STV-6410-R01 datasheet(93/105 Pages) STMICROELECTRONICS

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CMOS Sensor; Customer Datasheet, Rev 3.0, 28 September 2000

VV5410 & VV6410

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12.2.1 Noise Parameters and Dark Current

Various noise parameters are measured on the 410 device as follows:

•

Fixed Pattern Noise (FPN)

•

Vertical Fixed Pattern Noise (VFPN)

•

Random Noise

•

Fine Shading

•

Gross Shading

The parameters will be described in more detail below along with the data produced by the characterisation programme.

12.2.2 Blooming

Blooming is a phenomenon that does not affect CMOS sensors in the same way as CCD imagers are afflicted. With a CCD

blooming can cause an entire column/columns to flood and saturate.

CMOS imagers are however affected by a different type of saturation. If an intense light source, (e.g. Maglite torch), is shone at

very close proximity to the image sensor the pixel sampling mechanism will break down and rather than displaying a saturated

white light a black image will occur.

The 410 pixel architecture uses Correlated Double Sampling (CDS) to help reduce noise in the system. The pixel is read normally

first, yielding the true integrated signal information, then the pixel is reset and very quickly read for a second time. This normally

yields black information - as the pixel has had no exposure time - that can be subtracted from the signal from the first read. This

subtraction will remove much of the noise from the pixel leaving only the useful signal information.

In an example where a pixel has saturated in both the first and the second reads due to an intense light source. When the noise

cancellation subtraction operation is then performed the result is close to zero signal from the pixel therefore resulting in the

displayed black image.

We do not perform any test measurements for this phenomenon.

12.2.3 Dark Current

This is defined as the rate at which the average pixel voltage increases over time with the device not illuminated. The dark current

will be measured at a gain setting of 4 and a clock divisor of 16 at a fixed temperature and will be expressed in mV.

12.2.4 Fixed Pattern Noise

The FPN of an image sensor is the average pixel non-temporal noise divided by the average pixel voltage. The illumination

source will be white light that has been IR filtered, producing a diffuse uniform illumination at the surface of the sensor package.

The FPN will be calculated at coarse exposure settings of 0,10,150,250 and 302 with gain set to 1. 10 frames are grabbed and

averaged to produce a temporally independent frame before each calculation. FPN will be expressed in mV.

12.2.5 Vertical Fixed Pattern Noise

VFPN describes the spatial noise in an image sensor related to patterns with a vertical orientation. The VFPN is defined as the

standard deviation over all columns of the average pixel voltage for each column determined at zero exposure and zero

illumination. VFPN will be expressed in mV.

12.2.6 Random Noise

Random noise is the temporal noise component within the image. Random noise will be expressed in mV.

12.2.7 Shading

This describes how average pixel values per “block” change across the image sensor array. For fine shading calculations the

image sensor array is split into 30 pixel by 30 pixel blocks. An average value is then calculated for each block and the averages

are then compared across the whole device. The blocks are increased in size to 60 pixels by 60 pixels for the gross shading

calculation. Shading will be expressed in mV.

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