This interactive Flash tutorial explores the effect of apodized phase plates on image contrast and halo attenuation in phase contrast microscopy. Instructions for operation of the tutorial appear below the window.

Illumination from the light source passes through a hypothetical spherically-shaped specimen (phase object), whose size is adjustable globally with the slider, or by clicking on the arrow buttons to the left and right of the slider. The tutorial initializes with the specimen set to the smallest size position on the slider. As the thickness of the specimen is increased by moving the slider to the right, the phase difference between the specimen and the surrounding medium increases and the angle of light diffracted by the specimen becomes smaller. This diffracted light passes through an apodized phase plate (located in the rear focal plane of the objective), which contains two neutral density filter rings having a 50% transmittance. The width of the apodization absorption rings is based on the diffraction angle according to the calculation:

Where q is the angle of diffraction and d is the phase difference between the specimen and the surrounding medium. Contrast is controlled by the transmittance of both the apodized (50% transmittance) and the DL phase (25% transmittance) rings. Most of the light diffracted through large specimens passes through these phase rings, while much of the light diffracted through small specimens occurs at higher angles and passes through transparent portions of the phase plate. Light passing through the phase plates is attenuated and is represented by a decrease in color saturation for the arrows and waves in this tutorial.