Light gathering power

In Sect. 4.2.10 we saw that for a given resolving power the spherical FPI has a larger etendue for mirror separations r /Ad. For Example 4.19 with D — 5 cm, d = 1 cm, the confocal FPI therefore gives the largest product RU of all interferometers for r 6 cm. Because of the higher total finesse, however, the confocal FPI may be superior to all other instruments even for smaller mirror separations. 

In summary, we can say that at comparable resolving power interferometers have a larger light-gathering power than spectrometers. 

For D = 5 cm, RU is about 40 cm sr, and therefore two orders of magnitude larger than for a grating spectrometer. 

In summary, we can say that interferometers have a larger light-gathering power than spectrometers at comparable resolving power. However, the need for high-quality optical surfaces for mirrors and beam splitters and the inconvenience of careful alignment represent a certain disadvan- tage. 

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The light-gathering power of a lens is expressed in terms of F number, defined by... 

Etendue is a characteristic of an optical instrument. It is a measure of the light gathering power, i.e. the power transmitted per radiance of the source. A is the area of the source (or image stop) Q is the solid angle accepted from each point of the source by the aperture stop. 

Levi-Civita symbol 85 Lewis number 66 light gathering power 31 light second 110 light year 110 line width 31... 

The major advantages of FTIR instruments over dispersive spectrometers include better speed and sensitivity, better light-gathering power, more accurate wavelength calibration, simpler mechanical design, and the virtual elimination of the problems of stray light and IR emission. Because of these advantages, nearly all the new IR instruments are FTIR systems. 

Some workers who want especially high resolution use spherical mirrors rather than the flat mirrors described above. Spherical mirrors increase the light-gathering power and also allow easier scanning of a spectrum. 

The quality of a monochromator depends on the purity of its radiant output, its abiliiy to resolve adjacent wavelengths, its light-gathering power, and its spectral band-widih. The last property is discussed in Section 7C-3... 

Compare the light-gathering power of the lens described in Problem 7-16 with one that has a diameter of 37,6 cm and a focal length of 16.8 cm. 

Assuming that there is a uniformly bright light source near end of the fibre, its light gathering power P is proportional to (sinuc) and to the fibre cross-sectional area. By Eq. (11.24) this gives... 

The light-gathering power of a telescope depends on the area of the lens, i.e. it is related to the square of the apertme. However, the larger the relative apertme the greater the aberrations. In microscopy large-apertme objectives (corrected for aberrations) are preferred, since they reduce the... 

Numerical aperture A measure of the light gathering power of an optical system. 

This example shows that for a given light-gathering power, the confocal FPI can have a much higher spectral resolving power than the plane FPI. 

Resolving power and light-gathering power are not the only criteria by which a wavelength-dispersing instrument should be judged. A very important question is the attainable precision and accuracy of absolute wavelength measurements. 

The major advantage of the d/0° system compared to the 07d is in radio-metric system efficiency. In Fig. 2, the amount of light that is collected from the lamp is a function of the /-number // ) of the collimating optics. Those familiar with photography understand that f/ expresses the light gathering power of a camera lens. Lower values off/ are required for slower speed film. 

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