Camera resolution and contrast play an integral role in both the optics and electronics of an imaging system. Though camera resolution and contrast may seem like optical parameters, pixel count and size, TV lines, camera MTF, Nyquist limit, pixel depth/grayscale, dynamic range, and SNR contribute to the quality of what a user is trying to image. With tech tips for each important parameter, imaging users from novice to expert can learn about camera resolution as it pertains to the imaging electronics of a system.
PIXEL COUNT AND PIXEL SIZE
To understand a camera’s pixel count and pixel size, consider the AVT Stingray F-145 Firewire camera series. Each F-145 contains a Sony ICX285 sensor of 1392 x 1040 (horizontal x vertical) pixels on a 9.0mm x 6.7mm sensor. If one imagines the field of view as a rectangle divided into 1392 x 1040 squares (Figure 1), then the minimum resolvable detail is equal to two of these squares, or pixels (Figure 2). Tech Tip #1 is: The more pixels within a field of view (FOV), the better the resolution. However, a large number of pixels requires either a larger sensor or smaller-sized individual pixels. This leads to Tech Tip #2: Using a larger sensor to achieve more pixels means the imaging lens magnification and/or or field of view will change. Conversely, if smaller pixels are used, the imaging lens may not be able to hold the resolution of the system due to the finite spatial frequency response of optics, primarily caused by design issues or the diffraction limit of the aperture.
The number of pixels also affects the frame rate of the camera. For example, each pixel has 8-bits of information that must be transferred in the reconstruction of the image. Tech Tip #3: The more pixels on a sensor, the higher the camera resolution but lower the frame rate. If both high frame rates and high resolution (e.g. many pixels) are required, then the system price and set up complexity quickly increases, often at a rate not necessarily proportional to the number of pixels.
TV LINES
In analog CCD cameras, the TV Line (TVL) specification is often used to evaluate resolution. The TVL specification is a unit of resolution based on a bar target with equally spaced lines. If the target is extended so that it covers the FOV, the TVL number is calculated by summing all of the resulting lines and spaces. Equations 1 and 2 provide simple calculations for determining horizontal (H) and vertical (V) TVL. Included in Equation 1 is a normalization factor necessary to account for a sensor's 4:3 aspect ratio. Figure 3 shows an IEEE approved testing target for measuring TVLs of a system.
Figure 3: IEEE Approved Target for Measuring TV Lines (TVLs) [View Larger]
MODULATION TRANSFER FUNCTION (MTF)
The most effective means of specifying the resolution of a camera is its modulation transfer function (MTF). The MTF is a way of incorporating contrast and resolution to determine the total performance of a sensor. A useful property of the MTF is the multiplicative property of transfer functions; the MTF of each component (imaging lens, camera sensor, display, etc.) can be multiplied to get the overall system response (Figure 4).
The MTF takes into account not only the spatial resolution of the number of pixels/mm, but also the roll off that occurs at high spatial frequencies due to pixel cross talk and finite fill factors. Tech Tip #4: It is not the case that a sensor will offer 100% contrast at a spatial frequency equal to the inverse of its pixel size. For a complete discussion of MTF and its importance, view Modulation Transfer Function.
Figure 4: System MTF is the Product of the MTF of Each Individual Component [View Larger]
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