|THE HEINE CONDENSER (Part
Some thoughts concerning a very useful substage accessory
By Paul James (uk)
In this first part, I will describe the physical aspects of this condenser and the second will be concerned with the operational pro's and con's and some through the 'scope imagery too.
Leitz made a special form of condenser way back in the earlier years of the last century to circumvent a patent infringement problem they might have had regarding Zeiss's use of their newly devised Zernike phase contrast condenser system. This condenser is known as the 'Heine'. Though it was designed to provide a continuously variable annulus of light to the specimen so as to illuminate any phase objective, it also provides darkfield illumination and COL* too. In short it is a very useful device as all these illumination techniques can be implemented in seconds by altering one simple control on the body of the condenser.
(* - COL is circular oblique lighting, see earlier articles.)
Leitz did in fact advertise it as being able to elicit phase contrast, darkfield and brightfield, but to my way of thinking the brightfield they speak of is more akin to COL, as the central component that bright field normally issues is absent. COL is for my purposes a superior illumination technique to BF in some circumstances and thus I saw the Heine condenser as a definite step forward in illumination technology.
In practice this condenser can be used with any phase optic, and though Leitz's phase objectives of the day would have been matched to this condenser, other makes of phase optics work very well too. The virtue of having a 'thin' walled annulus from the 'Heine' ensures that it fits well within other makers objective phase rings .
The condenser actually provides an annulus of light through the specimen which can be continuously varied from about 0.2-1.0na and also to above 1.0na with an additional optic top element and oil contact with underslide. Thus darkfield can be raised easily in any objective by expanding the annulus to beyond the na of the latter, and that this can be accomplished in a second or two makes for a highly desirable substage accessory indeed.
At the heart of the condenser is a special optic which appears to use reflective and total internal reflective principles similar to a number of darkfield condensers made by various manufacturers such as Leitz and Beck etc.. The top of this optic however is very deeply concave and its upper peripheral rim issues the annular cone of imaging light onto the specimen. The actual light path within this optic is not easy to understand however since the general consensus of opinion seems to indicate that there are 'moving' optics within this condenser which control the na of the annulus. It is a fact however, that simply lowering the whole condenser with the substage housing of the microscope elicits exactly the same varying effects that the Heine condenser produces by its own control knob, which suggests a static optical array system?
It is housed at the top of a hollow 'piston' which can be raised or lowered by the control knob through a rack and pinion assembly. The housing 'cylinder' has a threaded upper portion which carries another optic presumably to allow oil contact to slides for objective apertures in excess of 1.0 na. I do not have this fitment and so cannot comment accurately on its construction or performance.
The images below show how the inner 'piston' moves taking the upper optic away from the underslide position.
The engineering is first class as you might expect from Leitz; the optic piston plunges very smoothly throughout its range. There is a simple plano-convex lens covering the bottom of the body which brings about a change of the ascending Köhler light to work in conjunction with the all important upper optic :-
This image below is how the top optic looks by eye when held up to daylight, which clearly shows the annulus, and when close to an objective the obliqueness amounts to approximately 1.0 na. at its highest setting.
How does the Heine condenser work ?
In order to try and resolve the mystery regarding the optic I cut a piece of white card so it fitted neatly into its concave depression and positioned it centrally so as to trace all the light issuing from the inner surface.
The image below gives an idea as to what is going on in the Heine condenser. It appears that there are 2 distinct emissions of light. First the broad top cone tipped with yellow spot is the light which is seen as an annulus which becomes smaller the lower the condenser is moved. This varies from about 0.2na to a theoretical maximum of 1.0na: the latter is realised when the condenser optic is raised 2-3 cm. If the objective in question has a lower na than that of the annulus it will of course experience DF. When the condenser is raised higher a zone of no illumination is experienced which lies just a little higher than the red spot. When raised further it elicits a more highly intense DF from the zone of light focussed at the red spot. Below this is another zone which probably is an artefact ?
In essence therefore, Leitz seem to have designed not a variable optical assembly to bathe the specimen in a number of illumination configurations, but have actually managed to 'stack' the requisite optical elements to allow each in turn to function by changing the proximity of the condenser to the specimen. Thus phase contrast, COL and medium power DF are provided by the upper conical annulus, and the highpower DF and COL with a very oblique discrete cone below. (The oil immersion top screwed optic, which I unfortunately do not possess, must utilise one or more of these cones of light to transfer it through oil to the underside of the slide).
The Heine condenser is simple in use yet its make up is deceptively ingenious, and the layered optical assembly which must I suspect be cemented up from 2 or 3 elements, some having very small reflective optical surfaces and deeply spherical shapes probably accounts for its initial high cost.
To be continued next month.
All images taken with a Nikon Coolpix 800 2.1 megapixel digicam
|All comments welcome to the author Paul James|
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