from opaline glass mountable on the tip of the optical fiber creating a dependency on the incidence angle that is nearly proportional to the cosine. For spectrometry, it is common to consider each wavelength or frequency in a spectrum separately. Figure 4 clearly reveals how important it is to use appropriate optics, calibration and to take into account the detector characteristics. On the RCL spectrometer server, the acquisition of spectra is done by a windows service. The same program than computes the spectral irradiance. The acquired and the computed data will than be send to a client and there be displayed. A special challenge was to make mobile devices like smartphones and tablets able to display the spectra and to control the experiment. Therefore, the client program is build for HTML5 web browsers, nowadays available on any device. In German schools, network restrictions are very high and we have to consider firewalls. Therefore, we use a commercial WebSocket service (pusher.com) to deliver spectral data and control data. Before a user may control the experiment authentication is necessary. This is done by a PHP script running on a usual web server that also provides the website for the experiment. An IP cam watches the experimental set-up and sends the captured video stream directly to the client where the video stream is displayed in the browser. The advantage of using the WebSocket service is that no static IP is needed for the RCL server. Only the IP cam needs direct Internet access. Several parameters of the experimental set-up may be changed by the user. For example, the acquisition parameters for the spectrometer may be set, like the integration time, the boxcar width (averaging over neighboring pixels), or the samples to average (averaging over two or more sequentially recorded spectra). Furthermore, users can choose from six standard light bulbs mounted on a carousel like tungsten incandescent light bulbs, halogen incandescent lamps, cold white and warm white compact fluorescent lamps, light-emitting diode lamps or special bulbs. The cosine-corrected probe may be moved and positioned in front of the bulb in a field of 1 x 1.5 square meters. The probe may be rotated too. Many different objectives may be followed by students. They may compare spectra from different lamps and thus rate the usability of the light sources for a certain purpose. Students may assess color temperature and color fault. They may compare the radiated light with the color sensitivity of the human eye. Students may distinguish between physical and physiological quantities. They can analyze the energy efficiency of usual lamps. The free positioning of the probe allows further experiments like analyzing the decrease of spectral irradiance with the squared radius or the spatial spectral radiant emittance. With compact fluorescent lamps, differences can be noticed between the light coming from the gas discharge and light coming from the fluorescent layer. The presented remote-lab experiment on optical spectrometry is highly educative and instructive for an introduction to atomic ...