CCD Camera System
A CCD camera system is most commonly used for PSP and TSP measurements in wind tunnel tests. Figure 1.4 shows a schematic of a CCD camera system. The luminescent paint (PSP or TSP) is applied to a model surface, which is excited to luminesce by an illumination source such as UV lamp, LED array or laser. The luminescent emission is filtered optically to eliminate the illuminating light before projecting onto a CCD sensor. Images (wind-on and wind-off images) are digitized and transferred to a computer for data processing. In order to correct the dark current in a CCD camera, a dark current image is acquired when no light is incident on the camera. A ratio between the wind-on and wind-off images is taken after the dark current image is subtracted from both images, resulting in a luminescent intensity ratio image. Then, using the calibration relation for the paint, the distribution of the surface pressure or temperature is computed from the intensity ratio image.
Scientific grade cooled CCD digital cameras are ideal imaging sensors for PSP and TSP, which can provide a high intensity resolution (12 to 16 bits) and high spatial resolution (typically 512×512, 1024×1024, up to 2048×2048 pixels). Because a scientific grade CCD camera exhibits a good linear response and a high signal-to-noise ratio (SNR) up to 60 dB, it is particularly suitable to quantitative measurement of the luminescent emission (LaBelle and Garvey 1995). The major disadvantages of a scientific grade CCD camera are its high cost and a very slow frame rate. Less expensive consumer grade CCD video cameras were used in early PSP and TSP measurements (Kavandi et al. 1990; Engler et al. 1991; McLachlan et al. 1992); the intensity resolution of a CCD video camera is typically 8 bits with a conventional frame grabber. When there is a large pressure variation over a model surface, a consumer grade video CCD camera can be used as an alternative to give acceptable quantitative results after the camera is carefully calibrated to correct the non-linearity of the radiometric response function of the camera (see Chapter 5). The low SNR of a video camera can be improved by averaging a sequence of images to reduce the random noise. In addition, film-based camera systems were occasionally used in special PSP measurements like flight tests (Abbitt et al. 1996).
The performance of a CCD array is characterized by the responsivity, charge well capacity and noise. From these quantities, the minimum signal, maximum signal, signal-to-noise ratio and dynamic range can be estimated (Holst 1998; Janesick 1995). These performance parameters are critical for quantitative radiometric measurements of the luminescent emission, which can be estimated based on the camera model and noise models (Holst 1998). Here, the most relevant concepts are briefly discussed. The responsivity, the efficiency of generating electrons by a photon, is determined by the spectral quantum efficiency Rq(h) of a detector. The full-well capacity specifies the number of
photoelectrons that a pixel can hold before charge begins to spill out, thus reducing the response linearity. The maximum signal is proportional to the full – well capacity. Normally, the well size is approximately proportional to the pixel size. Therefore, in a fixed CCD area, increasing the effective pixel size to enhance the SNR may reduce the spatial resolution. The dynamic range, defined as the maximum signal (or the full-well capacity) divided by the rms readout noise (or noise floor), loosely describes the camera’s ability to measure both low and high light levels.
The minimum signal is limited by the camera noises, including the photon shot noise, dark current, reset noise, amplifier noise, quantization noise, and fixed pattern noise. The photon shot noise is associated with the discrete nature of photoelectrons obeying the Poisson statistics in which the variance is equal to the mean. The dark current is due to thermally generated electrons, which can be reduced to a very low level by cooling a CCD device. The reset noise is associated with resetting the sense node capacitor that is temperature-dependent. The amplifier noise contains two components: 1/f noise and white noise; the array manufacturer usually provides this value and calls it the readout noise, noise equivalent electrons, or noise floor. By careful optimization of the camera electronics, the readout noise or noise floor can be reduced to as low as 4-6 electrons. The quantization noise results from the analog-to-digital conversion. The fixed pattern noise (the pixel-to-pixel variation) is due to differences in pixel responsivity, which is called the scene noise, pixel noise, or pixel nonuniformity as well.
Although various noise sources exist, for many applications, it is sufficient to consider the photon shot noise, noise floor, and fixed pattern noise due to pixel nonuniformity. Thus, according to the Poisson statistics, the total system noise < Пу > is given by
where < ns2hot > , < n2loor > and < n2pattern > are the variances of the photon shot
noise, noise floor and pattern noise, respectively, npe is the number of collected
photoelectrons, and U is the pixel nonuniformity. Accordingly, the signal-to-noise ratio (SNR) is
SNR = npe Цnpe +< n2floor > + (Unpe)2
Figure 4.5 shows the total noise, photon shot noise, noise floor (readout noise), and fixed pattern noise of a CCD as a function of the number of photoelectrons for < n2floor >1/2 = 50e and U = 0.25%. For a very low photon flux, the noise floor
dominates. As the incident light flux increases, the photon shot noise dominates. At a very high level of the incident light flux, the noise may be dominated by the fixed pattern noise. When the photon shot noise dominates, the SNR
asymptotically approaches to SNR = ^npe, and the dynamic range is
(npe )max/ < nfloor >, where (npe )max is the full-well capacity. The dark current only affects those applications where the SNR is low. In most applications of PSP
and TSP, the pressure and temperature resolutions are limited by the photon shot noise. Table 4.1, which is adapted from Crites (1993), lists the performance parameters of some CCD sensors.
Table 4.1. Characteristics of CCD Sensors
The selection of an appropriate illumination source depends on the absorption spectrum of a luminescent paint and optical access of a specific facility. An illumination source must provide a sufficiently large number of photons in the wavelength band of absorption without saturating the luminescence and causing serious photodegradation. It is desirable for a source to generate a reasonably uniform illumination field over a surface such that the measurement uncertainty associated with model deformation can be reduced. A continuous illumination source should be stable and a flash source should be repeatable. A variety of illumination sources are commercially available. Pulsed and continuous-wave lasers with fiber-optic delivery systems were used in wind tunnel tests (Morris et al. 1993a, 1993b; Crites 1993; Bukov et al. 1992; Volan and Alati. 1991; Engler et al. 1991, 1992; Lyonnet et al. 1997). Lasers have obvious advantages in terms of providing narrow band intense illumination. Very stable blue LED arrays were developed for illuminating paints (Dale et al. 1999). LED arrays are attractive as an illumination source since they are light in weight and they produce little heat; they can be suitably distributed to form a fairly uniform illumination field. In addition, they can be easily controlled to generate either continuous or modulated illumination. Other light sources reported in the literature of PSP and TSP include xenon arc lamps with blue filters (McLachlan et al. 1993a), incandescent tungsten/halogen lamps with blue filters (Morris et al. 1993a; Dowgwillo et al. 1994) and fluorescent UV lamps (Liu et al. 1995a, 1995b). The spectral characteristics of illumination sources can be found in The Photonics Design and Applications Handbook (1999). Crites (1993) discussed some available light sources from a viewpoint of PSP application.
Optical filters are used to separate the luminescent emission from the excitation light, or separate the luminescent emissions from different luminophores. There are two kinds of filters: interference filters and color glass filters. Interference filters select a band of light through a process of constructive and destructive interference. They consist of a substrate onto which chemical layers are vacuum deposited in such a fashion that the transmission of certain wavelengths is
enhanced, while other wavelengths are either reflected or absorbed. Band-pass interference filters only transmit light in a spectral band; the peak wavelength and spectral width can be tightly controlled. Edge interference filters only transmit light above (long pass) or below (short pass) a certain wavelength. Color glass filters are used for applications that do not need precise control over wavelengths and transmission intensities. The ratio of transmission to blocking is a key filter characteristic. All filters are sensitive to the angle of incidence of the incoming light. For interference filters, the peak transmission wavelength decreases as the angle of incidence deviates from the normal, while the bandwidth and transmission characteristics generally remain unchanged. For color glass filters, an increase of the incident angle increases the transmission path, reducing the transmission efficiency.