Transmissivelighting
Transmissivelightinghastwoforms:centrallightingandobliquelighting:
(1)Centrallighting:Thisisthemostcommonlyusedtransmissiveilluminationmethod,whichischaracterizedbythecentralaxisoftheilluminationbeamandtheopticalaxisofthemicroscopeonthesameline.Itisdividedinto"criticallighting"and"Kohlerlighting"twokinds.
A.Criticalillumination:Thisisanordinaryilluminationmethod.Thecharacteristicofthiskindofilluminationisthatthelightsourceisimagedontheobjecttobeinspectedafterpassingthroughthecondenser,andthebeamisnarrowandstrong,whichisitsadvantage.However,thefilamentimageofthelightsourcecoincideswiththeplaneoftheobjecttobeinspected,whichcausestheilluminationoftheobjecttobeinspectedtoappearuneven.Thepartwiththefilamentisbright;thepartwithoutthefilamentisdim,whichnotonlyaffectstheimagequality,butalsodoesnotItissuitableforphotomicrography,whichisthemaindrawbackofcriticallighting.Theremedyistoplaceamilkywhiteandheat-absorbingcolorfilterinfrontofthelightsourcetomaketheilluminationmoreuniformandavoidlong-termexposureofthelightsourcetodamagetheinspectedobject.
B.Köhlerlighting:KöhlerwasanengineerattheZeissfactoryattheendofthenineteenthcentury.Inordertocommemoratehisoutstandingcontributioninthefieldofoptics,latergenerationscalledhisinventedsecondaryimagingKöhlerlighting.Köhlerlightingovercomestheshortcomingsofcriticallighting.Idealilluminationmethodinresearchmicroscope.Thisilluminationmethodisnotonlygoodforobservation,butalsoanecessaryilluminationmethodforsuccessfulphotomicrography.Afterthefilamentofthelightsourcepassesthroughthecondenserandthevariablefielddiaphragm,thefilamentimagefallsontheplaneofthecondenserapertureforthefirsttime,andthecondenserformsthesecondfilamentimageatthebackfocalplaneatthatpoint.Inthisway,nofilamentimageisformedontheplaneoftheobjecttobeinspected,whichdoesnotaffecttheobservation.Inaddition,thelightingbecomesuniform.Duringobservation,thesizeofthecondenseraperturediaphragmcanbechangedsothatthelightsourcefillstheentrancepupilofdifferentobjectives,andthenumericalapertureofthecondensermatchesthenumericalapertureoftheobjectivelens.Atthesametime,thecondenserwillimagethefielddiaphragmontheplaneoftheobjecttobeinspected.Changingthesizeofthefielddiaphragmcancontroltheilluminationrange.Inaddition,thethermalfocusofthiskindofilluminationisnotattheplaneoftheinspectedobject,andevenlong-termillumination,itwillnotdamagetheinspectedobject.In2004,Zeisslaunchedafull-systemapochromaticlightingtechnologywithareflectivebowlonthebasisoftraditionalKöhlerlighting,whicheliminatesthechromaticaberrationofthelighting,enhancesthereducibilityofthelight,andimprovestheresolution.Atthesametime,thelightingisuniformandthelightefficiencyishigh..
(2)Obliqueillumination:Thecentralaxisoftheilluminationbeamisnotinastraightlinewiththeopticalaxisofthemicroscope,butformsacertainanglewiththeopticalaxistoilluminatetheobjectobliquely,soitisobliqueillumination.Phasecontrastmicroscopyanddarkfieldmicroscopyareobliqueillumination.