2.2. Optinen mikroskopia

Vaihekontrastimikroskopia
Vaihekontrastimenetelmällä voidaan
värjäämättömien biologisten näytteiden
(vaiheobjektit) kontrastia parantaa ilman, että
resoluutio heikkenee merkittävästi.
Vaiheobjekteista taipuvat säteet kokevat ¼
aallonpituuden vaihesiirron, mutta suoraan läpi
menevät säteet säilyvät muuttumattomina.
Vaihekontrastimenetelmässä suoraan
meneville säteille aiheutetaan lisäksi -¼
aallonpituuden vaihesiirros, jolloin kuvatasossa
nähdään objektin yksityiskohdat tummina
valaistua taustaa vasten.
As light travels through a sample, interaction with this medium causes its amplitude and phase to change in a way
which depends on properties of the medium. Changes in amplitude = absorption of light, which gives rise to colours
when it is wavelength dependent. The changes in phase are not directly observed by human eye, yet often these
changes in phase carry a large amount of information.
The phase variations introduced by the sample are preserved by the optical microscope, this information is lost in
the process which measures the light. In order to make phase variations observable, it is necessary to combine the
light passing through the sample with a reference so that the resulting interference reveals the phase structure of the
sample.
In optical microscopy many objects such as cell parts in protozoans, bacteria and sperm tails are essentially fully
transparent unless stained (and therefore killed). The difference in densities and composition within these objects
however often give rise to changes in the phase of light passing through them, hence they are sometimes called
"phase objects". Using the phase-contrast technique makes these structures visible and allows their study with the
specimen still alive.