In 1957 Herbert Pychlau had the idea of a new diagnostic measuring device determining the patient load during medical X-ray examinations. This measuring device was meant to show young radiologists working in teaching hospitals the patient dose consequences of radiological procedures also considering minimizing techniques like collimation or filtration adjustment. Herbert Pychlau invented a new measuring quantity based on the inverse square law, which he named the "Dose Area Product (DAP)". The DAP is the product of the multiplication of the entrance dose and the area of the irradiated field. Doubling the distance increases the radiation area by factor four, while the dose decreases simultaneously by factor four. Both effects cancel out each other and therefore the new unit [R*cm²] was ideal for educational measurements as it is independent of the focus patient distance.