Technical Application Note 0003

Setting up the camera

Revision: 1.0
Date: 2018-05-25
Contact: support@carrida-technologies.com
Copyright: 2017-2018 Carrida Technologies GmbH, Ettlingen, Germany
Author: VC Support

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Table of Contents

1   Introduction

This document describes how you can set up the program for the first time and optimize the image capturing process in order to achieve good reading results with the Carrida 4 ALPR engine. Here you will find tips on how to set up the most important parameters, as well as how to start the ALPR detection and where to find the live stream result.

Achieving a reasonable image quality is sometimes not trivial, and a careful consideration of an installation must include

  • illumination conditions at the location
  • required sensor resolution
  • gain and shutter settings

This document explains those factors and provides information on how to achieve the best possible reading results.

1.1   The camera setup

Taking control of the camera imaging sensor is most important when it comes to obtaining a good image quality day and night and under all possible illumination conditions.

A camera in an outdoor environment or close to that (e.g. the exit of a parking garage) is subject to harsh illumination from the sun which might create reflections, or from headlights of cars which may beam directly into the camera mounted at the gate.

Under all conditions, images should be sharp, with minimal motion blur, and of proper exposure. The license plates should not at all be overexposed - if you have to make a choice, its better to have slightly darker images than too bright ones.

Gain and shutter can be changed on the Carrida Camera homepage under 'Camera configuration-Camera'.

./images/imageTan5.png

It is our recommendation to keep the Flash option enabled.

Minimum shutter refers to minimal exposure time in microseconds. It should be changed according to the vehicle speed. If the vehicle is approaching in high speed, the exposure time should be set to a lower level. If set to high it may cause lower reading performance because of the blurry images. For access control the lowest shutter value can safely be set to 1, which is the shortest possible exposure time.

As a general recommendation, the following exposure (shutter) times are advised:

  • a shutter time of 0.1-3 ms on highways, for high speed vehicles
  • a shutter time of 0.1-3 ms in urban areas, for medium speed vehicles
  • a shutter time of 1-20 ms in parking situations, for slow vehicles

Maximum shutter, when set to high, can produce blurred images. It is recommended to set this parameter higher during lower light exposure. It will raise the visibility of the plates, as more light will come trough the lens.

1.1.1   How to estimate the maximum allowed exposure (shutter) time

The maximum possible exposure time can be calculated from the viewing geometry and image resolution. The basic principle is that a vehicle should not move more than 1 px in the image during the exposure time at the maximum possible speed.

It is possible to either compute this value exactly using measurements of the visible street section, viewing angles, etc. or it can be approximated. By measuring the time T it takes a vehicle from top to bottom, or left to right of an image - whatever time is shorter - and the image resolution in this direction (i.e. number of pixels), the maximum allowed exposure time in ms can be computed as follows:

exposureTime(ms) = 1000*(T/resolution)

Hint

Hint Sign An average image brightness between 120-180 gray values is a good starting point for the overall image brightness.

The last important factor that affects image quality is the gain setting on your camera. The gain affects the sensitivity of your camera and it may be dynamic or not, depending on the camera's brand and model.

The higher the gain, the grainier the images become, and ALPR will become harder. The limits of gain depend on the device, reasonable upper limits are typically 6-10 dB.

1.1.2   Limiting exposure time and camera analog gain

A typical application for ALPR will limit the allowed exposure time and gain setting on the camera so as not to create blurred or noisy images. This can be achieved by enforcing a shutter and gain limit in the camera configuration either programmatically from the camera driver, or in the camera configuration dialog.

The VC |car| Pro camera offers a setting in the configurator program to limit shutter and gain in auto exposure mode. Camera configurator allows you to set the shutter time and gain completely manually.

Gain equals sensitivity, it is expressed in decibel (dB). It allows you to boost the signal by some amount. It can make the picture brighter.

If gain is set too high, it will distort the image, as it boosts the noise as well with the signal itself. The higher the gain, the grainier the images becomes, and ALPR will become harder. The limits of gain depends on your device, reasonable upper limits are typically 6-10 db.

We recommend a maximum gain value of 10 for the Carrida Camera.

1.2   Using external illumination

Under Camera Configuration settings you will find parameters for the ALPR Auto-luminance and its region of interest (ROI).

We suggest to always enable the Use Anpr-Autolum parameter.

The external illumination reduces the effects of glare from the sun or headlights from cars very efficiently.

The ROI is a field enclosed by the green lines. Limiting the ROI the ALPR is allowed to focus on a more specific area, so the effect will be greater if it is directed to a minimal surface where the LP is expected. You can do so by clicking on any of the yellow corners and dragging them to a specific area on the image.

./images/imageTan6.png

It is always better for the plates to be darker then too lite.

After defining the ROI, following parameters should be adjusted:
  • Anpr-Atolum brightness low
  • Anpr-Atolum brightness high
  • Anpr-Atolum maximum saturated pixels
  • Anpr-Atolum minimum correct pixels and
  • Anpr-Atolum adjustment factor.

Depending on the outside conditions set the brightness low and high parameters. All the px with lower and higher brightness values than set here will be considered on one hand not illuminated enough or on the other hand over- illuminated.

The amount of maximum saturated px and minimum correct px is set in percentage (0-100%).

If 'Anpr-autolum brightness high' is set to 150, that means that all px with higher brightness will be considered over illuminated. When maximum saturated px is set to 20% but the overall percentage of px over 150 is 50%, the ALPR engine will reduce the brightness through the shutter and gain parameters.