Optics: measuring and testing – Of light reflection – With diffusion
Patent
1988-04-26
1991-01-29
McGraw, Vincent P.
Optics: measuring and testing
Of light reflection
With diffusion
G01N 2147
Patent
active
049882066
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and apparatus for monitoring the diffuse reflectivity of a surface which, in the field of banknote handling, can provide a measure of the degree of soiling of banknotes.
There is a well known requirement for the automated sorting of used banknotes based on the amount of dirt or contamination, otherwise known as soiling, which has built up on the notes. Notes deemed to be sufficiently clean are re-issued and the remainder are withdrawn from circulation. Methods which are used to reliably transport used banknotes are not normally compatible with restraining the plane of the banknote within better than about .+-.2 mm from its nominal position while still allowing a clear view of its surface. The note itself may have fold lines and crinkles which locally remove its surface from the nominal position.
It is important that any means of measurement should be able to make comparative measurements which are significantly more precise and repeatable than may be judged by the un-trained human eye, otherwise notes that are too soiled will be re-issued and notes that are too clean will be withdrawn.
Previous attempts at measuring the soil state of banknotes have either accepted the large inaccuracy in measurement resulting from the uncertainty in the position of the plane of the note, or have used a long optical path between the note and the sensor, such that variations in the length of this path by a few mm do not produce a significant variation in the measured reflectivity. These long optical path length solutions imply large dimensions for the system, or a complex optical system to fold the optical path back on itself many times within small dimensions Long optical path systems also suffer from an intrinsic lack of efficiency in collecting the reflected light, thereby requiring the use of high intensity illumination with the attendant problems of space and heat dissipation requirements.
There are several other constraints on a system intended to measure the soil level of banknotes.
1. The system must have a resolution sufficiently high to enable measurements to be made on unprinted or very lightly printed areas of the note. This is because the human observer naturally makes a judgement on the basis of these unprinted areas, and also because an attempt to make measurements on areas containing significant printing would be significantly influenced by normal variations in the density of the inks.
2. The system should not be sensitive to light specularly reflected from the surface of the banknote. This is because specular reflection is highly angular dependent and would be greatly affected by any tilt in the plane of the banknote, and also because any shiny adhesive tape which may have been used to repair the note would have the effect of making the note appear cleaner than it really is.
An example of a known monitoring system is described in GB-A-No. 2117897 in which a pair of sensors detect diffuse radiation reflected from a surface. This system is designed to look for discontinuities in the surface such as creases and requires a complex detection system to operate successfully. Another example of a known monitoring system is described in US-A-No. 4,092,068 which is concerned with obtaining an indication of topographical surface characteristics such as roughness. In neither example is account taken of the practical problem of movement of the sheet towards or away from the sensing assembly. This is a real problem particularly in the case of high speed sheet feeding apparatus.
In accordance with one aspect of the present invention, a method of monitoring the diffuse reflectivity of a portion of a surface comprises exposing the surface portion to a beam of radiation; sensing the intensity of a part of the diffuse radiation, reflected by the surface portion, at a sensing position situated such that the intensity of the diffusely reflected radiation sensed at the sensing position is substantially invariant within a working range of distances between the sensing position and the surface porti
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Melleney Derek A.
Potter Michael
Reeves David C.
De La Rue Systems Limited
McGraw Vincent P.
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