Human Eye L2

Human Visual SystemDigital image processing is built: Foundation of mathematical and probabilistic formulations

Human intuition and analysis play a central role: The choice of one processing technique versus another

The choice is made based: Subjective visual judgments

Fundamental Steps in DIP

Result is more suitable than the originalImproving the appearanceExtracting image componentsPartition an image into its constituent parts or objectsRepresent image for computer processing

Basic understanding of human visual perceptionInterest: The mechanics and parameters related to how images are formed in the eye The learning of the physical limitations of human vision in terms of factors that also are used in digital image

Structure of Human EyeShape: Nearly sphere

Average diameter: approx. 20 mm

Three membranes enclosed the eye: The cornea and sclera outer cover The choroid The retina

Image Formation in the EyeIn an ordinary photographic camera:The lens has a fixed focal length.Focusing at various distances is achieved by varying the distance between the lens and the imaging plane, where the film is located.

In the human eye:The distance between the lens and the imaging region (retina) is fixed.The focal length needed to achieve proper focus is obtained by varying the shape of the lens (for eye 14 mm to 17 mm)The fibers in the caliary body and lens accomplish this.The distance between the center of the lens and the retina along the visual axis is approximately 17 mm.

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Continue.Dimension of image formation on the retina:If h is denote the height of the object (see figure) in the retinal image.To calculate the size of the retinal image of the object, We can write, 15/100 = h/17 h = 2.55 mm

Images with different focal length

Brightness adaptation and DiscriminationSubjective brightness: Intensity perceived by the human visual systemBrightness adaptation: The range of intensity levels to which the human visual system can adapt. Brightness discrimination: The ability of the eye to discriminate between changes in light intensity at any specific adaptation level.

A plot of light intensity versus subjective brightnessPhotopic vision alone: the range is about 106 The transition from scotopic to photopic vision is a range from -3 to -1 mL (millilambert)

Brightness discriminationThe ability of the eye to discriminate between changes in light intensity at any specific adaptation level. Weber ratio (WR) = Ic /I (small means good brightness discrimination) large means poor brightness discrimination

Typical Weber ratio plot as a function of intensityThis curve shows that brightness discrimination is poor (WR is large) at low levels of illumination, and it improves significantly as background illumination increases.

Perceived brightness is not a simply function of intensity

A regions perceived brightness does not depend simply on its intensity(simultaneous contrast)

Optical illusions (wrongly perceived)

Problem 1: Estimate the diameter of the smallest printed dot that the eye can discern if the page on which the dot is printed is 0.2 m away from the eyes. Assume for simplicity that the visual system ceases to detect the dot when the image of the dot on the fovea becomes smaller than the diameter of one receptor (cone) in that area of the retina. Assume further that the fovea can be modeled as a square array of dimensions 1.5 mm 1.5 mm, and that the cones and spaces between the cones are distributed uniformly throughout this array.

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Solution:

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Problem 2: You are hired to design the front end of an imaging system for studying the boundary shapes of cells, bacteria, viruses, and protein. The front end consists of the illumination sources and corresponding imaging cameras. The diameters of circles required to enclose individual specimens in each of these categories are 50, 1, 0.1, and 0.01 micrometer, respectively. Can you solve the imaging aspects of this problem with a single sensor and camera? If your answer is yes, specify the illumination wavelength band and the type of camera needed. Identify the camera as being a color camera, far-infrared camera, or whatever appropriate name corresponding to the illumination source.

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(b) If your answer in (a) is no, what type of illumination sources and corresponding imaging sensors would you recommended? Specify the light sources and cameras as requested in part (a). Use the minimum number of illumination sources and cameras needed to solve the problem.

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Solution:

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