New Technique

[apochronaut]

The most common 6v. 20 watt halogen bulb used on microscopes is the ESB. It is about 1/2 an inch long, with a 2 pin socket ( G 4), and a little dollop of hardened glass on the end that makes them look like miniature soft ice cream cones. They are installed sideways. There is another version of a 6v. 20 watt halogen, known as the GE 778. It is almost the same, except that little dollop of hardened glass, where they pull vacuum on the bulb envelope is at the bottom between it's pins, making them look like Pinocchio on a particularly exciting morning. The GE 778 is intended to face end on to the illumination axis, with the filament projecting through the clear rounded glass at the top of the bulb.
In the modern world of downsizing and consolidation of resources and effort, the ESB is widely being substituted for the 778 by bulb companies. They even show pictures of an ESB but call it a 778. Same volts, same socket , same watts, more or less the same height, except for the nipple on top, so why not?
Well, for one thing, it doesn't work properly. You can only use a 778 in a microscope designed for that bulb. The beam exiting through the ice cream cone top on an ESB is distorted and since the end of the correct 778 bulb is designed to get very close to the heat proof filter behind the field lens, the glass dollop on the ESB can hit the filter, scratching and even gouging it over repeated futile attempts to focus the bulb.
Such was the fate on a microscope I was setting up recently. A horrible dark smudge and numerous dark spots blackened the background when adjusted for Köhler, the cause of which upon inspection, turned out to be gouges in the center of the glass heat shield.
The heat shield was in fact a ground glass plano/convex lens, with the plano side facing the bulb. I had just purchased a new heavy 240 grit water stone, with which to hone the nicks out of some old chisels. It was brand new and dead flat. What better tool was there with which to grind a lens? If you are unfamiliar with abrasive grit sizing, 240 is coarser than sand, more like Charles Bronson's beard stubble. Comet cleanser is much finer. 240 grit is about 70 microns and jeweller's rouge is 1/2 to 3 microns.
Anyway, worked like a charm. In 5 minutes, the grand canyon of lenses was down to the size of a Sussex swale, leaving about the same degree of dispersion as the original convex side has. No more apparitions in the background looming over the oblivious, innocent protists. Talk about stage fright!

[macnmotion]

That was a good read

[charlie g]

Thanks, apo, for suggesting hands on practical treatments of a basically sound stands light-path optics. You encourage all of us to try and correct issues in our optics...when " we" can.

My last wonderful workhorse stand, an Reichert/Austria Biozet trinocular stand was my workhorse microscope for decades. This circa 1960's (?) stand used the wonderful ( IMHO) tungsten square filament bulb for Kohler illumination.


On eBay...these (? Wotan square-filament tungsten bulbs?) became scarce...I found a low cost batch of: 'bayonet-style square filament tungsten bulbs'..and with grinding stones on a handheld electric drill...I easily sculpted the batch of

replacement bulbs to nest in the illuminator for my Reichert/Austria trinoc stand. Here I am, a dinosaur equipped with tungsten bulbs that will outlast me.

My current workhorse stand (trinoc Nikon Labophot-pol, ? circa 1980's?) uses halogen bulbs...and here too, I replaced the 20 watt bulb with a higher watt tungsten bulb (?30 watt, 50 watt?)...nothing 'melts' if you turn this stands illumination off

when you end observations.

Thank you apo for yet again sharing hands on practical microscopy optics advice. BTW, working with a variety of grades of gold, platinum, and silver...we polished metals with ever more exactiing 'rouge polish grades'...red-bar rouge, grey-bar

rouge, then white bar rouge.

Thanks, Phil/apo for all your useful microscopy threads, charlie g..today an 'upsetting' 60's degree F. in Febuary.

[Phill Brown]

G4 sockets allow for error in filament placement.
Custom bulbs with a collar require a jig to focus the collar fitment precisely.
Probably not a good idea to run 6v 30w on a supply intended for 20w.
6v @ 30w is 5 amps.
6v @ 20w is 3.5 amps.
Either the voltage will be low and give a warm tint or the supply could quit out of the project.
Light path alignment can be done on the dining table but it's not the simplest thing to get right.

[apochronaut]

That is definitely true of G4 sockets. in fact one of the easiest ways of focusing a G4 bulb is just to position the filament by bending the little pins this way or that. In this case though, the illuminator is a slide in from the back up to an adjustable stop which can be set to allow the end of the 778 bulb to get as close to the heat shield lens as possible without the bulb envelope hitting it. The filament would be about 4 or 5mm away. If the bulb blows and a replacement has been received from a supplier that is in fact the ESB with that vacuum extraction nipple on the end, the bulb is now physically about 3mm longer and when installed by a naive user will hit the heat shield, likely before the illuminator housing comes to a full stop against it's collar. Any subsequent attempts to either seat the illuminator housing properly or to focus the bulb with the focus mechanism will damage the heat shield surface as happened in the case of this microscope.