Six methods of brightness matching in optical pyrometry were compared experimentally each other, using a laboratory-type optical pyrometer and tungsten ribbon lamp. The measurements by these methods were compared in accuracy each other and significant differences amog the results obtained by four observers were evaluated stochastically. The conclusions were summarized as follows:
1. Values obtained by increasing the filament brightness to attain the matching and values by decreasing for the same purpose differed significantly each other for all the observers.
2. Increasing method, decreasing method, method of taking the mean value between the two values by the increasing and decreasing methods, and method of finding the mid-point value in the course of changing alternatively the filament brightness too high and too low, all showed a same degree accuracy. But the differences among observers' values were apt to be significant in the former two methods.
3. Method based upon the judgment of the equality of the brightness contrast between the filament and the background by means of a modified lamp circuit which facilitates to alter the filament brightness by a definite fraction, showed the highest accuracy at the 5% contrast. It was also the highest among by all the methods. Moreover, no significant differences among observers' values were found in the 5% contrast method.
4. Contrast method using a glass-plate in the optical path of the objective system of the pyrometer, resulted in a serious difference between the values obtained by a particular observer and the values by others.