A new module on the characterisation of linearity errors has been added to the EMVA 1288 Standard, which describes how a manufacturer must detail the specifications of vision equipment.
The EMVA 1288 Standard was established last year to provide a standard way of measuring different parameters, such as the sensitivity of a camera. In the past, these values could vary wildly depending on how they are measured, meaning they meant very little when compared to another manufacturer that used a different method.
The standard, however, ensures that all datasheets containing the EMVA 1288 logo are based on the same principles so that it is easy to compare datasheets. The measurement of parameters such as sensitivity, noise, and the signal-to-noise ratio - basic values that are key to describing a camera - have already been specified in past modules.
Now linearity has been added to the list, which measures how well an imaging sensor represents the brightness of a scene. A perfect sensor would provide a response that is directly proportional to the amount of light that reaches it – so if the source is twice as bright, the sensor’s electrical response should be twice as large too.
However, most sensors do not have a perfectly linear response, and over- or under-compensate for certain intensities and manufacturers normally provide details of these errors on product specification sheets. The new module has specified a mathematical way of defining this property, and a very precise way of performing the evaluation that includes the use LED illumination to accruately vary the amount of light that reaches the camera.
‘A lot of know how from the participating companies was used in setting this up,’ Martin Wäny, chair of the EMVA 1288 Standard Working Group, told imveurope.com. ‘Linearity is an important parameter for image sensors, particularly for those used for brightness measurement, and for 3CCD and 3CMOS cameras.’
These sensors split the incoming light into three different sensors, which respond to different colours, and the information is then recombined to give the overall image. If one sensor responded differently to brightness than the others, it could create a bias in one of the colours.
This new module also represents a shift in the way the EMVA 1288 Standard operates. Unlike previous modules on the sensitivity, noise, and signal-to-noise ratio of a camera, which were compulsory for manufacturers, the new module is optional, and companies may chose not to describe linearity if it is not relevant to their product.
‘It’s their freedom not to specify anything about it,’ said Wäny, who will be giving a talk about the standard at the Vision Show in Stuttgart this November. ‘But if they do specify linearity, they have to do it by this standard to use the EMVA 1288-compliant logo.’
