Thanks Guys. We also spoke to Axis Tech support and they are saying pretty much the same thing that you guys are. Exposure settings, manual focus, and field of view as the main settings. Here is their response and their recommended settings for LPR. We adjusted today on-site and we will check the video on Monday. I'll let everyone know the results.
Q1765-LE LPC Case Study
Filip Johansson
Axis Communications
June 3, 2015
1 Introduction
The purpose of this document is to briefly outline some key settings and considerations to be made when setting up Q1765-LE for License Plate Capture.
There are not many settings that need to be considered, however due to the
complexity of the camera settings page there is a lot of confusion on how to do
this properly. Even though Axis cameras are thoroughly tested to work well in
most situations out of the box, LPC is a very specialized application and there
are some physical limitations that need to be addressed through camera settings
to make it work properly.
The scope of this document is not how to setup a complete solution with
an Automated LPR system, camera considerations etc. That setup is assumed
to have been done properly. The key components to successfully deploying an
LPC solution are eld of view (FOV), illumination, focus, exposure time, gain
and frame rate. Each will be treated in its own section.
2 Field Of View
It is critical to get the correct number of pixels on the license plate.Recommendations on the pixel density vary between vendors and solution providers. In
general, follow the recommendations from the solution provider but increase the
density for night time LPC situations when NIR illumination is needed. As an
example Milestone recommends 125 pixels per foot (A license plate is one foot
wide) but the author would recommend at least 150 pixels per foot when NIR
is involved. The NIR easily washes out over the characters when the resolution
is lower.The vertical angle is also important. Try getting the camera mounted at
least 14 above the point where the license plates are to be read. This is to
avoid the glare from the car headlights. Also, try to mount the camera as
straight on as possible.
3 Illumination
Daytime LPC usually requires no extra lighting. With current technologies NIR
or white light LED:s are usually required to get enough light from the license
plates in night time applications. The Q1765-LE provides a very good solution
for moderate distances. Do not use WDR technologies unless general overview
is equally as or more important than the LPC application. Figure 1 shows an
example where a Q1765-LE was used to capture license plates from a freeway
overpass with cars passing at speeds of around 55 mph.
Figure 1: LPC using a Q1765-LE. Shot taken from a height of around 15 feet and
distance estimated to 60 feet. Cars passing at approximately 55 mph. Pixel density
of 150 pixels per foot.
4 Focus
Fixed focus is always recommended for LPC. The best way to focus the camera
is to have somebody else parking/stopping a car where the plates are to be
detected and manually setting the focus on the plate, alternatively auto focus
can be used temporarily and turned o when the correct focus is achieved. A
special case occurs with non-NIR compensated lenses like the Q1765-LE. Two
presets for day and night respectively should be setup. Using the event system
the camera can be configured to toggle between the presets as appropriate.
Make sure the focus is good before changing other settings.
5 Exposure Time
The most critical factor for a successful night time LPC application. If the exposure time is too long, the plate will be obscured by motion blur. The settings
usually included in the camera settings page are not enough in most cases. There
is a setting in plain config called root.ImageSource.I0.Sensor.MaxExposureTime
that needs to be configured. This limits the maximum exposure time that the
camera can use, thereby providing a hard limit for the motion blur. The units
are μs. So to for example configure a maximum exposure time of 1/250 s (which
is a good starting point) one would use the following calculation:
MaxExposureTime = 1000000/250 = 4000
To find this value in day time the author recommends using fixed exposure times,
this is because in day time the exposure algorithm might choose a shorter value
which can fool the user in to thinking a correct value has been found. Once
a good configuration has been found it is advisable to decrease the value by
a factor of 2 to give some margin for speeders. Go to "System Options" -
"Advanced" - Plain Config" - "ImageSource" to find these settings.
6 Gain
This does normally not need to be tweaked for daytime LPC but for night time
LPC with NIR or white light illumination it is often needed to avoid saturation
on the plates. Once again use plain cong and tweak the Sensor.MaxGain
parameter until a good exposure is achieved. It is easiest to tweak this after
focus and exposure time has been configured using a still standing car. The
units are 0-100 in an internal scale, 0 representing no gain and 100 representing
the maximum gain in the camera system.
7 Frame Rate
Use a sufficient frame rate so that there is no chance that cars passing by will
not appear in at least 2-3 video frames. Please note that the exposure time
settings discussed in section 5 will take care of motion blur, there is no need to
use extremely high frame rates for LPC.
