Area array and line scan cameras are each suited for unique machine vision applications. Area array cameras, for all intents and purposes, are “conventional” cameras that use sensors with two-dimensional pixel arrays. The square or rectangular shaped sensor captures an image in a single pass with the resulting image having a width and height corresponding to the number of pixels on the sensor, for example, 640x480. Because of this, area array cameras are ideal for machine vision tasks where objects are small or have approximately the same size in both dimensions. However, the size of PCBs, LCD panels, and wafers has increased beyond the speed, accuracy, and resolution capabilities of many area array cameras. Line scan cameras offer a better solution.
A line scan camera uses a single row of light-sensitive pixels that image across the object, line-by-line, accompanied by high intensity lighting. Resolution is specified in the horizontal axis since the achievable resolution in the vertical direction will depend on the clock rate of the camera and the speed of the web. A completed image is built by stitching together the lines, much like a fax machine. Because only a one-dimensional correction needs to be applied, line scan cameras are much easier to correct for lens shade, photo response non-uniformity (PRNU), or dark signal non-uniformity (DSNU), than an area array camera.
Line scan pixels accumulate photoelectric charges relative to the light from the object imaged onto that pixel. Next, a readout register amplifies, adjusts, and digitizes the charges, all while the next row of pixels is being exposed. The maximum rate at which exposure and readout can occur is the “line rate,” calculated in kilohertz (kHz) — the number of lines exposed in one second. In production, the faster an object is moving, the higher the required line rate. To avoid under- or over-sampling an object, a programmable encoder, often connected to a conveyor or web, measures speed and precisely synchronizes the camera in pulses. A predetermined number of lines of the image are then stitched together to form a frame that is analyzed with software. Any defects are recorded on roll maps.
Line scan cameras excel at producing a flat image of cylindrical objects, at imaging very large objects with high resolution and at producing images of objects in continuous movement past a fixed point, such as parts on an assembly line or web applications. Line-scan applications include paper, rolls of metal, fiber, railway inspection, solar cells, textiles, pharmaceuticals, semiconductors, and postal sorting. Another advantage is that the cameras can fit into tight spaces, for example when they must see through rollers on a conveyor to acquire images of the bottom of a part.
In certain applications demanding both high scan rates and high contrast however, the sensitivity of line scan cameras using single x1 linear sensors can fall short. Increased sensitivity requires multi-line scan cameras. Dual-line scan designs feature two parallel arrays of pixels, capturing twice the number of photons and doubling sensitivity. To improve sensitivity further, time-delay integration (TDI) is frequently incorporated into line scan cameras. TDI-based cameras have several vertical integration stages, resulting in the capture of multiple exposures of the same object. Integrating the output from these stages increases sensitivity.
Single-line monochrome line scan cameras have linear sensors consisting of multiple pixels in a x1 configuration. To obtain a color image from a single-line scan imager, a linear R-G-B-R-G-B filter can be applied to the sensor with the pixels merged to create a color image. Unfortunately, this approach produces an interpolated image with lower resolution.
A “trilinear” approach calls for each of three arrays to capture one primary color simultaneously but at somewhat different locations on a moving object. The channels are then combined to form a full color image. Spatial correction compensates for the separation — the first and second arrays are buffered to match the third. The downside of using only three channels is relatively low spectral resolution. Manufacturers have improved the performance with image-based color measuring approaches that enable color to be measured on the whole surface of the object, not just on one spot, as with traditional spectrophotometers.
For truly accurate color inspection, line scan cameras with more than three color channels are required. Modern multispectral line scan cameras feature 6 – 12 spectral channels in the 360 – 960 nm range. Multi-channel imaging provides accurate spectral and color output on varying substrates such as paper, plastics, films, and foils.
Color imaging may no longer be enough, however, for inspection where specific wavelengths are required that are either outside the visible spectrum or in between the RGB color bands. Multispectral cameras can be used from near IR up to 960 nm in that case.