Optics: measuring and testing – By shade or color – With color transmitting filter
Patent
1994-09-02
1995-12-05
McGraw, Vincent P.
Optics: measuring and testing
By shade or color
With color transmitting filter
359220, G01J 351
Patent
active
054734380
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method of measuring optical radiation, in which method the intensity of a radiation coming from an object to be measured and to be lighted by collimated radiation is measured at several wavelengths by focusing the radiation by an optical means and a mirror means on a detector group comprising several detector elements.
Spectroscopic measurements are based on an investigation of a spectrum of radiation coming from an object to be measured. It depends on the properties of the object to be measured how the spectral distribution of the radiation, which is reflected or emitted from the object or which has penetrated the object, is within a frequency range. For instance, the thickness of a film of slushing oil or impurities on the surface of the object influences the distribution and amplitude of the spectrum reflected from the object or, in practice, from the film on the surface.
Typical of spectroscopic measurements is that the radiation obtained from a spot-like light source is collimated, i.e. made parallel, and the object to be measured is lighted by this radiation.
Detectors used in spectroscopic measurements comprise several detector elements connected to each other, each detector element measuring radiation coming from an object within different wavelength ranges. For a successful spectroscopic measurement, each detector element should see the object at the same incidence angle, in order that the multichannel detector formed by the detector elements may be used for measuring relative intensities of different wavelengths of the spectrum of the radiation reflected. In practice, it is, however, necessary to position the detector elements relatively far from each other for structural technical reasons, due to which the detector elements see the same object at slightly different angles or the radiation comes to the separate detector elements at the same angle, but has started from another part of the object than the radiation arrived at the rest of the detector elements. These problems appearing as error factors in measurements cannot be eliminated by fixed optics.
To eliminate these problems, it is known to use a diffuser in front of the detector elements. A parallel radiation bundle coming to the diffuser is diffused in the diffuser, which leads to that the radiation coming to the elements is mixed in different directions. On account of the operating principle of the diffuser, a large part of the radiation, i.e. of the light, passes the detectors, and the diffuser attenuates also the intensity of the radiation, because the more light rays come to the diffuser, the more the radiation is attenuated.
U.S. Pat. No. 4,792,684 discloses a horizontal scanner, which is used for instance in satellites or missiles to follow their movements. The device described in this publication directs the light rays collected by the device from several different directions and objects to one detector element. By this solution, it is not possible to avoid dimensional errors, because the light rays come from different places, due to which a ratio measurement taking place within a certain wavelength range would give somewhat erroneous measurement results. The mirror means to be used in the solution according to this publication comprises two separate levels, which are positioned at an angle with respect to each other. By means of this solution, it is not possible to circulate a focus on a detector level formed by several detectors. In this solution, moreover, the reflecting mirror is tilted to form a big angle of about 30.degree. in relation to a level perpendicular to its rotating axis. The so-called split mirror structure causes a halving of the light intensity into two separate radiation beams.
U.S. Pat. No. 4,748,329 describes a method of and a system for measuring the thickness of a light transmitting film, whereby a multichannel detector measures radiation reflected from the surface. Several reflector means are used in this solution, by means of which the focus of radiati
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Alajarvi Tapani
Keranen Heimo
Pietarinen Pasi
Vali Veli
McGraw Vincent P.
Rautaruukki Oy
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