Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
development of high-resolution cameras with active areas containing more than 1,000,000
pixels. These types of cameras can be employed to cover large fields of view, whilst still
achieving high accuracy.
The CCD active area is normally sensitive to wavelengths of light from around 400 nm to
1000 nm (visible to infrared wavelengths). In general, the camera can discern between 256
graduations (8-bits per pixel) of contrast information called grey scale, in addition to
planar image information. More recent developments have seen the introduction of 10 and
12-bit greyscale cameras, although for dimensional measurement 8-bit cameras are
normally more than adequate. Most modern CCD cameras are capable of outputting the
pixel clock, providing synchronisation between it and the frame grabber. They usually
also provide other features such as Automatic Gain Control (AGC)*, which adjusts the
sensitivity of the CCD sensors to match the image brightness and an electronic shutter,
which allows selection of the exposure time of the CCD array.
*Note: AGC should never be used when performing dimensional measurement as it may cause
variations in the image intensity levels.
5.1 LINEARITY
When choosing a camera it is worth investigating the linearity of its sensitivity response.
This is particularly important if the application involves imaging over a wide brightness
range. In such cases it is advisable to check the linearity of the camera’s response with
neutral density filters or a calibrated step wedge.
5.2 DATA TRANSMISSION
Analogue CCD cameras use the American RS-170, or the European CCIR video standards.
The cameras output a voltage signal that corresponds to the brightness level of individual
pixels. Both standards use 0-700 mV for the output signal, 0 mV being black and 700 mV
white. The full frame transmission rates are slightly different, with RS-170 at 30 frames per
second and CCIR at 25 frames per second.
Digital cameras, by their discrete nature, require their own separate video standards, RS-
422 and RS-644, which is the LVDS (Low Voltage Differential Signalling) format of RS-422.
They offer data transmission rates of up to 400 MB per second. The main drawback to
these standards is the requirement for bulky and inflexible cables for the data
transmission. Added to this is a lack of inter-changeability between cameras and frame
grabbers, which often results in very expensive, bespoke cabling solutions.
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