Light sources intensity measurements

Fig. 11. Schematic design of a fluorescence sensor. A strong light source creates radiation with low wavelengths. Optics like lenses and filters extract and focus the desired excitation light which is sent through the window into the measuring solution. Only a small fraction of the fluorescent light arrives at the window, passes this, and is collected by appropriate optics and fed to a sensitive detector (usually a photomultiplier). Variations in the light source intensity can be compensated by a comparative measurement. When optical fibers are used inside the instrument, the dichroitic mirror shown is obsolete...

It has to be noted that in situ calibration is absolutely necessary for every experimental setup. If pure TLC substance is used, it must be applied fresh by taking into account the purity of all substances and materials. The light source intensity, the angle of illumination, and all camera parameters must be set before the calibration procedure. This is important, since small variations of these parameters can falsify the measurement results in a wide range. 

We can evaluate the impact of indeterminate error due to instrumental noise on the information obtained from transmittance measurements. The following discussion applies to UVWIS spectrometers operated in regions where the light source intensity is low or the detector sensitivity is low and to IR spectrometers where noise in the thermal detector is significant. 

The intensity of a light source is measured in candela . This comes from the term standard candle . 

Ideally, using the reference beam would make measuring the baseline unnecessary. In practice, due to differences in the optical elements efficiencies and alignment between the two paths the baseline correction is usually still necessary for accurate measurements. The main advantage of the dual beam spectrophotometer is to compensate for short term (seconds to minutes) variations on the light sources intensities and the efficiency of the optical comptments common to both beams. Most variations in this time scale are due to light source intensities and detector efficiency changes due to temperature variatirai. 

The S/N of any light intensity measurement varies as tire square root of tire intensity (number of photons) produced by tire source during tire time of tire measurement. The intensities typical of xenon arc lamps are sufficient for measurements of reasonable S/N on time scales longer tlian about a microsecond. However, a cw lamp will... 

The amount of light emitted by a source is measured by its luminance or by its luminous intensity, which are defined in Figure 18.2. Intrinsic light emission relates to the amount of light emitted per unit area (luminance). Table 18.1 lists approximate luminances for some common light sources. 

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

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