Optimal Mounting Position For LPC Cameras

I've been reading up on the optimal mounting position for LPC cameras. Geovision prefers mounting the camera at about 3m (9 feet) high with a 30º angle looking down at the front plate of the car. Other manufacturers advise one should avoid the headlights and instead mount the camera low to the road and capture the rear plate. Presumably the differing recommendations depend on the techniques the cameras use for reliably capturing plates.

I'd appreciate hearing how people have mounted different brands and models of LPC or LPR cameras and specifically whether they were mounted high or low and which plate was being captured. Thank you for your help.


Almost certainly back license plates are easier since the lights on the back are less intense than the front. Of course, this is impacted by where plates are in one's region. Some regions only have front plates so you are forced to do that.

In our testing, avoiding aiming directly at the headlights can help (as it reduces the amount of direct light that needs to be overcome to capture the plate), though if you have strong enough IR and a fast enough shutter you should be able to get through that.

Thanks John, that's very helpful. Front and rear plates are required in Australia so I can set up plate capture for either end.

If capturing rear plates, can you provide any guidance as to the optimal height above road level for the camera and also any beneficial distances or angles or any to avoid?

I'm still puzzled as to why Geovision recommend capturing the front plates of vehicles, in their LPR Camera Installation Guide, given the general consensus appears to be that capturing rear plates is easier.

I'm still puzzled as to why Geovision recommend capturing the front plates of vehicles...

Can't say for sure what Geo is thinking, but reading what other fans of front reading say, the theory goes like this:

Shooting the front plate is better because the bright headlights put the camera in low gain mode that won't overexpose the plate because of too much reflected IR.

Whether it is true to some or any extent, dunno...

Thanks for finding this information Undisclosed 1, even if we're not sure if it is true!

Are your license plates background mainly white?

Have you ever got images back where the plate looks like a solid white rectangle with no numbers?

Hi Undisclosed 1, most plates here have a white, reflective background. I have not used LPC cameras before so haven't encountered the problem you mentioned but would like to know why you mentioned it. Would that occur if IR lighting was too strong?

To the right or left, or above the Lane. Within 20-30 degrees of the center line of the car at the point you want to capture the plate is typical. The more pavement the camera sees in advance of a car coming through the better. This quiescent state is needed to minimize processing, and make a dramatic difference in the scene as the car passes through the focal point of the camera. Using an IR pass through filter, ample IR illumination, coupled with a LENS/FOCAL length that has a very short depth of field (IRIS FULLY OPEN) is the key to getting good results.

...make a dramatic difference in the scene...

Why that is important?

Thanks very much Andrew. That's great information!

"Dramatic Difference" The LPR process will work best when it is idle as it looks at the pavement. Then when a car passes through it processes that scene looking for the shape of a plate and the edges of fonts.... Then back to looking at pavement. The fewer good frames the better.. (short focal length)

Do you hard set all exposure parameters, like AGC?

Yes....

What do you fix gain control to? Off? If you fix it to a certain level, besides off, it risks ruining the image during the day?

So do you fix it off or do you cap or?

So one fixed setting for night, and another fixed setting for day?

With pass through IR filters, its all the same.

What's all the same gain? Can you please answer directly what specifically you set gain to?

Using IR pass through filters, and ample IR illumination, you're not likely to need much gain. However, every camera is going to behave differently, and you will not need day/night settings.

Like so?

Not much gain or no gain all? I can understand turning it off with ample IR illumination but leaving it fixed and on more than very low levels will generally cause day time images to be washed out.

I can't share entire plate captures, but did we discuss the camera being in B/W Mode?

Since I am only experienced with Avigilon LPR, you mileage may vary... however,

I did a screen grab from a client of the first three digits from a plate that is really washed out, but all of the digits are reading correctly. Two images of same plate at different times are shown. The day images are almost identical with a deep IR pass through filter. Regarding the sample above, and reviewing some installations the horz. width and height of the plate is between 1/7th and 1/8th of the screen width / and height. I think these look good, and would expect good results.


fine plate2