Reverse engineering of biometric technology.
Biometrics use several algorithms to identify a target, for the purposes of this study we have chosen the ANPR licence plate recognition system. What we hope to do in this study is examine the processes that are key to its operation in detecting and defining licence plates. The reason for this study is to allow people to eductae them selves on what may obscure plate recognition so they can avoid using such products. It is dangerous to obscure your plate, you may not be able to receive fines or be recognised by tolling booths, this can be dangerous.
The use of luminescence which is facilitated by the paint used on licence plates. The back ground paint is luminescent and the lettering is not. This gives a definitive contrast between the two surfaces to identify characters. If the luminescent paint were replaced with a matt non reflective paint the reader would lose distance capability.
Increasing over all luminescence so that the back ground and the foreground read similarly in grey scale may confuse the reader but would increase the efficiency of radar and laser detection equipment.
Matt Black back ground and blue lettering are the most difficult colour schemes to differentiate in terms of contrast.
The plate shape is next most important being Rectangular, if the plate is not rectangular it may not identify the plate. If the border of the plate were oval then the reader would find it difficult to locate the plate. The height it is from the ground must be at normal standard height, any lower or higher will interfere with reading the plate, it must also be centralized, it is stated when you drive through a toll reading gantry in the middle of 2 lanes the ANPR reader may not see the plate, this may be true however its not fool proof as cameras will detect plates that ANPR systems don’t. If the plate is to the side and lower then it is definitely harder to read.
If the surface behind the plate were to have a series of triangle shapes that are luminescent in contrast to the colour of the vehicle this may cause disruption to the reader. Diagonal lines behind the plate may confuse the reader into thinking its not detecting a rectangle especially if the diagonal lines are predominantly more luminescent than the plate. This can be achieved by contrasting same colour schemes from back to foreground in terms of shape luminescence to background.
Plate flatness is also important. If the plate is rounded it will throw reflections at a lower rate back to the reader especially if the luminescent paint has been removed and replaced with a similar matt colour.
Colour Scheme between the back ground and lettering. Having a plate back ground the same colour as the car can confuse the reader if the border is not present. Highly polished Clear Perspex in an oval shape at a reflective angle downward or upward differing from the plate can send the reading awayfrom the point of source. Most disruptive is if the plate cover is bubbled. The lens distortion of a curved plate cover is disruptive to the reader.
when a plate reader finds a plate it will put it into grey scale to increase the contrast between the back ground and lettering. Colour Schemes that offer lesser luminescent difference between them may cause the grey scale reader to malfunction especially if one of the characters is un readable because of mud or dirt obstructing part of its definitive character recognition.
Blue Back ground with black lettering is over all most difficult to read in grey scale. You can download colour scheme luminescence checking programs. Luminescence lower than 1.9 difference is very poor for plate reading. Successful reading must be over 4.5.
The distance between characters must be no less than 8 mm. If they are closer the reader may not be able to differentiate. If there is an obstruction in the centre of the plate ( as opposed to the border ) this may make it difficult for the reader to successfully read the plate.
Light diffusing membranes can cause problems if the diffusion is not consistant. Curved diffusion is most disruptive to reading. If you pull apart an old computer screen you will notice 3 or 4 sheets of diffuser, if placed inside a plate frame against the glass or perspex it will diffuse light reading units such as radar, laser and ANPR readers. It does this by diffusing the returning reader beam, this inhibits its accuracy causing letters to seem larger or distorted , perhaps even over lap. Diffracting reflection also inhibits distance range.
There are certain products available to cause your entire plate to over contrast when light is shined upon it , the common misconception is this will confuse the reader, Safety Cameras have a flash will blow out the plate making it hard to read however Safety Cameras have back up video camera running all the time, they can also take the frames before the flash hits at its peak and the plate is perfectly visible. So these do not work, in fact they make the reflection of the plate better for Radar.
Angled Deflection Matting
Using this over a plate can succesfully obscure it from ANPR readers and Safety Cameras, because Safety Cameras and most Gantry Toll readers need the plate visible from a 45 degree angle above, a deflection matting will obscure the plate completely making it illegible. This is commonly found used as privacy screens for computers. A sheet of this inside a plate frame holder under the perspex will make the plate look black from an above or side angle but look completely normal from behind.
We hope you found this study helful in being a law obiding citizen.