A method and apparatus of priming or rinse back of an extracorporeal circuit using a dialysis machine comprising a dialysis liquid preparation system. The subset of image data may be a delta subset that represents those areas of the image that have changed since the previous transmission. An image is formed by the lens system at a spherically curved image plane that substantially matches the concave surface of the photosensor. The current path through the channel is approximately equal to twice the height of the pillar plus the width of the pillar.