Heat treatment method

Claim 1

In a method for heat treating an object wherein the object is carried into a heating furnace and is heat-treated by irradiation with light which is emitted from a light source disposed facing at least one side of said object in accordance with an output program which calls for a rise in temperature followed by a decrease in temperature, the improvement comprising: (a) preheating the furnace prior to carrying the object into the furnace by irradiating the furnace with light in accordance with the output program; (b) measuring a first elapsed time between a first time when a first set-up temperature is reached in the course of the rise of furnace temperature, and a second time when a second set-up temperature is reached in the course of the decrease in furnace temperature; (c) repeating a cycle of steps a and b to measure a second elapsed time; (d) comparing the difference between the first elapsed time and the second elapsed time with a specified value, wherein if the difference between the first elapsed time and the second elapsed time is less than the specified value the preheating is terminated and an object is carried into the furnace, and wherein if the difference between the first elapsed time and the second elapsed time is greater than the specified value, step c is repeated to measure a subsequent elapsed time which is compared according to step d with the elapsed time measured in the immediately preceding cycle.