IMAGE ACQUISITION, CAMERAS

Transcription

1 IMAGE ACQUISITION, CAMERAS 1/39 Václav Hlaváč hlavac

2 IMAGING SYSTEMS 2/39 Pohled na celek: from the observed property of interest through radiance L and irradiance E to an electrical signal and finally to a digital image. Two options of image acquisition: Direct observation there is one-to-one correspondence between a point in the 3D scene and its 2D image (e.g., a ray in projective transformation). Indirect observation provides also a spatially dependent radiance L but there is no one-to-one correspondence between 3D and 2D information (e.g., radar, tomography, spectral imaging techniques, magnetic resonance).

3 LIGHT POLARIZATION (1) 3/39 Radiance is expressed as oscillating electrical and magnetic field in the theory of electromagnetic field. Vector fields describing the intensity of electric field E and the intensity of magnetic field B are solution to the system of Maxwell s linear differential equations. The direction of vector E in 3D varies in general. Sun light is a random mixture of short emission phenomena on the Sun and contains all orientations, i.e. unpolarized light.

4 LIGHT POLARIZATION (2) 4/39 Harmonic planar wave is the solution to Maxwell s differential equations a free space (without electric potentials and currents). The unpolarized light is polarized after passing through a polarization filter. E.g., Iceland spar (in Czech dvojlomný vápenec). Practical polarized filters = parallel fibres of elongated molecules oriented in one direction. Examples: polarized spectacles for fishermen. Polarized filter for a camera lens.

5 ILLUMINANTS ACCORDING TO EMISSION (1) Day light no flickering, unstable in time and color, very good viewing colors. Incandescent lamp does not flicker, warms the device, big energy input, big starting current, should be changed often, good for viewing colors. Halide lamp no flickering, should be changed often, good for viewing colors (better than incadescent lamp), smaller than incadescent lamp. Fluorescent lamp flickering (it is possible to power it with high frequency current or synchronize it), needs special power source, long life, bad for viewing colors, close to surface source (in Czech plošný zdroj). 5/39

6 ILLUMINANTS ACCORDING TO EMISSION (2) 6/39 LED Light Emitting Diode, modulated light, no warming, small size, low power consumption, monochromatic (also infrared, white color), long life. Laser Light Amplification by Stimulated Emission of Radiation). Device producing light of a single (pure) color = monochromatic. Can be modulated, coherent (=same phase) problems with interferences, low power consumption, long life for semiconductor lasers. Flash tubes e.g. xenon lamps, used in applications in which big power is needed, very expensive.

7 ILLUMINANTS ACCORDING TO SIZE (1) Point sources e.g. halid lamp, LED, laser. Emphasize the roughness of the surface. Strong highlights. 7/39 Surface sources, diffuse e.g. reflection from a white opaque wall, paper, fluorescent lamp, illuminants with large focusing screen (in Czech matnice). Suppress the roughness of the surface.. Back light diffuse of advantage in the cases in which only the silhouette of the object is of interest and the object is thin (metal sheet, animal skin,... ). Very often used in applications as it simplifies segmentation to objects and background significantly. Suitable for gauging (in Czech měření rozměrů).

8 ILLUMINANTS ACCORDING TO SIZE (2) 8/39 Blacklight, telecentric illuminants with collimators. Can be used only for small objects (up to diameter of the lens aperture), to be combined with telecentric lenses. Suitable in cases in which silhouette of thin objects is of interest. Dark field oblique illumination when rays are not directed to the lens, there is a reflection from object to the lens.

9 OPTICAL TRICKS 9/39 Monochromatic filter can suppress ambient light and decrease influence of color abberations. Polarized filter removes or enhances polarized image, e.g. reflection from glass cover of the device).

10 INFLUENCE OF POLARIZED FILTER There is a clear glass positioned vertically in front of a camera and it is tilted with respect to optical axis by about 45. A double refraction on the glass is visible in both images. 10/39 Vertical polarization. Window reflected in a glass. Horizontal polarization. Reflection suppressed. Visibility through.

11 DIRECTIONAL ILLUMINATION 11/39 Irradiance of the opaque surface (ideal case: Lambertian) depends on the surface direction. That is the reason why the tilt of the surface can be measured (shape from shading). One of the first applications was in measuring shape of planetary surfaces, e.g. Venus craters. Shadows can generate edges in images which can be confused with object boundaries.. Mirror component of reflectance causes highlights. If this the problem then directional illumination is not suitable.

12 DIFFUSED ILLUMINATION 12/39 Natural day light with overcast sky, fog. Solution in devices: circle from LEDs, semi-sphere from LEDs. Useful for surfaces with significant mirror component of reflectivity.

13 BACKLIGHT ILLUMINATION 13/39 Useful when the silhouette of the non-transparent object is sought. Simplifies segmentation. Useful also for semi-transparent objects where a range of interactions between light and matter can be observed (refraction, absoption, diffusion of light). Local inhomogeneities in the matter can be detected. Examples: X-ray. Spectral analysis when absorption depends on frequency.

14 LIGHT FIELD ILLUMINATION 14/39 Rays from the illuminant are directed to the camera. Objects between illuminant and camera look darker due to refraction, absorption or diffusion. Objects are dark on the light background. Used for viewing small particles.

15 DARK FIELD ILLUMINATION 15/39 Rays from the illuminant are not directed to the camera. Refraction, reflection, diffusion of light which falls to the camera is visible. Objects are light on the dark background.. Used to visualize small particles, metallic surfaces in microscopy (e.g., aluminium conductors in microelectronics).

16 TELECENTRIC ILLUMINATION 16/39 A collimator secures parallel rays. Lenses of big parameters have to be used if objects are large (often Fresnel lenses = steps-like lens from concentric elements). The measured gauge is invariant to the distance of the object from the lens.

17 NORMAL LENS 17/39 Distance to the object focal length. Normal, wide-angle, telephoto lens. Object F Chip Lens

18 SHARPNESS OF THE IMAGE 18/39 open diaphragm closed diaphragm lens chip F

19 MICROSCOPIC LENS 19/39 Image is increased, short working distance (approx. 1 mm), however can be also bigger (approx. 100 mm). Wide observation angle and small depth of focus. Object F Lens Chip

20 TELECENTRIC LENS 20/39 Only principal rays used, i.e. those parallel with optical axis. The input lens has to have bigger diameter than measured object. Useful when measured object changes its position or the object is thick. Collimator Chip F Light source Object Lens Diaphragm

21 PARAMETERS OF LENSES (1) Focal length fixed, adjustable (zoom) manually or motorized. Working aperture, diaphragm (also speed of the lens) the smallest and the greatest aperture. Diaphragm fixed, adjustable manually or motorized.. Lens connecting C the distance between the back of the lens and the chip is approx. 17 mm. CS approx 12 mm, the other parameters are the same. Lens for C mount can be adjusted to CS mount by an extension ring 5 mm thick, not possible in the other direction CS to C. 21/39

22 PARAMETERS OF LENSES (2) Focusing Fix focus (e.g., web cameras), manual or motorized focusing. Distances in which object is in focus can be changed by extension rings in the expense of deteriorated optical properties. Format which is the biggest chip usable; 1, 2/3, 1/2, 1/3, 1/4. Thread for a filter clear filter is used to protect the lens. Radial distortion is not given in technical sheets but it is important for measurement applications. Lenses with short focal length have typically bigger radial distortions (several pixels). 22/39

23 PRINCIPLE OF PHOTOCONVERSION IN SEMICONDUCTORS 23/39 Incoming radiation (photons) in converted in the semiconductor mass into charge couples, electron-hole. The semiconductor is in a static electric field. The Electron-hole couples are converted into a short current impulse. The current impulse must be amplified and processed. E.g., in a CCD element the impulse is used to charge a capacitor.

24 PHOTODIODE AND MOS STRUCTURE 24/39 Cross cut of two main principles for current generation and storing the charge.

25 CCD ARCHITECTURES 25/39

26 CCD CHIP, PROPERTIES OF THE TECHNOLOGY + Linearity: CCD sensors explore conversion of a photon to the couple electron-hole. The obtained charge is integrated in a capacitor. + Low noise: is given by the integral character of the measurement. Uncooled chip with TV read-out has SNR approx. 60 db. + Efficiency: Current sensors have hight energetic efficiency approx. 40%, i.e. every third photon generates one couple electron-hole. Read-out: only from the whole chip at once. Limited range of intensities: is given by the maximal capacity of individual capacitors.. 26/39

27 CMOS CHIP, PROPERTIES OF THE TECHNOLOGY alt/hdrc ima.html /39 + Logarithmic sensitivity: CMOS sensors are based on the photo diode principle. They measure a current in a read-out instance. + Read-out: possible in arbitrary order, e.g. only the region of interest can be read-out. + Camera and processor on the same chip: CMOS technology is well mastered (processors, memory). Smart cameras. Higher noise:

28 CCD CAMERAS, USER S VIEW (1) Spatial resolution: number of pixels in a row and in a column. TV CCIR/PAL TV RS170/NTSC Non-television cameras also , keep increasing. Resolution in intensity: given in bits for digital cameras, output typically 8 bits also 12 bits. For analog cameras SNR, usually >50 db. Sensitivity: v lux. Should be recalculated according to used diaphragm and AGC. AGC: Automatic Gain Control; yes/no, can be switched off?, manual control of gain. Shutter: commonly from 1/50 s to 1/10000 s. 28/39

29 CCD CAMERAS, USER S VIEW (2) Format: size of the photosensitive chip. Given either in inches of the equivalent vidicon tube diameter or in mm. 1/2 corresponds to mm. Shape of a pixel: square pixel vs. non-square pixel. Output for automatic diaphragm: AWB: Automatic White Balance. Changes ratio of R and B with respect to G. Gama correction: fixed/adjustable. Direct signal γ = 1. Typicky γ = 0, 45 (enhances black). Compensates intensity conversion function of the CRT (Cathode Ray Tube) and adjusts it to the sensitivity of a human eye. Lens thread: C mount / CS mount. 29/39

30 INTERLACED/NON-ITERLACED SCANNING 30/ Interlaced. Non-interlaced.

31 SIGNAL INTERLACED/NON-INTERLACED SCANNING 31/39 field frame frame odd even odd even odd even odd even ~ Interlaced. Non-interlaced.

32 ELECTRONIC SHUTTER Shortened exposition is used either if there is too much light or if fast events have to be captured. 32/39 I exposition frame (noninterlaced) field (interlaced) t

33 I FLASH LIGHT AND SUPPRESSION OF AMBIENT LIGHT The shutter time is shortened. 33/39 I I ambient light intensity flash intensity exposition t t The instant of the flash is set when the shutter is open. The ration between the integral of ambient intensity during the shutter opening and integral of the flash intensity gives the influence of ambient light. I flash contribution t LED are often used as cheap flash light. ambient light contribution t

34 TYPES OF CCD CAMERAS 34/39 Line cameras: Used both in B/W and color modifications. Used often in industrial applications, scanners, faxes and copying machines. TDI A variant of a line camera used for synchronous capturing of moving scene using more lines. Increased sensitivity. Television cameras, black and white CCIR 50 Hz, 625 lines, 768x576; RS-170 (EIA) 60 Hz, 525 lines, 648x484. Television cameras, color one PAL, SECAM 50 Hz; NTSC 60 Hz. Progressive scan non-interlaced. Digital cameras contain A/D converter, there are high quality ones, prices drop down both for industry or for multimedia.

35 COLOR CAMERAS SETUPS 35/39 Manual change of color filters in front of the monochromatic camera lens. Three chip cameras an incoming light is divided to a appropriate chip using color filters and semitransparent mirrors. One chip camera has filters directly on a chip. Spatial resolution in color resolution is smaller than coresponds to the number of pixels.

36 ARRANGEMENT OF COLOR FILTERS IN A SINGLE CHIP CAMERAS 36/39 R G R G C Y C Y G B G B M G M G R G R G C Y C Y G B G B M G M G Additive color model. Subtractive color model.

37 COLOR SCANNER 37/39 scanned document glass Illuminant mirror lens chip movement direction c

38 FIRE WIRE (i.link u Sony) A fast serial link. IEEE types of transmission: 1. isochronous, e.g. images; 2. asynchronous, e.g. sending parameters to a device. Used also for disks, cameras, interconnection between domestic electronics pieces (e.g., audio system Sony). Non-television camera. Example: color camera , 15 snmk/s, 40 thousands K. Two types of connectors. 4 pin and 6 pin one including power sourse. Kabel max. 4.5 m. Repeaters The younger competitor to fire wire (IEEE 1394) is USB 2. 38/39

39 A NEW BUS 1394b 39/39 Substantial innovation. Speeds up to 3.2 Gb/s. Up to hundred meters 100 meters if transmitted on the optical cable. Full backward compatibility with currently used specifications and 1394a.