Camera T3

“Analog” RS-170 Cameras – Video information is transferred from the camera to the imaging electronics (frame grabber) according RS-170 television standard. The timing (synchronization) of a video system is controlled by either the camera or the frame grabber. In standard RS-170 mode, the camera outputs timing signals and the frame grabber follows these signals. In the “external sync” mode, the frame grabber provides the signals.

Each line in the transmitted image is conveyed as an analog signal. There are no pixels. The position of a feature along the line is determined by the time difference between it and the horizontal sync pulse. The frame grabber must resample each line into pixels. The resampling rate may be such as to generate a different number of pixels (usually less) from the number of pixels generated by the image sensor. The vision system uses “frame grabber” pixels for image processing and analysis. As the vertical relation between lines is fixed, each line of pixels in the image sensor corresponds to a unique line in the transmitted image.

In a synchronous analog system resampling is entirely controlled by the camera with line ready, frame ready, and pixel clock signals. For these cameras, the camera and frame grabber pixels are the same.

The position of the features along each horizontal line depends on the time between the horizontal sync pulse and that feature. As the sync pulse is carried on the same wire as the video data, level change in the video signal may cause a shift in the sync pulse. Hence, the sync pulse change may move the image over by several pixels. The pixel jitter has to be kept to a minimum, especially if the system is used for measurements.

The typical analog camera resolution is 640 x 480 pixels. Higher resolution cameras (up to 4096 x 4096) are available.

Digital Camera – Digital cameras are analog cameras with an analog-to-digital converter incorporated into the camera itself. This improves the signal and reduces noise, which results in higher accuracy. As they operate in a progressive-scan mode (non-TV standard), they can support larger image sizes, faster frame rates, and higher resolution.

Improved video signal allows for higher than 8-bit resolution. While 8-bit cameras allow for 256 gray levels, 10-bit allow for 1024 gray levels, and 12-bit allows for 4096 gray levels. Higher resolution cameras (14-16 bits) are typically used for scientific imaging and require special cooling techniques. Digital color cameras have typically a 24-bit output – 8-bit for each R, G, and B channel.

Digital cameras transfer pixels directly to the imaging electronics; hence, the camera and frame grabber pixels are the same.

Parallel Digital Cameras - Most digital cameras have been using a parallel interface standard for connection to imaging electronics. The parallel interface can have different types of data transfer signal - TTL, RS-422, and LVDS (RS-644). The TTL can only be used for extremely short cable runs. The RS-422 and LVDS are differential signals which are much more robust, with LVDS allowing longer cable runs and lower signal voltages.

Camera Link Cameras – Digital cameras, using the camera link interface standard and LVDS hardware standard, are becoming the standard - especially for high-resolution and high-speed cameras. The benefit of the camera link interface is having a standard simplified (less cabling) connection between cameras and imaging boards.

IEEE-1394 (Firewire) Cameras – These are digital cameras with a serial bus standard for use with a PC. The IEEE-1394 interface supports data rates of 400 Mbits/second. A single cable combines power, control and data. Because IEEE-1934 is a shared bus, there is bandwidth limitation and it also requires processor control to acquire image data, which limits processor availability for image processing.

Infrared Cameras – Infrared, or thermal, cameras measure the infrared, or thermal, energy emitted from objects. They are used in applications that require detecting the difference in a temperature of across inspected object – e.g. inspecting glass while it is being molded, or finding hot spots on electronic boards.

UV Cameras – Ultraviolet light is part of the light spectrum with a shorter wavelength than the visible light spectrum.

It is invisible to the human eye. The UV camera, which is sensitive to the UV light, can typically capture greater detailed data – e.g. surface scratches and blemishes, than the standard (visible spectrum) camera.

X-Ray Cameras – X-Rays are electromagnetic waves with an even shorter wavelength than UV light and therefore have higher energy.

X-rays tend to act more like particle than a wave. When passing through a material, x-rays are partly absorbed in the transmission direction. The relation between absorption and penetration depends on the kind of material (its atomic number) and on the energy of the X-rays. X-ray detectors collect actual photons of x-ray light.

An X-ray system has three main components: an X-Ray Generator, an X-Ray Camera, and the imaging electronics used to acquire and process data. X-Ray Generator emits a stream of x-rays into a fan beam that passes through the object before entering an X-Ray sensor – typically an x-ray sensitive image intensifier (scintillator) - producing a visible image on its output phosphor. The image is relayed into a CCD camera whose output is passed to the imaging electronics. The scintillator material can be coated on a tapered fiber optic bundle - which is directly coupled to a CCD camera.

In recent years, flat sensor panel imagers have been introduced which offer many advantages over image intensifiers. The 16-bit panel offers superior resolution with 65,536 possible gray levels. It also incorporates a windowing technique that enables it to display image data at multiple gray level settings for each imaging position. This allows details to be gathered from thick as well as thin sections of an object without blooming effects. The sensor panel imager incorporates an amorphous silicon photodiode array that is coupled to a scintillation material that fluoresces when hit by x-rays. When the x-rays strike the scintillator material, they are converted to visible light that is detected by a photodiode array and transformed into electrical signals. The electrical signals are extracted from the sensor and a digital image is produced.

Image Intensifier

Fiberoptic Bundle

Sensor Panel

The LDAs (Linear Diode Arrays) use some type of scintillator to transform the incoming radiation into visible light and photodiode arrays to measure the amount of light generated by the scintillator. The scintillator is mounted on the photodiode surface. Diode arrays can be arranged in various different shapes (e.g. L-shape, U-shape, Arc). When the x-ray energies are low (bellow 30 – 35 keV), a plain photodiode can be used to detect radiation. In high-resolution LDAs, photodiodes are connected to a CCD or CMOS sensor for additional signal amplification.

Different applications may use x-ray sources operating in different ways. Typically the X-ray generator provides a continuous flux of a broad spectrum of x-rays whereas a Pulsed X-Ray Generator provides a few microsecond broadband pulses several tens or hundreds of times a second. Isotopic sources are based on natural radioactive decay and produce monochromatic x-rays. In an application where different materials, typically organic and metal, have to be distinguished, dual-energy detection is used. Instead of using two separate x-ray sources and detectors, the dual-energy information is gathered by one detector comprised of two diode arrays.

Type of Cameras