Apparatus for optical inspection of metallic surfaces

An apparatus and method for inspection of metallic surfaces before subsequent treatment thereof (for example, painting) are disclosed. In accordance with the invention, several light sources illuminate the inspected surface with light beams having different wave lengths, reflected radiation is detected and converted to electrical signals, the electrical signals are processed digitally, and the results of processing are displayed. Some of the light sources are mounted so as to produce light beams inclined with respect to the inspected surface. These light beams are defined by an angular divergence of not more than 5 degrees. Display of results can be presented, in terms of the ISO standard, for inspection and assessment of industrial steel surfaces (i.e., in terms of preparation grades).

FIELD OF THE INVENTION 
The present invention relates to the field of technological quality 
control, and more particularly, to the inspection of steel surfaces before 
or during preparation thereof for painting or other surface treatment 
steps. 
Surface preparation here means cleaning of the surface from rust, mill 
scale, oil, grease and/or other contaminants. This step is an essential 
part of different technological processes, such as anticorrosive 
protection, welding, soldering, etc. 
Preparation of metallic, in particular, steel, surfaces is associated with 
imparting of certain roughness, which may influence the quality of coating 
if the roughness is excessive or insufficient. On the other hand, 
excessive or insufficient cleaning, for example, by shot blasting, is 
associated with superfluous consumption of materials and labour 
expenditure. Therefore, inspection enabling correct assessment of the 
surface condition in terms of its cleanliness and roughness is a 
sufficient precondition which should be met in order to prepare the 
surface in a most efficient and economical way and to ensure the achieving 
of reliable and durable coatings. 
BACKGROUND OF THE INVENTION 
For the inspection and assessment of industrial steel surfaces, the ISO 
Standard 8501, qualitatively specifies the cleanliness of steel surfaces 
in terms of grades designated as Preparation Grades. In the form of 
pictures this standard classifies said Preparation Grades depending on the 
amount of rust and mill scale, which remains on the surface, after the 
cleaning process. 
Furthermore, the ISO Standard 8501 also defines said Preparation Grades 
with respect to the type of cleaning technology to be chosen (e.g., shot 
blasting, power tool cleaning, flame cleaning, etc.) and with respect to 
the initial condition of the surface before cleaning, which is designated 
as Rust Grade. 
Besides Preparation Grades and Rust Grades the above standard also 
specifies requirements of the inspected surface with regard to the absence 
of oil contaminant thereon over a particular threshold limit. 
The main difficulty associated with implementation of the above standard in 
practice lies in the fact that attribution of the inspected surface to 
certain Preparation Grade and Rust Grade, as well as excessive oil 
presence has to determine by visual assessment whether the surface in 
question is identical or not to a particular picture, corresponding to a 
particular grade. It can be readily appreciated, that this method suffers 
from subjectivity and cannot be considered as suitable for reproducible 
industrial quality control. 
Also known are some attempts to make the attribution procedure independent 
of subjectivity, but inevitably associated with visual assessment by the 
naked human eye. 
For example, there is a known method and an optical device for 
determination of amount of rust on the surface of a metallic tube after 
its cleaning, as disclosed in SU 256440. 
In accordance with this method the inspected surface is illuminated by a 
visual light beam so as to receive a reflected light beam. The intensity 
of the reflected beam is measured by means of a photo sensor in terms of 
current, and this value is used for calculation of the amount of rust by 
an empirical formula. The formula has been developed by empirical study of 
the correlation between intensity of reflected light exhibited by steel 
surfaces with the known amount of rust, determined beforehand by chemical 
analysis. 
The disadvantage of this method and device for implementation thereof is 
associated with the fact that the above empirical relationship is valid 
only for those surfaces which were taken as standard without, however, the 
possibility of linking this relationship with Rust Grades, as designated 
in the above mentioned ISO standard, since the amount of rust is not taken 
into consideration. 
Furthermore, the above method does not enable unequivocal selection of 
surfaces coated by red rust (hematite) from those coated by black rust 
(magnetite) or unrusted surfaces. 
It should be emphasized, that the above method and device is suitable only 
for arbitrary attribution of steel surfaces depending on the amount of 
rust present thereon and is incapable of attributing the surface by amount 
of oil contaminants, preparation grade and surface roughness. 
Another known device for determination of cleanliness of a metallic 
surface, is as disclosed in SU 1393057. This device consists of an 
infrared light source, an elliptic light collector, and a photo sensor. 
The device measures the intensity of diffusive radiation obtained after 
illumination of the inspected surface by IR light waves. The assessment of 
the inspected surface by means of this device is based on the assumption 
of proportionality between diffusive radiation and purity level of the 
metal surface under inspection. 
The disadvantage of this approach lies in the fact that it does not take 
into consideration different influencing factors that destroy said 
proportionality. Among them are the strong influence of roughness, caused 
by such preparation methods like shot blasting or scraping used for highly 
rusted surfaces, the effect of excessive oil presence, etc. Furthermore, 
measurement of IR diffusive radiation intensity does not allow for 
selection of rust grade depending on whether two or three valent iron ion 
is present in the rust or whether the surface is clean of rust. The known 
device is merely suitable for providing the customer with the value of 
radiation intensity but without giving the surface attributions as to 
certain preparation grade, detection of presence of oil, or measurement of 
roughness. 
In conclusion it should be stated that despite the known methods and 
devices for optical inspection of metallic surfaces which enable separate 
detection of contaminants thereon or measuring roughness thereof, there is 
no known universal device which enables simultaneous assessment and 
attribution of the inspected surface as to certain rust grade and/or 
preparation grade and/or oil contaminants presence and/or to surface 
roughness quality. 
OBJECT OF THE INVENTION 
The object of the present invention is to provide a device for optical 
inspection of metallic, in particular, steel, surfaces in which the above 
mentioned drawbacks of known devices are sufficiently reduced or overcome, 
however without losing their inherent benefits. 
In particular, the first object of the present invention is to provide a 
new and universal device for optical inspection of steel surfaces which 
enables simultaneous detection of rust and/or oil contaminants on the 
inspected surface as well measuring of roughness of the surface with 
subsequent automatic attribution of the inspected surface to a certain 
rust and/or preparation grade. 
Another object of the present invention is to provide a new, simple, 
compact and convenient inspection device suitable for manual operation by 
an operator. 
Still a further object of the present invention is to provide an improved 
optical inspection device which ensures reproducible automatic attribution 
of the inspected surface to a certain grade. 
Another object of the present invention is to provide for a new, simple and 
versatile optical inspection device suitable not only for detection of 
contaminants and attribution of the inspected surface to a certain grade 
but also for displaying results of inspection in terms of ISO standards, 
including rust and/or preparation grade and/or oil contamination presence 
and/or surface roughness. 
The above and other objects and advantages of the present invention can be 
achieved in accordance with the following combination of its essential 
features, referring to different embodiments thereof. 
An apparatus for optical inspection of metallic, in particular, steel, 
surfaces, said apparatus comprising a housing, provided with a window, 
enabling viewing therethrough of said inspected surface, 
characterized in that said apparatus comprises 
a plurality of light sources, enabling illumination of the inspected 
surface via said window, wherein the inner surface of said housing is 
capable of reflecting total radiation, caused by said inspected surface 
after it has been illuminated by at least one of said sources; 
a detection means, comprising at least one photo responsive cell, enabling 
detecting of radiation, reflected by said inner surface of said housing 
and generating of corresponding electrical signals thereupon; 
a control means, enabling operating of said light sources and digital 
treatment of said electrical signals; 
a display means, showing thereupon said digital treatment results of 
inspection of said inspected surface in terms of presence of oil and/or 
cleanliness grade (rust grade, preparation grade) and/or roughness. 
In accordance with one of the preferred embodiments said plurality of light 
sources comprises 
a first light source, enabling illumination of the inspected surface via 
said window by a light beam, having wave length in UV region 
a second light source, enabling illumination of the inspected surface via 
said window by a visible light beam, having a wave length of less than 500 
nm; 
at least two additional light sources, enabling illumination of the 
inspected surface via said window by a light beams, having a wave length 
of more than 550 nm. 
In accordance with another embodiment, said second light source and said 
additional light sources are formed as sources, producing collimated light 
beams, wherein said beams are defined by their angular divergence of not 
more than 5 degrees, and said beams also not being parallel. 
In a third preferred embodiment, at least one of said light sources is 
mounted within said housing with the possibility of varying direction of 
the light beam, produced thereby. 
As per still another preferred embodiment, said second light source is 
mounted within said housing so as to direct the light beam produced 
thereby with respect to said inspected surface at an angle of 60-70 
degrees; 
at least one of said additional light sources is mounted within said 
housing in close proximity to said inspected surface so as to direct the 
light beam, produced thereby with respect to said inspected surface at an 
angle of not more than 12 degrees; 
and the second of said additional light sources is mounted within said 
housing remote with respect to said inspected surface so as to direct the 
light beam, produced thereby with respect to said inspected surface at an 
angle of 60-70 degrees. 
According to yet another preferred embodiment. The inner surface of said 
housing is provided with a region capable of absorbing part of the 
specular component of said total radiation. 
In still a further preferred embodiment, said apparatus is formed with a 
handle to be gripped by an operator, said apparatus being provided with 
frontal and rear extremities, wherein the frontal extremity thereof is 
brought close to the inspected surface so as to enable viewing thereof 
through said window; the rear extremity thereof faces an operator and said 
display means is situated thereon. 
In another preferred embodiment the inspection results are presented by 
said display means in terms of ISO standard. The present invention in its 
various embodiments has only been summarized briefly.

THE INVENTION DESCRIPTION 
With reference to FIGS. 1,2,3 the apparatus for optical inspection of steel 
surfaces comprises the main body portion 1, provided with a handle 2 to be 
gripped by the operator's hand. At the frontal extremity 3 of the main 
body portion a cylindrical housing 4 is mounted and provided with a window 
5. In the inspection mode the handle 2 is gripped in the operator's hand 6 
and the apparatus is brought close to the inspected surface so as to 
enable viewing thereof through the window. The rear extremity 7 of the 
main body portion is provided with a screen 8 for displaying results of 
the inspection and with different knobs, enabling operation of the 
apparatus. In FIG. 3 shows an example of inspection results. In accordance 
with these results the inspected surface is attributed to ISO preparation 
grade ASa. 
One can also see that the value of surface roughness in terms of Rz as well 
as oil thickness can be viewed on the screen. 
In one of the preferred embodiments as shown in FIG. 1 the main body 
portion communicates via wiring 9 with an external source of energy, e.g. 
a replaceable pack of dry or rechargeable batteries 10. In the embodiment 
shown in FIG. 1 the source of energy is formed as an external autonomous 
unit, attachable to a belt 11 to be worn by an operator. By virtue of this 
provision the apparatus becomes portable and can be easily manually 
operated. It should be readily appreciated that the source of energy can 
also be formed as a dedicated built-in unit or as a net AC/DC power 
supply. 
With reference to FIGS. 4,5 construction of the housing as well of other 
components of the apparatus, mounted within the main body portion 1, will 
now be explained. 
The housing 4 is formed as a cylindrical case and its rear part, situated 
adjacent to the main body portion is closed by an upper cover 12. The 
opposite part of the housing is provided with a window, covered by cover 
13, situated within a case at a certain distance from the foremost 
extremity thereof. This cover is made of a material, for example, glass, 
which is translucent to UV, visible and IR radiation. By virtue of this 
provision the inspected surface 14 can be easily viewed through the window 
without any danger of breaking the cover when the foremost extremity of 
the housing is brought in contact with the inspected surface, as shown in 
FIG. 4. 
In accordance with the present invention the main body portion and the 
housing are provided with a plurality of light sources, enabling 
illumination of the inspected surface by light beams, having different 
wave lengths. These light sources include a first light source 15, capable 
of radiating UV light, a second light source 16, capable of radiating 
visible light with short wave length not more than 500 nm and at least two 
additional light sources 17,18, capable of radiating light with long wave 
length of more than 550 nm, preferably IR light. 
In practice it is recommended, that the above light sources employed in the 
apparatus in accordance with the present invention will be collimated 
light sources, capable of producing light beams with an angular divergence 
not exceeding 5 degrees. 
It has also been empirically established that, in accordance with the 
present invention, location of the second light source 16 and both 
additional light sources 17, 18 should be chosen so as to satisfy certain 
requirements as follows. 
The second light source should be situated within the housing remotely with 
respect to the surface 14 being inspected and the light beam, produced 
thereby should be slanted with respect to this surface at an angle of 
.alpha.=60-70 degrees (see FIG. 4). 
The additional light source 18 should be situated between the cover 13 and 
the inspected surface 14 so as to be in close proximity thereto. This 
light source should be slanted so as to produce a light beam, directed 
with respect to the inspected surface at an angle .phi., which is not more 
than 12 degrees. 
The other additional light source 17 should be situated remotely with 
respect to the surface being inspected and the light beam, produced by 
this source should be slanted with respect thereto at an angle 
.beta.=60-70 degrees. This beam should be inclined in an opposite sense 
with respect to the direction of light beam, produced by the second source 
16. 
It is schematically shown in FIG. 4 that all the above mentioned sources 
15,16,17,18 are energized via respective wirings 15',16',17',18' by means 
of the above mentioned source of energy 10. Programmed operation of light 
sources is enabled by means of a controller 20, the functioning of which 
will be explained later on. 
In the cover 12 there are formed openings which enable the connection of 
light sources 16, 17 with respective wirings 16',17'. 
The cover is also provided with an opening 15", in which there is situated 
a filter means F1 which enables passage of UV light only. This light is 
produced by source 15 for illumination of inspected surface 14. An 
additional opening 19 is formed within the cover, and this opening is 
provided with a filter means F2. Mounted within openings 15", 19 
respective filters F1,F2 are provided for retention of long waves, with 
lengths of more than 400 nm and for retention of short waves, with lengths 
less than 420 nm. 
When the surface 14 is illuminated by at least one of the above light 
sources, total back radiation, consisting of diffusive and specular 
components, is effected. This back radiation is returned to the housing 
via window 5 and translucent cover 13. The major part of the inner wall of 
the housing is provided with a surface which enables reflecting of this 
back radiation and directing it to the upper cover 12, where it is 
detected by one of two photo sensors PS1,PS2, situated on the cover. The 
minor part of the inner wall is not provided with a reflecting surface, 
but constitutes a region, coated with a strip 4' made of light absorbing 
material. The significance of this provision will be explained further. 
Upon illumination of the inspected surface by the above light sources, the 
electrical signals are generated by photo sensors, which are corresponding 
to intensity of back radiation. These signals are transmitted via 
respective wirings 21',22' to amplifier 23 and then to converter 24, which 
converts them to digital form. 
Upon conversion these signals are transmitted to microprocessor/controller 
unit 20, where they are digitally treated according to a preprogrammed 
algorithm, enabling calculation of several quantitative parameters, 
corresponding to the condition of the inspected surface and simultaneous 
attribution of the inspected surface to certain roughness and/or 
preparation grade, preferably in terms of ISO Standard 8501. 
In accordance with the present invention it has been found, that the 
intensity of back radiation, caused by each of the above sources can be 
used for qualitative and quantitative assessment of all the above 
mentioned parameters, required for attributing a certain cleanliness grade 
and roughness value to the inspected surface. In particular the first 
light source refers to detection of presence of oil and measuring of oil 
film thickness. 
The second light source is relevant to detection of rust presence and type 
of rust. Two additional sources are employed for measuring of surface 
roughness. 
It should be emphasized, that by virtue of the above constructional 
features of the apparatus in accordance with the present invention the 
functioning of the plurality of separate light sources, employed therein 
is not independent in the sense that intensities of back radiation caused 
by these sources become interrelated and can be used for correction of 
independent assessment parameters, thus improving reliability and 
versatility of attribution. These parameters refer to the presence and 
type of rust, the presence of oil contaminants and surface roughness and 
they are displayed on screen 8 together with the grade type. The 
algorithm, which enables calculation and correction of these parameters 
upon individual intensities of the back radiation caused by the above 
light sources, is omitted here for the sake of brevity. 
Having described, in general, the concept underlying the apparatus in 
accordance with the present invention, how this concept is implemented in 
practice will now be explained in more detail with reference to FIGS. 6-8. 
In accordance with the sequence of events prescribed by the algorithm the 
first light source 15 is energized and illuminates the inspected surface 
14 by UV light passing through filter F1. The illuminating light consists 
of light waves with lengths less than 400 nm, preferably 355-365 nm. The 
presence of grease/oil contaminants on the surface 14 will cause 
luminescent back radiation LBR. Said luminescent back radiation will be 
associated with light waves, having lengths more than 420 nm and their 
intensity will be measured by photo sensor PS2 after filtration from 
exciting UV-rays by filter F2. If the measured intensity of the 
luminescent back radiation exceeds some programmable threshold value, then 
the further sequence of events is interrupted and results in terms of an 
excessive amount of oil will be displayed on the screen. 
In the case of a tolerable intensity of luminescence back radiation the 
UV-source 15 is de-energized, and the control unit will energize the 
second light source 16. This light source illuminates the inspected 
surface 14 by visible light with short waves having lengths less than 500 
nm, for example blue light (see FIG. 7). 
The light beam, produced by the second source 16 reaches the inspected 
surface and causes back radiation being collected by reflecting mirror 
walls of housing 4. This back radiation is detected by photo sensor PS1 
and its intensity will be measured. 
The incidental light beam, produced by the second light source 16 causes 
total back radiation, which consists of specular back radiation component 
SBR, directed by the surface 14 being inspected to the mirror walls of the 
housing and of diffused back radiation component DBR. In accordance with 
the present invention it has been found, that for correct measurement of 
intensity, associated with total back radiation and unequivocal 
attribution of the inspected surface thereupon one should ensure that 
intensity, associated with the diffused back radiation be approximately 
equal to that associated with the specular back radiation. By virtue of 
the inclination of the second light source at the above mentioned angle 
.alpha., a certain part of the specular back radiation component SBR is 
directed towards the absorbing strip 4' so as to be absorbed thereby. It 
is not shown in detail, but should be understood, that this light source 
can be mounted within a housing with the possibility of varying this 
inclination angle by a dedicated conventional means (not shown). 
Due to this provision the intensity of specular back radiation component 
SBR is partly reduced, and thus, the above requirement is satisfied. The 
particular configuration, dimensions and location of the light absorbing 
strip can be established empirically so as to achieve most efficient 
absorption of the specular radiation component. 
After measurement of back radiation intensity, associated with illumination 
by light source 16 the additional light sources 17,18 are consequently 
energized, and the intensity associated with the back radiation caused 
thereby is measured by photo sensor PS1. 
These intensities are used for measuring surface roughness by the two 
angles method, which is known per se and according to which the surface 
roughness is calculated as a ratio between them. 
It has been established in accordance with the present invention, that in 
order to improve reliability of the assessment of surface roughness, the 
intensity of diffusive back radiation component DBR associated with light 
source 18 and measured by photo sensor PS1 should be approximately equal 
to the intensity of the specular back radiation component SBR. By virtue 
of inclination of the light source 18 at an angle .phi. not exceeding 12 
degrees, the light beam produced by this source and the corresponding back 
radiation component satisfy the above requirement. By virtue of this 
provision, the apparatus can be advantageously used for reliable 
determination of surface roughness of different surfaces, irrespective of 
preparation technology, including shot blasting. 
Therefore, by virtue of the present invention the inspection apparatus is 
universal in the sense that it enables the simultaneous obtainment of all 
necessary data defining the inspected surface and attributes it in terms 
of particular rust grade and/or preparation grade and/or presence of oil 
contamination and/or surface roughness. This data in terms of ISO standard 
designations is displayed on screen 18. 
It can be readily appreciated, that the above described apparatus has very 
simple construction and its operation is easy and convenient for an 
operator. 
It should be understood, that the present invention should not be limited 
to the above embodiments. 
It should be understood also that changes and modifications can be made by 
one ordinary skilled in the art without deviation from the scope of the 
invention. 
For example, it may not be necessary that components, responsible for the 
treatment of electrical signals produced by photo sensors will reside in 
the same main body portion where the light sources are located. These 
components can reside in a separate location, remote from the inspected 
surface. Communication between these components and photo sensors can be 
effected either through wirings or cordless. 
Instead of two additional light sources illuminating the inspected surface 
by a long light waves and employed for measurement of surface roughness, 
one can use three or even more light sources so as to improve reliability 
of measurement. 
It would be advantageous if the third light source were situated with 
respect to the light source 18 in such a manner that the light beam 
produced by the third source is perpendicular to the light beam produced 
by source 18. 
The scope of the present invention is defined in the appended claims. 
The features disclosed in the foregoing description, in the following 
claims and/or in the accompanying drawings may, both separately and in any 
combination thereof, be material for realizing the invention in diverse 
forms thereof.