Design of a laser pyrometer for the measurement of unsteady temperature distribution during laser welding

Pyrometers using a single element detector, 3x1 linear array and 3x3 area array were designed for the purpose of measuring temperatures of a spot in a pulsed laser weld-pool. The design is predicated on sampling at rates sufficient to track weld-pool temperature changes introduced by a 5 kHz pulsed laser system. The pyrometer design permits electro-optical interfacing with the Center for Laser Applications' (CLA) Rofin-Sinar-3000 high power CO_{2 laser}.

For every temperature measurement the pyrometer measures the reflectivity of the target (weld-pool) using a 25 millimeter (mm) aperture optics and a liquid nitrogen cooled Indium Arsenide (InAs) detector. Emissivity is determined from measured reflectivity. A 1.535μm low power cw laser source is incorporated in the system for this purpose. The detector is highly sensitive and linear over the entire temperature range. It has a passband from 1 to 3.1 fim and converts the optical signal to electrical, which is then amplified and recorded as 12 bit data on the CLA's Masscomp computer.

The pyrometer will scan a strip whose width (100 μm) is decided by the Instantaneous Field Of View (IFOV) of the detector and whose length is governed by the weldment traverse speed and the laser duty cycle. It can measure sample temperatures from 300 °C to 2500 °C with an accuracy of one degree °C.

A calibration black-body and a reference gold coated surface are to be used for calibration of the pyrometer. The emissivity of the target weld-pool is calculated using the reflection calibration. The black-body calibration is the basis for calculation of an overall system constant from which the true temperature of the target is calculated.