Thermal measuring and testing – Calorimetry – Total radiant energy or power measurement
Patent
1996-08-23
1998-04-28
Bennett, G. Bradley
Thermal measuring and testing
Calorimetry
Total radiant energy or power measurement
374179, 374208, G01K 1720
Patent
active
057436410
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an apparatus for power measurement of laser irradiation.
BACKGROUND OF THE INVENTION
Laser irradiation, particularly CO.sub.2 -laser irradiation, is used for varied purposes in industry, medicine and research. An important field of application is the treatment of materials, especially metals, with the aid of laser irradiation. In the case of the CO.sub.2 -laser, the radiation energy is directed through optical elements such as mirrors for example onto material surfaces in order to expose the same to heat effect of the laser irradiation. Laser irradiation is introduced in the material surface to be treated continuously or in pulse form. One condition for the practical application of laser irradiation in the field of material treatment is the observance of a pre-selected variable or constant power output. For example, cooling systems of a CO.sub.2 -laser produced by Rofin Sinar Laser GmbH and rated 5 kW output are equipped with a heat accumulation tank holding 5,000 liters in order to minimize temperature fluctuations of the cooling water. A cooling water temperature fluctuation by 0.5.degree. C. would cause a power change on the laser by approx 2%. Such power changes during the treatment of workpieces may lead to considerable damage and render the treated workpieces unserviceable.
To realize the sufficiently exact observance of the power curve or the constancy of laser irradiation power the power of the laser irradiation is checked on the last beam outlet opening prior or subsequent to a working step. The measuring is effected for example calorimetrically. In this, a heat insulated submass m, in particular a copper mass, is heated by means of a laser pulse by a temperature amount of .DELTA.l. Then the temperature increase of the mass is measured after reaching the isothermal temperature distribution in the metal mass. The power of the laser irradiation is then determined according to the following equation: ##EQU1##
The pulse time can also be a time in which a metal mass wiping through under a continuous laser beam is hit by the laser beam and is heated by .DELTA.l. However, this method of measuring is subject to considerable errors: Ideally, the total given laser irradiation energy is introduced in the metal body by absorption of the laser light on the surface of the metal body to be heated; the value s as the ratio between the energy which is introduced in the metal body and the energy totally applied then is 1. Reflections of the laser light on the surface of the metal body however prevent complete absorption and cause corresponding errors of the measuring method. Attempts to reduce the reflection to sufficiently small values by blackening and roughening the surface of the metal body to be heated do not lead to satisfactory results. One reason therefor is that the body optically cannot be brought to a lasting blackening degree of almost 100%. Disadvantages of the known measuring method also reside in that under the effect of the laser irradiation, particularly at higher power densities, layers generated for the purpose of blackening are changed or applied layers are stripped so that the absorption coefficients are destabilized and insufficiently maximized.
SUMMARY OF THE INVENTION
Therefore, the invention aims at providing an apparatus for power measurement of laser irradiation which produces sufficiently exact measuring results.
According to one aspect of the invention, there is provided an apparatus for power measurement of laser irradiation comprising: a first elongate body forming a hollow space and being comprised of a metallic wall consisting of a material of small thickness and good thermal conductivity, said body having an opening for receiving the laser irradiation on a first longitudinal and thereof, a black and/or rough surface at least in an area covered by the laser irradiation in the hollow space thereof, and a heat detector each on the first longitudinal and a second longitudinal thereof, said heat detector being respectively adapted to be coupled to a means
REFERENCES:
patent: 4429999 (1984-02-01), Bimberg et al.
patent: 5098195 (1992-03-01), Halyo et al.
patent: 5209567 (1993-05-01), Loosen et al.
Electrical Review, "Research and development", vol. 188, No. 2, (8 Jan. 1971), pp. 59-60.
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