MichaelG. wrote:apochronaut wrote:The 31-33-85-11 lamp using that bulb was originally designed for critical illumination, not Köhler. That's what Bausch & Lomb says [ ... ]
With the PR-27, the modernized successor research illuminator, they kept the same system going, albeit in a more convenient package.
[ ... ]
Köhler, really isn't a designed illumination system. it is a condensing and focusing technique with which one can make a bad situation better. [ ... ]
I don't have any argument with what you say
... but it's interesting to note that the PR-27 was also described as "ideal for providing Köhler type illumination"
IMG_2394.JPG
Was this just a matter of following the trend ?
MichaelG.
Presumably PG stands for Professional General as opposed to PR ; Professional Research? Köhler is just a case of adjusting the illuminator condenser in such a way as to get as distorted an image of the filament as possible at the image plane, in order to broaden and even out it's illuminating charactersitics. B & L does seem to be following a somewhat necessary marketing tact by mentioning Köhler because by the time the coiled filament had invaded the microscope world almost exclusively, the use of Kohler had become a necessary fact of life. In the B & L catalogue, I linked some scans to above, the 31-33-85-11 is specifically referred to as being for critical illumination, whereas in the scan you provide there is mention of critical work. Same bulb. Theoretically, with enough travel in the condenser, you could adjust for critical illumination or köhler at your whim.
Interestingly, Spencer, who made a very parallel line of products; the two companies were only 70 miles apart and no doubt passed employees back and forth a bit, took a very different approach. By the time the 31-33-85-11 B & L 108 watt ribbon filament lamp was out, Spencer , in addition to their electric arc lamp, exclusively used coiled filament bulbs, either 100 watt or 400 watt. In their catalogues, they make no mention of either critical or köhler illumination, just that the condensing apparatus can be used to provide an intense concentrated beam of light suitable for DF work.
Later they produced the model 367 research lamp, which had 100 watts , and a condenser. It had similar characteristics to the B & L version PG 26 but like the older 31-33-85-11, recommended Kodak Wratten filters and had a water cell, protecting the filters , as well as heat sensitive delicate living structures. One key difference was that several of the Spencer illuminators were on poles, so that the entire body of the illuminator could be elevated to be used for incident illumination.
Later yet, Spencer produced a very similar lamp to the PG 26, the 735, also with a coiled filament.
Spencer seldom mentions the concepts of Köhler illumination or critical illumination in their literature. Certainly not in terms of illuminators. The one exception is that they mention it particularly in the case of phase microscopy.
The 6v. 108 watt bulb was probably never very cheap. My last catalogue that I keep for bulb cross references, lists it at 89.50. 50 hour service life.
It is a P28s base, widely used as a projector bulb. 2200 lumens or thereabouts.
