Why Use A CCD In An LPR Camera?

I noticed with intrigue that amongst Dahua's incredible number of new products, there are a couple of LPR cameras with 1" and 1/1.8" image sensors. Both of these cameras use CCD image sensors. As far as I know, these are the only cameras in Dahua's offerings that use CCD's rather than CMOS. Why would they prefer CCD's for LPR cameras? I tried using the camera finder to search on LPR cameras but couldn't see an obvious way to do that. However the ones that I did find used CMOS sensors so hence my intrigue.

IMHO, they are using CCD's because of their global shutter. CCD's always have a global shutter and CMOS sensors usually have a rolling shutter, though global shutter options for CMOS have become more widespread in the last couple years.

The difference is that a global shutter works much like an ideal shutter would; when the shutter is open light falls on the photo-cells and starts to create a charge on each pixel to the degree each one is illuminated.

Then when the shutter 'closes', no more light is collected and instead the sensors accumulated charges are transfered off the sensor chip and processed. This charges represent one frame's worth of data for the duration of the shutter speed.

Note that in this scenario the frame truly is a snapshot representing an single moment in time.

Rolling shutter works differently; mainly to accommodate the way CMOS stores and transfers charges, the shutter reads and transfers out one single horizontal line of pixels at at time, and by the time it has gotten to the bottom line some number of micro-seconds have elapsed.

This means that technically the pixels at the top of the frame are ever so slightly earlier than the ones at the bottom. How much earlier varies, but as a limit no more than 1/fps, so 1/30 second, and in practice much less.

Which is not a lot, really. And normally you would never notice unless looking at things like helicopter rotors and fans. Then it can be quite amusing actually:

One thing to note is that there is of course no physical shutter in either case, just an electronic one that can allow charges to accumulate or not.

The claim that a license plate capture can be significantly impacted by this artifacting though is dubious. But it is technically true that any motion will cause a rolling shutter to display some artifact, so they overstate it a bit as a technical selling point.

Besides this, most other advantages that CCD'S have historically enjoyed over CMOS have been eliminated.

Hi A, thank you for your thoughts about the use of CCD's possibly being due to global shutter support. I don't think Dahua chose CCDs for their global shutter as these are new cameras which could easily take advantage of the availability of CMOS sensors with global shutter. I believe CCDs to be more expensive than equivalent CMOS sensors so that's another reason why I am intrigued they have been used in this case.

I believe CCDs to be more expensive than equivalent CMOS sensors...

I don't believe this is the case for CMOS sensors with global shutter, not yet at lease.

With CMOS, global shutter is not easy to do. In the approaches I have seen you need to put a little charge memory next to every pixel on the sensor.

With CCD's, its the natural way they work.

I could be wrong though since there is always something newer and cheaper. What surveillance cameras with a CMOS sensor and global shutter have you seen?

I'm familar with this one, but its a bit pricey.

One other thing I just saw, it looks like some of Dahua's LPR cameras are designed to work with short burst strobe lights.

Depending how short the burst, this is actually a valid reason for requiring global shutter (CMOS or CCD), since the burst may be shorter than the time for the shutter to roll thru the rows.

Whe you see these LPR cams at the intersections in China, the lights are strobing in view of the driver. It's quite distribing. I don't believe we've going to see them over here for reasons of safety.
Flash visible light speed cameras here are typically rear facing.
IR ones are front facing.

It's quite disturbing. I don't believe we've going to see them over here for reasons of safety.

I would have agreed a year ago, but for some unknown reason in Southern California, the Toll Roads on-ramp have been strobe flashing on every car. Used to be only for the unfortunate ones without transponders but not anymore.

The red-light cameras, of course do it also, but at least only when they detect a violation.

Still, it seems a little dangerous; Anyone running a red light has enough problems to deal with, why blind them with a strobe too? ;)

If you're trying to make a dent in a niche market like LP, any selling point helps, no matter how theoretical the benefits. If you DID deploy a Dahua LP cam and the rolling shutter DID cause an issue, you'd probably never use one again. In light of that thought, it probably makes sense that they'd prefer CCD.

Related: John just linked, in an unrelated discussion, to a video about strobe lights and integrated IR. Although the video has nothing to do directly with CMOS vs CCD, it shows what happens with strobe lights and rolling shutter.

Check out how the strobe only lights up part of the frame.