Setting Up In-line Digital Holography

Question

I would like to set up an in-line digital holography system to image plankton samples. The set up consist of a laser that project through an objective which will transmit through a pinhole. In the papers that I have read, the pinhole sizes have varied from .1mm to .001mm.

I do not understand how the size of the pinhole is chosen? From what I understand different sized pinholes will focus the light at different distances and if this is correct then, how does this effect the resolution?

Second, how do you determine the distance of the sample and the distance of the camera sensor? From what I understand, there is a maximum length for the sensor, otherwise it will not meet the Nyquist criteria. Is that correct?

Thanks.

Answer 1

The purpose of a pinhole at the focus of an objective is not to focus the beam. In fact, its purpose is to filter out "noise" in the beam due to dirt and imperfections upstream from the pinhole. The proper selection of pinhole size depends on the focal length of the objective and the width of the beam that enters the objective. A wider beam entering the objective requires a smaller pinhole.