In Pursuit of Constant Illumination Intensity in Brightfield

A simple tale of cause and effect

By Paul James


I decided recently to modify a substage condenser, and though it proved successful enough, the experiment raised some curious sentiments I'd not considered before. The following notes might be of use to those observers of organisms that are sensitive to changes in illumination intensity that brightfield incurs through manipulation of the substage iris diaphragm.

The Objective

The Idea of trying to modify a substage condenser's light intensity output so that it remained constant at any setting of its iris diaphragm seemed to me an interesting challenge. Whatever method was used it would have to be directly linked in some way to the motion of the iris diaphragm's lever control, and in theory at least rid the observable field and specimens of distracting variations of light levels.

Though we all accustom ourselves to the fact that the iris diaphragm controls the resolving capability of a given objective, the varying light intensity produced as a consequence becomes instilled into the psyche to remain for the duration. I strongly suspect that there is a subconscious tendency for the observer to adjust the iris diaphragm's setting for both resolving and light intensity requirements, most particularly if the intrinsic illumination output from the lamphouse is too high in the first instance. There are a number of well tried solutions such as the use of rheostats to reduce the current flowing through the lamp, or of course making use of crossed polar screens...............but both techniques are post operative in nature. What I really wanted to witness was the change in image quality as the iris diaphragm opened and closed without any distracting alterations to the field intensity.

The obvious solution in theory I thought would be to use the crossed polar principle which can be used to control the overall light output from the lamphouse, but link one of them instead to the lever of the condenser diaphragm or its internal ring assembly. So I initially decided to place a circular section of a polar sheet on top of the substage condenser's iris leaf assembly as a start, to actually see if the light intensity variation of the crossed polars counter- matched that of the closing iris. So when the iris was closed or opened the rotary motion of the diaphragm assembly would be directly imparted into the polar disc. It seemed likely to work since both polar and iris diaphragm's diminishment of light are basically realised by 90 degree rotation, though diaphragms do vary of course. The other polar sheet would be placed below the substage on the field lens and would be orientated to 'tune' the light variation system.

The relative axial angle of the lower polar screen was adjusted in order to bring about as constant level of light output as it was possible to attain by the sweeping of the diaphragm's lever control......a sort of auto cancellation effect that I was intending to achieve. Thus the substage condenser's polar screen's axial setting was such that about 50% of the light from the lamphouse was absorbed whilst the diaphragm was at maximum aperture. So as the diaphragm closed its coupled polar screen rotated too and let more light through to compensate for the field darkening that the iris diapragm's closure gradually incurred. Ideally therefore the lamphouse illumination intensity needs to be about 50% brighter than for normal brightfield observation, as well as some losses through the polar pairing process etc.. The partial diminishment of field intensity by the semi crossing of the polars at the onset of diaphragm closure is essential, as there must be a reserve of light intensity which is utilised as the diaphragm's polar screen let's through more light as the iris gradually closes. This can be subtly tweaked by the axial setting of the field lens polar screen.

It worked reasonably well after sorting out the polars relative axial angles so that the light intensity was fairly constant at all NA settings, save when the iris was closed well down, and then the light intensity fell abruptly, but this was of little consequence as it occured well beyond the range of usefullness anyway. So far the idea seemed to work satisfactorily, though the method of supporting the condenser's polar screen needed to be refined.

Matching the Light Attenuating Characteristics of the Cross Polars and Diaphragm

Since the light variation through the iris diaphragm is non linear, being exponential in nature on opening, it does not match the polar variation which cycles through 90 degrees rotation from clear to blackout. However in practice the external polar screen below the condenser is set so that the absorption of light is well underway when the iris diaphragm is wide open. So in effect the polars are blocking some light at the onset of iris diaphragm closure and then continue to pass more light through as the diaphragm closes in compensatory fashion. The setting of the bottom polar is critical and governs the constancy of illumination intensity. I was able to achieve a fairly evenly illuminated field from the condenser's full aperture of around 1.0 NA to around less than 0.2 NA.

But having got somewhat excited at realising the potential, there were 2 niggling problems. The first, a physical dilemma caused by the polar sheet inside the condenser, where its tendency to curl caused unwanted friction even though it was loosely sandwiched between the iris leaves and upper condenser body. A glass polar screen as used in photography would be best, but would require more effort and ingenuity to install into the condenser body.

The second problem : Having briefly experienced the virtues of the constant light ouput from the substage condenser, I could see the change in the specimen's fidelity as the condenser's NA varied, but it wasn't as obvious as I had expected it to be. The psychology of association of field brightness and resolution had become so deeply entrenched into my subconscious over time that it has become perceived as a single phenomenon, when of course they are in fact two separate entities. It seemed therefore that a lifetime's experience of varying field intensity isn't easy to ignore, and nigh impossible to unlearn in a few minutes.

Conclusive Thoughts

If there is any virtue in all this then I'd imagine it might be of use in situations where organisms with high sensitivity to light variations are under observation, or it could be specifically tweaked for a favourite object/objective combination where variation of the substage's iris diaphragm could be utilised more subtly in constant lighting. Since there are no definitive settings, this technique is one of those disciplines which can be entirely designed around the individual's requirements. With an even brighter output from the lamphouse, a 3rd polar screen could be installed beneath the field lens's polar screen to control the overall light intensity.

The 'jury is still out' on this. I'm going to install a glass polar disc into the condenser and persevere and see if this technique passes the test of time.........The real test !


All comments welcome by the author Paul James


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