From: vporguen@unl.edu (victor porguen)
Newsgroups: sci.optics
Subject: Re: Objective lenses that work in UV
Date: 25 Jan 1993 04:17:33 GMT
Organization: University of Nebraska--Lincoln	
Distribution: world
NNTP-Posting-Host: unlinfo.unl.edu

Writes hnr@ecs.soton.ac.uk (Harvey Rutt):

> I seem to recall UV transmitting microscope objectives are used 
> in biology where they excite fluorescence with UV.  

Nearly any microscope objective can be used for this purpose 
(excitation of fluorescent dyes used to stain tissue)  because the 
"UV" in this case is near UV, to which all those lenses, including 
ones designed for the visible only, are fairly transparent. 

> Also there are certainly some so called 'fluorite' objectives 
> (?CaF2), although I *think* this was used for superior chromatic 
> aberration compensation in the visible, not for its good UV 
> transmission.

It was (and is) used for both purposes.  Fluorite (crystaline CaF2) 
is _both_ a low-dispersion material and a UV-transparent material. 
Both of these properties have been used in optical systems design. 
So-called "fluorite objectives" and so-called "apochromatic 
objectives" are characterized by the employment of fluorite 
elements because of its very low dispersion.   Since the fluorite 
elements used in complex objectives are usually strongly biconvex 
elements, and account for much of the total lens thickness of the 
objective, the objectives usually end up being also more transparent 
to UV radiation.  Some new objective designs (including some 
high-school microscope objectives!) also use fluorite but 
manufacturers' marketing departments do not necessarily refer to 
the resulting systems as either "fluorites" or "apochromats"...  ;-)

> As another contributor pointed out, true diffraction limited 
> performance in the UV with high NA is probably not on 
> commercially.

Not so, as zrepachol@cc.curtin.edu.au also points out in another 
article.  It's been available for quite some time. There is little 
special about diffraction-limited performance in microscope 
objectives, whether visible or near-UV.  The higher cost of good 
far-UV microscope objectives arises mostly out of the rarity of 
their use, not from design difficulties -- they're not mass-
produced.

The original question posed here was for UV-transmitting optics at 
380 nm.  This is no problem for most microscope objectives, 
including the conventional, visible-range ones. Any "fluorite" or 
"apochromatic" microscope objective should give excellent-quality 
images at this wavelength. 

The special Schwartzchild-type catadioptric microscope objectives 
give a much higher transmission at shorter wavelengths, at which 
glass optics are essentially opaque. They have been made with N.A.s
as high as 1.00, I seem to remember, and consisted of quartz/
fluorite lens elements and aluminized glass mirrors. It is not
necessary to use these costly objectives for 380 nm work.

Writes also zrepachol@cc.curtin.edu.au:

> A 'standard' micro mix is a special lens correct solely for one of 
> the Hg emision lines ( can't recall which one, but is in the 
> extreme blue/near UV range ) with glycerine imersion. Zeiss 
> have/had a lens for this, but I can't find my catalogue, so I have 
> refrained from posting till now.

Yes. There have been various "monochromats" such as this. The use of 
glycerin as an immersion medium for high-NA UV objectives is VERY 
old - goes back to the days of cedar oil.  Cedar oil fluoresces 
strongly under UV, which glycerine does not.  Newer synthetic 
immersion fluids do not fluoresce either but I don't know whether
anyone has designed UV objectives to take advantage of them.

> Names to try: Zeiss, Wild, Perkin Elmer, Olympus, Nikon, 
> Rodenstock, Schneider.

Yeap. I would try Olympus especially as a possible top-quality but 
lower-cost alternative over Zeiss. 
 
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    Victor Porguen                                      
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