Shutter Speed In LPR Situations

Hello, I´m working on setting up LPR camera along with an IR illumination.

What are your thoughts in ideal exposure in total darkness to catch a license plate number on a car travelling maybe 60-80 km/h?

My thought was about 1/1000 , with a high power IR light and a zoomed camera to a fixed field of maybe 2-3 meters where the cars plate will always pass.


Gudjon,

That is probably in the right range for the shutter speed. Does your camera have options to adjust the shutter speed to different levels? I am assuming so. If so, simply experiment with a few values say from 1/500s through 1/2000s to see what works best for that specific scene / setup.

My thinking is that you'd want to lock your exposure first so that the image is completely or almost-completely dark in all ambient lighting conditions the view will be in - this removes the need to change exposure due to lighting levels, or to have it overexpose in the brightest conditions.

Then you'd add the illumination to create the proper image with that exposure, so you'll get the same picture regardless of ambient light.

Note that, as John indicates, you'll probably need a pretty fast shutter to avoid blurring at upwards of 80km/h, and that means a stronger illuminator.

Also, the faster the shutter, the wider the aperture you need to compensate, and the wider the aperture, the lower the DOF. This is part of the reason red light cameras use a high-powered strobe - they need to have substantial DOF to get sharp pictures through the width of an intersection, which means a tighter aperture, and less light. Of course, if you're darkening the image with a tight aperture, you can use a slower shutter... the flash itself then "freezes" the image. In your instance, you probably don't need that much DOF, either.

The other thing to remember is that the more straight-on your camera is to the plate, the less motion blur you'll get, allowing a slower shutter.

What camera and lens are you using? And how far from the camera will the capture take place?

HIK-HV0733D-6MP lens on a 2MP Hikvision Darkfighter Camera and a Raytech IR illuminator.

Distance from camera to License plate will be around 35-50m at about 15° angle

Something we didn't mention is one major issue could be that the IP camera could remain in color all the time with blurry effects you can imagine if there are any sources of light in the neighborhood even far from the lenses (street lighting, neon signs..)

I have tried to summarize other key points in LPR:

- 5-50 mm focal lenses with headlight filter to avoid any glare issues depending on your camera angle and location.

- 5-50 (or 12-40mm..) Gives you a good capture distance, generally 20 m and more to neutralize part of the car speed up to 90Km/h

- AGC (IrisxShutterxGain) should be disable and put to manual value. As said previsoulsy, if you are using good IR projectors your can setup shutter to 1/800, 1/1000. Gain control should be also set to lower value , between 20 -30 instead of maximum in most factory defaults

- Switch to B/W and IR should be automatically monitored to avoid that your camera wait 1 Lux before switching to B/W this will occur blurry effects in the morning and evening. So you have to force your camera to switch tp BW much sooner than the sensor usually do.If you put 1/500 or 1/1000 but your camera doesn't switch or too late in BW mode, your are done.

... The system I know uses IR Projectors with sensors to switch in B/W at 300 lux. That way , your camera will never run out of light in color mode (the camera switchs back in color at 600 lux)

- Two IR long range projectors 10/15° each able to cover 50 m or more" depending on your lenses and the number of lanes you want to take (IR equipped with sensors to remote control the camera day/night profile ?)

- Disable Wdr and DNR and any other post capture processing generating blurr and delay

- Avoid High 264/265 profiles which consume more CPu (BAsic /Main should be sufficient) , use a reduced GOP,move to Mjpeg if you still have issues

- fps : try 25/30 fps and test it ,(don't choose 50 or 60 fps by default which consums much more of everything)

- make sure your bit rate cap is reasonable versus Fps/resolution/compression, and will not decrease your image quality (test it as IPVM does) Be careful is your transmission is Wireless, with lower bandwidth than wires.

- In general: factory settings are your main enemy!

Something we didn't mention is one major issue could be that the IP camera could remain in color all the time with blurry effects you can imagine if there are any sources of light in the neighborhood even far from the lenses (street lighting, neon signs..)

That's why you set up the exposure so the scene is completely or nearly-completely dark even in bright daylight.

Switch to B/W and IR should be automatically monitored to avoid that your camera wait 1 Lux before switching to B/W this will occur blurry effects in the morning and evening. So you have to force your camera to switch tp BW much sooner than the sensor usually do.If you put 1/500 or 1/1000 but your camera doesn't switch or too late in BW mode, your are done.

I'd disable it entirely - IR illuminators won't be effective in color mode, so just lock it at B&W, dial down the exposure so ambient light isn't a factor, and blast it with IR so ALL your exposure is produced by the illuminators. That gives you total control over the light level.

Sorry Matt, I do not agree. With1/1000 you keep on receiving much more than 5 lux during a large part of the afternoon even when the sun is gone and your camera will be in color or in B/W without IR , (so IR cut filter On) so completly out to read anything at 20 meters or more. If you force in B/W with IR Cut filter Off during the day, IR & UV from the sun will create disturbances. Moreover, during the day , most customers and police forces prefer color mode to get the color and brand of the car and eventually identify the driver. Better keep the color mode as long as we can.

Bellow 300 lux on a target far from the lenses (>15m) and at 1/1000, it's becoming very tricky to get something sharp even in B/W (I have tried). We are obviously talking about standard FHD cameras bellow 0,8K$ all included (camera, casing, heater, IR, headlight lenses..)

With 1/1000 you keep on receiving much more than 5 lux

Not if you close down the iris as well, which has the added benefit of increasing DOF and sharpness. And who says you have to stop at 1/1000? The higher your shutter, the less problem you'll have with motion blur.

Moreover, during the day , most customers and police forces prefer color mode to get the color and brand of the car and eventually identify the driver

That's not the point of LPR though. Have a second wide-FOV camera to get all that. You're trying to make one camera do too much, when you NEED it to do one specific thing very well.

I've played with LPC at my previous residence watching the end of our street. Simple IQ511 with basic (non-MP) 5-50mm lens. It captured plates very well... when the exposure was right. Even at night, if the car was going slow enough, I could get a plate (and these cameras are color-only and not at all good in low light). Biggest problem I had was the changing exposure. A black car driving through would cause the exposure to bloom and everything around would be washed out. A white car on a sunny day would make the whole scene dim too much. I played with exposure zones, I endlessly tweaked the AE response speed... nothing was 100% effective.

Solution: take ambient light out of the equation entirely.

Solution: take ambient light out of the equation entirely.

Or get a dual shutter Hik (and move to China).

no, no. Double profiles and day/night shutter speeds aren't sufficient to get sharp plates from 00:00 to 23:59 whatever the ambient light is, as Matt said.

It's much more tricky when your speed is 90 Km/h. (56 Mph) and headlight filters are also not an option when you face the car headlight (if you take the car rear it's less an issue)

Double profiles are recent at Hik and some others (Axis..) but exist since a while in Taiwan. (I don't quote brands..) i's good for slow speed : profile with WDR, profile without WDR.and so and so.

I will try Matt's trick to mamually reduce Iris to force BW switch. I less concerned by DoV as we are zooming on a reduced area. Moreover I'm using most of the time P-Iris.

no, no. Double profiles and day/night shutter speeds aren't sufficient to get sharp plates from 00:00 to 23:59 whatever the ambient light is.

Check again, I never said ambient light was enough to get sharp plates. I agree that 24/7 IR illumination is key.

All I said was you don't need to take ambient light "out of the equation entirely" with this Hik camera, as you can get the plate with the one exposure and the car and the car make with another.

OK. I got it now. Double simultaneous shutters, color and B/W with a single sensor and lenses, and 7MP and probably 30 fps or more (it's not described) , embedded vehicule shape AND plate reco (chineese or worldwide), uh ! Nice on the paper if it's not a funny vaporware! The list is more a wish list than a technical reality I presume. (zoom and read the features... ah ah ah)

I remind to dreamers that the current CPU power of the best IP camera is very limited when setup in 3D with complex tracking and low light conditions (whatever analytic editors say) . So recognize a vehicule by its shape and extract its plate in parallel, It should also recognize colors of trafic lights...Not sure but could be a nice hoax? May be in 5 or 10 years

Yes, I am skeptical as well that does what it claims well. But it does appear to be a real camera that is sold in China.

btw, the spec sheet (use the link in the other article) says 25fps. Also, the operating temp range is -40 to 80C, which is very insane.

1 inch ccd global sensor can't be cheap either.

I think there ARE cameras in the US like this, possibly from ATS, but their specs are not published and they likely cost tens of thousands.

Related: The 29MP Red Light Cameras Of ATS

I have a question about the assumption for shutter speed recommendation. Are you assuming that the LPR engine is capturing images (snapshots) or operating on a video stream. Are you aware of vendors that can detect LPR on a video stream at high speed? What would be the recommendation for the camera setting in that case.

Aren't they still working on individual frames regardless?

Or are you saying that they are synthesizing multiple consecutive frames into a super-frame, and then using that for the LPR?

yes I meant the latter for better accuracy.

ALPR software detects plates mostly on key frames and analyse on key frames with short gop. Some system working on Mjpeg. Like for face reco, system takes the best image to do the analysis. Some professionnal ALPR camera do a local OCR with a specific DSP