Microscopy Forum

I apologize if this is not too clear: I tried posting as text, then posting as a Word document, then I typed it in manually, but every time it causes some words near the right margin to be cutoff or changed even though in EDIT mode there are no obvious problems; I have no idea how to fix it.  Hopefully it will be sufficiently clear, though.

After the discussion about the advantages and disadvantages of tungsten-halogen vs. LED micoscope illuminatorshttp://www.microbehunter.com/f…..ge-2/#p779 ,   I thought I would try to determine if adding an IR-absorbing filter to a halogen-illuminated microscope would have a significant beneficial effect.

What I did:  Nikon Labophot microscope fitted with a 6 V, 20 W halogen bulb.  The bulb is located at the back inside the microscope base, so heat is conducted by the metal of the stand to the stage (and slide), in addition to the heat radiated to the slide.

I set the bulb voltage to 5 V (nominal).  Condenser aperture and field diaphragms were fully open.  I cut a square piece of black paper and taped it to a slide and put it on the stage.

I turned on the microscope lamp and recorded periodically the temperature of the stage close to the slide and the warmest point on the upper surface of the black paper taped to the slide.  The room temperature was 24°C and fluctuated over time by about 0.3°.  The measured temperatures do not necessarily correspond to the temperatures that microbes on the slide would experience, of course; the idea was simply to see how effective an IR-absorbing filter would be, at least on my microscope.

When the temperature rate of rise started to decrease, I added the IR blocking filter over the field lens at the base of the microscope and continued the measurements (I should have waited for the temperature to reach a constant level before adding the filter but I was too impatient ).

I plotted temperature vs. time of both the paper on the slide and the stage, as shown in the figure below.  The IR-absorbing filter was added at 4.5 hours after turning on the microscope lamp.  The filter resulted in a fairly rapid reduction of the temperature of the slide, to a value within about 1 degC of the stage temperature.

Thank you, Avidor.  Yes, the IR-blocking filter is one I bought from the surplus website I mentioned in the other thread.  It was 45 mm in diameter, and about 3 mm thick.  The temeperature rise of the slide must be interpreted very narrowly I think, and would apply approximately to the conditions of the test, including a ~20 mm black square paper** taped to the slide and measured with an IR thermometer aimed at the (exposed to air) top of the (not-so-good heat conducting) paper, and the same microscope and lamp voltage, and not necessarily to a microbe on the slide.  What was interesting, I thought, was that the warming resulting from conducted heat along the body and stage of the scope was larger than that due to the light radiation, and that would be very dependent on the design and construction of the scope.

I have later wondered about the effect of introducing the filter on the image.  It does change the color temperature making it cooler (as would be expected), so the white balance of the camera must be adjusted.  Also, it does absorb enough light to require a slight exposure increase (I think a little over +1 EV). 

The possible bad news is that I have a suspicion that it has an adverse effect on the image resolution.  If true, it may be because the filter is introduced in a light beam that is not composed of parallel light.  I will try to confirm whether there is perceptible image degradation (I need to take a larger sample of images to make a credible conclusion).   In the meantime, or unless you do your own evaluation, it may be wise to remove the filter when taking photographs.

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** I think I should have used a piece of aluminum foil with the bottom surface painted black.