.::APHESA SPRL - imaging, electronics and software consulting::.

Why using variable exposure time in EMVA1288 measurements

The EMVA1288 standard offers two methods to sweep the total exposure of the camera or sensor under test. The first methods maintains the irradiance constant and sweeps the exposure time, the second method maintains the exposure time constant and sweeps the irradiance. As what we are interested in is only the total exposure, i.e. the product of irradiance and the exposure time, do the two methods really give the same results?

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When using fixed exposure time, the camera requires less controls and the test is therefore easier. However, it won't give the same performance as variable exposure time and will not allow to perform all tests. It may also be slower. Here is why.

Maintaining the exposure time fixed will require the light source to reach multiple levels. The measurement is therefore affected by the stability, linearity and repeatability of the light source process. Changing exposure time is a digital control that is more reproducible, perfectly predictable and perfectly linear. However, we have seen some strange behavior of exposure time registers. Setting the light source to a specific level is also slower than setting the exposure time because of the analog and feedback loop design inherent to the light source.

The biggest issue with not having a variable exposure time is the impossibility to measure dark current. Dark current is measured as the increase in offset or the increase in variance when exposure time increases, therefore the lack of exposure time control does not allow for a measurement of dark current.

When we perform EMVA1288 measurements, we also include additional tests that are not part of the EMVA1288 standard. For some of these additional tests, for example some specific types of defect pixels, we use more than one exposure time.

 
 

About us

Aphesa develops custom cameras and custom electronics including FPGA code and embedded software. We also provide EMVA1288 test equipment and test services as well as consulting and training in machine vision and imaging technologies. Aphesa works in several markets including industrial, medical, oil&gas and security.