This is description of this part:
Professional glass filter Carl Zeiss Jena U204g, 32 mm in diameter, intended for fluorescence excitation with ultraviolet radiation.
Basic parameters of this filter:
Manufacturer: Carl Zeiss Jena
Model: U204g
Filter type: glass excitation filter for UV fluorescence (the manufacturer calls it Erregerfilter)
Glass type: UG 1
Diameter: 32 mm
Thickness: 2 mm
Colour: black-violet, passes very little visible light
Toughened version: yes
The g at the end of the designation stands for the German gehärtet, meaning toughened. The manufacturer gave it to filters meant to work right next to high-power lamps, especially HBO mercury burners, where ordinary glass does not withstand the high temperature. With a UV excitation filter this matters especially, because it sits at the hottest point of the whole path and absorbs most of the burner energy.
Filters 32 mm in diameter can be used in most microscopes from many manufacturers. In Carl Zeiss Jena microscopes they can be used, among others, in the following fluorescence microscopes:
- Carl Zeiss Jena Jenamed Fluorescence (Fluoreszenz)
- Carl Zeiss Jena Jenalumar
- Carl Zeiss Jena Fluoval / Fluoval-2
- Carl Zeiss Jena Laboval Fluorescence
The manufacturer catalogue gives the following transmission values for this glass at 2 mm thickness:
- 0.015 at 302 nm
- 0.093 at 312 nm
- 0.435 at 334 nm
- 0.644 at 366 nm
Across the whole visible range, from 405 to 644 nm, transmission is zero. This means the filter works almost entirely on the 366 nm mercury line, exactly where the HBO burner is strongest in the ultraviolet.
One property of this glass is worth knowing: above 700 nm it starts to transmit again, and at 750 nm transmission reaches 0.210. This infrared leak is invisible to the eye, but it heats the specimen and spoils contrast in photographs.
In the excitation path U204 is also combined with a red-absorbing filter. The manufacturer documentation shows such a pair with the B226 filter, which cleans up the excitation band further.
The same UG 1 glass was supplied in two thicknesses, each with its own designation:
- The Carl Zeiss Jena U204 filter is UG 1 glass 2 mm thick
- The Carl Zeiss Jena U205 filter is UG 1 glass 4 mm thick
The thicker U205 cuts more sharply, but passes about a third less energy on the 366 nm line, so the image is noticeably darker.
A safety note: this filter passes ultraviolet, which is invisible and harmful to the eyes. When working with it, never look into the illumination path without a barrier filter in place.
What is this part needed for?
- if you know the U204g specification and this is exactly what you need (for example for a UV excitation set)
- if your microscope has a Carl Zeiss Jena U204g filter, but it is cracked, scratched or in very poor condition
- if you already have U205 and want to add the thinner option, to get a noticeably brighter image on the 366 nm line
- if you want to use this filter in your own DIY project or in a microscope from another manufacturer (for example Leica, Nikon, Leitz Wetzlar or Zeiss West Germany)
