Last month our feature article discussed the differences, pros and cons of Interlaced Scan and Progressive Scan sensors. If you missed it, or would like a refresher, click here.
This month we are discussing the Interline Transfer architecture
for CCD’s and how it compares to that of Frame Transfer and
Full Frame sensors. Almost all CCD industrial cameras available
these days use Interline Transfer sensors. Some cameras however
use Frame Transfer sensors because of their specific advantages
in some applications. There are few if any Full Frame sensors used
in modern industrial cameras because they lack significant advantages
over Frame Transfer.
The key difference between Interline transfer, Frame Transfer
and Full Frame sensors is where the pixel charge is stored after
it is acquired and during the next integration period while the
current image is being read out of the sensor. To explain it is
first best to describe the sensor image capture process in more
- The first step of the image capture process on CCD sensor begins
with a reset of the sensor, where any residual charge left from
the previous image is drained from every pixel.
- The second step is where light is captured from the image scene.
Photons that are reflected from objects in the scene are focussed
by the lens onto the sensor. The photons hit the sensor and are
converted to electrons at each pixel. Electrons accumulate in
each pixel (much like water molecules in a bucket) over the period
in which the sensor is acquiring light. This period is called
the integration period or shutter period.
- At the end of the integration period the electrons that have
accumulated are shifted out of the pixel, before the pixels are
reset and the process starts again for the next image.
The difference between Interline transfer, Frame Transfer and
Full Frame sensors is where the electrons (pixel charge) are stored
(if stored) before being read out of the sensor.
Interline Transfer CCD
In Interline transfer CCD sensors each and every pixel has a charge
storage area next to it. The charge storage area of each pixel is
masked so that photons cannot hit it. The pixel storage areas form
a vertical column to the bottom of the sensor. This vertical column
is used to transport the pixel charges down and out the bottom of
the sensor. The light sensitive pixel area (active area) is electrically
separated from the storage pixel area and so the active pixel area
can act independently of the storage area. At the end of the integration
period the charge in every active pixel is shifted to its adjacent
pixel storage area. At this point the active pixel area is reset
and the next integration period starts while the charges in the
storage pixels are clocked down to the bottom of the sensor. This
is analogous to a bucket brigade where you have a row of buckets
full of water (electrons) where the last bucket empties its contents
before it is filled by the water from the bucket before it, and
when the bucket before it is empty it in turn is filled by the water
in the bucket before that and so on. This happens with all pixels
on a line simultaneously so that lines of pixel charges are clocked
down together. This in fact is where a CCD gets its name from -
Charge Coupled Device - charges are coupled pixel by pixel and transported
out of the sensor.
When a pixel charge reaches the bottom of the sensor it is clocked down once more to a shift register. The pixel charges in the shift register at any one point in time represent the pixels acquired on a row of the sensor during the earlier integration period. Once the pixel charges are in the shift register they are clocked out at a rapid rate before the next row of pixel charges is clocked into the shift register. From the shift register the pixel charges are fed to amplifiers which convert charge to voltage. The voltage is then conditioned further, digitised in the case of digital cameras and then fed out the back of the camera.
The key advantage of Interline Transfer CCDs is that the charge from an integration period can very quickly be shifted to the storage pixel area. This allows the active pixels to be reset and to acquire the next image in the next integration period while the pixel charges from the previous image are being clocked down the storage pixel columns and out the sensor via the shift register. So both image acquisition and readout is happening in parallel which facilitates faster frame rates.
Interline Transfer CCD sensors avoid (minimise) image smear by the fact that the pixel storage area is masked so light cannot hit it. If they were not covered then photons hitting the storage area while a charge is being clocked down through that storage area, would have electrons added to it. This has an effect in an image that looks like and is called smear. If the image scene has a bright point, this bright point would be smeared vertically down the image.
Another advantage of Interline Transfer CCD sensors is that the
sensor area is relatively small and so relatively inexpensive to
manufacture. There is however a cost to covering up half the pixel
area for storage. It reduces the area of each pixel available to
collect light (this is called Fill Factor) and so is less sensitive.
Interline Transfer is therefore not the best sensor technology to
use for applications that are starved for light however are very
good in many or most other applications. Some Interline Transfer
sensors are fitted with micro-lenses covering every pixel which
direct light away from the masked storage area of each pixel onto
the active area. These sensors are called HAD or Hyper HAD and have
significantly higher effective Fill factor as compared to Interline
sensors without micro-lenses. The Jai CVM4+CL is an example of a
camera with hyper HAD sensor and is often used for light starved
situations such as night time intelligent traffic applications.
There two different types of Interline Transfer sensors:
Interline Transfer - Interlaced
Interline Transfer - Progressive Scan