Abstract
Fro the viewpoint that the effective focal spot size of a X-ray tube is a major factor affecting the image quality of X-ray pictures, we have inspected periodically the effective focal spot size using star chart. We concluded from our measuring results that no significant change exests in size by operating year. Despite our conclusion, ORITO reported in 1987 that a tendency of an increase existed in size by operating year. Our redetermination found that the change in size was within 10% at maximum and no relevance prevailed between the effective focal spot change and exposure counts not operating year because we admitted there are ±2%~±5% error in chart reading and also ±1%~±5% error in chart arranging, so we recognized these changes in value are within the measuring error. In consequence, the data above suggest that it has no significant meaning in effective focal spot size measurement.
