Thermal measuring and testing – Calorimetry – Total radiant energy or power measurement
Patent
1992-11-02
1994-06-21
Cuchlinski, Jr., William A.
Thermal measuring and testing
Calorimetry
Total radiant energy or power measurement
374183, G01K 1720
Patent
active
053223594
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for measuring laser power, wherein an electrically actuable measuring wire or the like having a temperature-resistant electrical resistance is subjected to a relative reciprocatory movement transversely over the beam cross-section.
To measure laser power, it is generally known to provide a detector to absorb the whole beam. During the measuring period it is impossible to process a workpiece using the laser beam. It is also generally known to measure a portion of the laser beam using a reflecting element or a partially transmissive optical system, so that the measurement can in principle be performed even during the processing of the workpiece. However, the reflecting element or the partially transmissive optical system may have an adverse effect on the laser beam or itself be destroyed or damaged by excessive radiation powers or intensities. For this reason as a rule it is impossible; to perform measurement between the focusing optical system of the laser beam and the workpiece.
EP B1 0050159 discloses a process having the steps mentioned hereinbefore. That process uses the temperature dependence of the electrical resistance of the measuring wire to obtain a measured value for the power of the laser beam. However, the measured value is adversely affected by interferences, for example, different heat transfer resistances depending on convection.
SUMMARY OF THE INVENTION
It is therefore an object of the invention so to improve the process of the type specified above so that the laser power can be measured free from interferences in an on-line measurement.
This object, as well as other objects that will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by moving the measuring wire in relation to the beam cross-section in a time substantially shorter than its thermal time constant. A parallel compensation wire is also provided, closely adjacent to the measuring wire, and which is also electrically actuable and has a temperature-dependent electrical resistance and which has a degree of absorption of the laser radiation which is low in comparison with the measuring wire. The measured values of the two wires are determined and their quotient is calculated as a measure of the laser power independent of the convectional cooling of the two wires.
This process starts from the fact that the radiation power absorbed by the measuring wire is given off in the form of heat to the surroundings of the measuring wire, a state of equilibrium being set up as a result. The heat is given off by radiation and convection, the proportion of radiation being negligible due to the relatively low temperature of the measuring wire. The heat given off by convection depends on the heat transfer resistance .alpha. of the measuring wire. The heat transfer resistance .alpha. depends to a small extent on temperature and is inversely proportional to the wire diameter or depends on the cross-sectional geometry of the measuring element used. Moreover, the cooling of the measuring wire by convection must not be regarded as constant, since currents of the convectional cooling flow are set up at the measuring place--i.e., flows of air or process gas, such as are used to influence processing at the place where a workpiece is processed. These flows result in a heavier cooling of the measuring wire, so that the measured value achieved thereby is too low. The radiation power is displayed with too low a value. By means of the compensation wire the influence of convectional cooling on the measuring result--i.e., the influence of the heat transfer resistance .alpha. on the cooling and measuring wire--can be eliminated. Since the two wires are close to one another, the influence of the air flow on the cooling of the two wires is identical in each case. If the measured values of the two wires are brought into ratio with one another, the influence of the air flow and also the dependence of the heat transfer resistance .alpha. on the conv
REFERENCES:
patent: 2442823 (1948-06-01), Poyle
patent: 2993138 (1961-07-01), Scott, Jr.
patent: 3282100 (1966-11-01), Baker
patent: 3387134 (1968-06-01), Treharne
patent: 3394258 (1968-07-01), Schleiger et al.
patent: 3622245 (1971-11-01), Rasmussen
patent: 4522511 (1985-06-01), Zimmerer
"Feindraht-Kalorimeter Zur Absoluten Messung Der Energie Von Lasern", Von G. Fenske, Berlin, Experimentelle Technik der Physik, XV, 1967, Heft 2, pp. 132-137.
Briesch Paul
Kramer Reinhard
Wissenbach Kurt
Bennett G. Bradley
Cuchlinski Jr. William A.
Thyssen Stahl AG
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