Fun with filters: Assessing a 400 nm bandpass interference filter on a tungsten lamp to improve resolution as an alternative to an LED.

by David Walker, UK



Comments to date

A narrow bandpass 400 nm filter looks useful for extending suitable objective performance with the microscope's tungsten lighting, with a quick switch from visual to short wavelength camera work by just adding the filter to the field iris. Normal size interference filters (eg 25 mm) are expensive but small ones just sufficient for the field iris aperture at higher mags are more affordable.

Quite modest NA objectives may give noticeably better results than with a typical deep blue glass / acetate filter and far superior than with green. The choice of objective can be important; 'fluorites / semi-apos' and apos which are corrected for spherical aberration for blue may be suitable.

The emission at 400 nm is smaller at lower filament temps, so 100W lamps would be most suitable although was under-running the Zeiss lamp. So lower wattage lamps may be sufficient if a camera is sensitive enough. I also tried a LOMO 6V / 15W lamp and gave sufficient output with the camera used when set at full gain. Although suspect it was using the upper end of the 420 nm band transmission at the low colour temp of ca. 2850K.

Using a 400 nm bandpass filter can't match the striking results some enthusiasts have been sharing with LED lighting emitting as low as 365 nm (near the transmission limit for normal glass optics), but for the hobbyist who doesn't wish to venture into non-visible wavelengths, a 400 nm filter can be worth a try.

For hobbyists looking for a narrow band deep blue - violet filter without going as low as 400 nm, a slightly higher wavelength interference filter for 420 nm could be tried with potentially higher light output from a tungsten lamp eg Knight Optical's 420FIB12 which has a similar bandwidth to an LED.

I'd be interested to hear of what short wavelength filters on a tungsten lamp other enthusiasts have found useful and cameras used.

Comments to the author David Walker are welcomed.




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Published in the August 2008 edition of Micscape.

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