![]() ![]() Computation of the modulation transfer function is a mechanism that is often utilized by optical manufacturers to incorporate resolution and contrast data into a single specification. The resolution and performance of a camera within an optical system can be characterized by a quantity known as the modulation transfer function (MTF), which is a measurement of the camera and optical system’s ability to transfer contrast from the specimen to the intermediate image plane at a specific resolution. In addition to the discrete pixels, other factors such as the quality of the imaging system and camera noise all limit the accurate reproduction of an object. The Nyquist theorem deals with 2-dimensional signals such as audio and electrical signals and it is unsuitable for an image, which has three dimensions of intensity versus x and y spatial dimensions. Nyquist's theorem, which states that the frequency of the digital sample should be twice that of the analog frequency, is typically cited to recommend a "sampling rate" of 2 pixels relative to the object image size. The quality of the image is also now independent of where the object image is centred, at a pixel centre or at the vertex of pixels. It is only when the object image covers three pixels do we start to obtain an image that is more faithfully reproduced, and clearly represents a circular object. If the diameter of the projected image is equivalent to one or even two pixel diagonals the image reproduction is still not a faithful reproduction of the object and critically varies on whether the centre of the image projection falls on either the centre of a pixel or at the vertex of pixels. Even if the object is imaged onto the vertices of 4 pixels the object will still be reproduced as a square only dimmer - not a faithful reproduction. If the image falls directly in the centre of a pixel then the camera will reproduce the object as a square of 1 pixel. Before we consider the most appropriate pixel size of a particular application, it is important to consider the relative size of projected image to the pixel size to obtain a satisfactory reproduction of the image.Ĭonsider a projected image of a circular object that has a diameter smaller than a pixel. The trend in cameras is for the sensor size to decrease, and cameras with pixels as small as 4 x 4 microns are currently available in the consumer market. CCD arrays of over 1,000 x 1,000 sensors (1 Mega-pixel) are now commonplace in scientific cameras. The resolution of a CCD is a function of the number of pixels and their size relative to the projected image. ![]() What is CCD Spatial Resolution & Pixel Size? ![]()
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