- Because of the increasing use of the CCD camera in optical metrology, this device is described separately in Section 5.8.. - Radiant flux density at a surface, M = E = ∂/∂A, is the radiant flux at a surface divided by the area of the surface. - Figure 5.1 The concept of radiance. - 2 /∂∂A cos θ Luminance lm sr −1 m −2 All of the radiometric terms have their photometric counterparts. - They are related to how the (standard) human eye respond to optical radiation and is limited to the visible part of the spectrum. - To represent the response of the human eye, a standard luminosity curve V (λ) has been established, see Figure 5.2. - Figure 5.2 The standard luminosity curve. - Lambert’s cosine law states that the intensity (flux per unit solid angle) in any direction varies as the cosine of the reflection angle:. - Since the projected area of the source also varies as cos θ, the radiance becomes inde- pendent of the viewing angle:. - d = (r sin θ dθ r dφ)/r 2 = sin θ dθ dφ (5.8) The total radiant flux reflected into the hemisphere therefore is given by. - The ratio of the total reflected radiant flux to the incident radiant flux d i = EdA defines the diffuse reflectance of the surface. - the spectral distribution of the emitted flux is not the same as for a blackbody. - where M is the radiant exitance of the source of interest and M is the radiant exitance of a blackbody at the same temperature. - Because of the last assumption, the flux onto B must equal the flux leaving B toward A. - Let us compare the light from a typical He–Ne laser and a blackbody with the same area as the output aperture of the laser. - Assume this area to be 1 mm 2 and the blackbody temperature to be 3000 K, close to the temperature of the filament of an incandescent lamp. - The radiance at the centre of the beam is therefore (cos θ = 1). - 5 mW and a wavelength λ = 0.6328 µ m, this gives L W m −2 sr −1 , a number clearly in favour of the laser. - Note that the radiance of the blackbody is independent of its area. - By decreasing the power of the laser by reducing its output aperture, the radiance decreases accordingly.. - The flux incident over an annular element of the entrance pupil is given by. - If θ m is the angle of the marginal ray passing through the entrance pupil, the flux incident over the entrance pupil is. - We can write a similar expression for the flux d i incident over the exit pupil from a fictitious Lambertian source L i , in the plane of the image. - the reversibility of light, we can say that this flux, leaving the exit pupil in the direction of the image, gives rise to an image plane radiance L i according to. - d i = π L i dA i sin 2 θ m (5.25) where dA i is the image area and θ m is the inclination of the marginal ray in image space.. - the square of the lateral magnification. - where D i is the diameter of the exit pupil and b is the image distance. - It indicates that image irradiance is inversely proportional to the square of the F -(aperture) number. - (5.34) where S is the area and D is the diameter of the entrance pupil, a is the object dis- tance and where we for simplicity assume the entrance and exit pupils to have equal area S. - If we take the image area dA i to be equal to the area of the Airy disc (see Section 4.6, Equation (4.33)). - The spectrum of the emitted radiation is continuous and approximates a blackbody. - In all tungsten filament lamps, the tungsten evaporates from the filament and is deposi- ted on the inside of the envelope. - Radiation is emitted as the electron falls to a lower energy level characteristic of the particular type of gas.. - The spectral distribution of the radiation is a combination of both the continuum and the line spectra. - Xenon lamps have colour temperatures of about 6000 K, close to that of the Sun. - E 0 may or may not be the ground state of the atom. - Therefore the population of both the upper and lower levels of the laser transition are important. - Dependent on the construction of the laser, one can obtain lasing from a single transition or from a multitude of transitions.. - Emission in other directions is lost out of the laser medium and the light is concentrated in a beam oscillating back and forth between the mirrors. - However, because of the high refractive index of many semiconductor materials (for GaAs, n = 3.6) most of the light suffers total internal reflection (see Section 9.5). - Thus, for n = 3.6, only 3.9% of the total generated photon flux can be transmitted. - If the thickness of the active region (the junction) could be reduced, the optical gain would be the same with a far lower current density. - the carriers tend to diffuse out of the region. - The first emits at 1.54 µ m and is therefore a candidate for eye-safe laser range finders (The 1.06 µ m wavelength of Nd-YAG poses a serious eye hazard.) The most important line of Er-YAG is at 2.94 µ m which is absorbed strongly by water, so it leaves a thinner damaged layer between healthy tissue and the zone removed by surgery. - laser line narrowing and alteration of the laser wavelength, and chang- ing of the pulse length.. - This is based on the nonlinear interactions between light and matter (usu- ally a non-linear crystal) which can generate harmonics at multiples of the light-wave frequency. - The magnitude of the non-linear effect is proportional to the square of the incident power. - The second is insertion of a modulator (usually electro-optic or acousto-optic devices) into the cavity, blocking off one of the cavity mirrors. - Each time they hit the partially trans- parent output mirror, part of the light escapes as an ultra-short pulse. - Most of the laser types described above have been used in numerous different types of applications. - Below we mention some of the more successful application areas.. - The 511 nm line of the copper vapour laser is suitable for underwater transmission.. - Thus, the spectral distribution of the light from a multimode He–Ne laser is typically as given in Figure 5.15.. - the contrast or visibility of the interference pattern will have a maximum. - the same locations of the minima, but with a more steeply varying visibility function | γ (τ. - Photographic films are often characterized by the slope γ of the linear portion of the D – log E curve.. - Because of the non-linearity of the t –E curve it is advantageous to have t b slightly lower (i.e. - The recording technique consists of a number of steps, beginning with establishing a uniform electrostatic charge on the surface of the thermoplastic with a corona discharge assembly.. - This is a quite complicated procedure, but complete camera units are commercially available, giving a hologram ready for reconstruction within 5 s of the exposure.. - When exposed, a part of the monomer is polymer- ized. - Of the other materials for hologram recording, we mention dichromated gelatin, pho- toresist, photochromic materials and ferroelectric crystals.. - of their relative unimportance in the field of optical metrology we name only some of the better known types – the thermocouple, the bolometer and pyroelectric detectors.. - kT ) of the atoms in the detector itself. - where the work function W is the energy difference between the vacuum and the Fermi levels of the material. - 0.1%) and high work functions (W = 2.1 eV for Cs) which makes them useful only in the visible and ultraviolet regions of the spectrum. - The contact electrodes are often placed on the same surface of the material in a geometry such as in Figure 5.21 to maximize the light transmission while minimizing the transit time. - In the photovoltaic mode, a voltage V p is produced across the device that increases as a logarithmic function of the incident light irradiance. - Other advantages of the photoconductive mode are faster response, better stability and greater dynamic range.. - (3) The low resistance of the metal decreases the RC time constant thereby increasing the speed. - Figure 5.24 shows a simplified sketch of one of the capacitors. - Figure 5.25 shows how the charge packets are transferred from one electrode to the next by proper clocking of the potentials of the electrodes. - a barrier is maintained behind the charge packet while a deeper well is formed in front of the packet. - We also assume two-phase clocking of the electrodes. - Figure 5.26(a) shows the organization of the so- called frame-transfer structure. - Figure 5.26(b) shows the organization of the so-called interline transfer structure. - Note that the effective integration time for the interline transfer structure is twice that of the frame-transfer structure because the integration in the interline transfer structure is performed in sites separate from the transport registers.. - where R(0) is the responsitivity at zero spatial frequency and the other term is the product of all of the modulation transfer function (MTF, see Section 4.6.2) factors that affect the frequency response of the chip. - These factors are: (1) the loss of frequency response due to the geometry of the integrating cell (MTF integ. - For FIIT approximately one-half of the chip area is photosensitive because the other half is occupied by the vertical transport registers. - The latter is, however, a fundamental effect on the chip responsitivity and is related to the finite size of the integrating cells.. - (5.46) Therefore the MTF of the integrating process is (cf. - It follows that the spectrum of the sampled function is given by F s (f x. - This does not matter so much as long as f x is well below 1/x, the first zero of the sinc function: see Figure 5.28(d).. - Overlapping of the spectra causes frequencies higher than the Nyquist limit (f n = 2/p) to appear in the passband. - The remaining six views are moir`e patterns produced by the interaction of the CCD structure and the bars of the test chart.. - To guide the scanning beam of the TV-monitor, this video signal also contains some timing information, see Figure 5.31. - Figure 5.29 Spatial frequency domain analysis of the sampling process. - In the AD converter of the frame grabber (see Section 10.2), this porch is used for adjusting the grey level to a known value (black). - The odd fields consists of the odd-numbered scan lines. - Of the 10.5 lines in a field, we can assume that 1 line is scanned during vertical retrace to have a convenient vertical flyback time.. - Table 5.4 Comparison of the black and white video systems RS-170 and CCIR. - When using non-interlaced video, the resolution in the vertical direction of the frame transfer CCD cameras is also doubled, see Section 5.7.2. - (b) Find M λ of the Sun’s surface at this wavelength.. - The area of the solar disc dA is given by dA = π α 2 s 2 /4 where s is the Earth-Sun distance.. - the irradiance) on the top of the earth’s atmosphere at the wavelength λ m. - A detector is located 120 cm to the right of the lens. - Neglect losses in the lens and find the value of the photocurrent coming from the cell.. - 5.3 The responsitivity of the integrating process, cf. - An accepted definition of the radius of the spot size of this beam is the distance at which the intensity has dropped to 1/e 2 = 0.135.. - (b) Find the spectrum of the sampled signal
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