Illumination of Extended Areas

In the case of extended detector areas, the total power received by the detector is obtained by integration over all detector elements dA (Fig. 2.10). The detector receives all the radiation that is emitted from the source element dA within the angles —u 0 - -u. The same radiation passes an imaginary spherical surface in front of the detector. We choose as elements of this spherical surface circular rings with dA = 2%r r = 2n sin cos6 d. From (2.29) one obtains for the total flux P impinging onto the detector with cos 0 =  

If the source radiation is isotropic, L does not depend on 0 and (2.32) yields 

A strictly parallel light beam would be emitted into the solid angle d 2 = 0. With a finite radiant power this would imply an infinite radiance Z, which is impossible. This illustrates that such a light beam cannot be realized. The radiation source for a strictly parallel beam anyway has to be a point source in the focal plane of a lens. Such a point source with zero surface cannot emit any power. 

Qi) Radiance of a HeNe laser. We assume that the output power of 1 mW is emitted from 1 mm of the mirror surface into an angle of 4 minutes of arc, which is equivalent to a solid angle of 1 xl0 sr. The maximum radiance in the direetion of the laser beam is then L = 10 /(10 -10 ) = 10 W/(m sr). This is about 50 times larger than the radiance of the sun. For the speetral density of the radiance the comparison is even more dramatie. Sinee the emission of a stabilized single-mode laser is restricted to a speetral range of about 1 MHz, the laser has a spectral radiance density Zv = lxlO W- s/(m sr ), whereas the sun, which emits within a mean spectral range of 10 Hz, only reaches Ly = l xlO W- s/(m sr ). 

A strictly parallel light beam would be emitted into the solid angle dQ = 0. With a finite radiant power this would imply an infinite radiance L, 

Radiance of the sun. An area equal to 1 m of the earth s surface receives 

Radiance of a HeNe laser. We assume that the output power of 1 mW is emitted from 1 mm of the mirror surface into an angle of 4 minutes of arc, which is equivalent to a solid angle of 1 x 10 sr. The maximum radiance in the direction of the laser beam is then L = 10 /(10 10 ) = 10 W/(m sr). This is about 50 times larger than the radiance 

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Plasmon modes with odd parity characters are dipolar forbidden because no polarization is created upon photo-excitation. Observation of the odd plasmon mode in Fig.4.7d indicates that the optically forbidden mode becomes optically allowed by the local illumination of the near-fleld. It is also noted that observation of the wavefunction image indicates that the coherence of the polarization wave extends from the tip position to the whole area of the nanorod. 

The incident radiation is provided by a laser pulse (wavelength 360-760 nm, duration 1-15 ns, pulse rate 1-100 Hz, energy 15-30 pJ). The illuminated area is typically 0.5 mm in diameter and the final temperature decay curve is accumulated following up to 10 000 laser pulses. This technique is suitable for the in situ non-destructive study of surfaces, and has been used to measure the thermal diffusivity of polyester films [13] and pigments [14]. More recently OTTER has been extended to the in vivo study of water concentration gradients in the stratum comeum [15]. The principle drawback of OTTER is that the values of the estimated parameters are entirely model dependent and improving the quality of experimental models is the major focus of current research in this field. 

For many spectroscopic applications only a small fraction of the cathode area is illuminated, e.g., for photomultipliers behind the exit slit of a monochromator. In such cases, the dark current can be futher reduced either by using photomulitpliers with a small effective cathode area or by placing small magnets around an extended cathode. The magnetic field de-... 

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