Natural Science Forum / Physics / Optics / August 2007

Dear Fellow Bug Eyes,

   I'm going through the process of buying my first personal microscope and
am somewhat perplexed. I'm not unfamiliar with microscopes but in my prior
incinerations as an undergrad and grad student someone else always set up
the gear and got the stuff. No lab runs without hidden lab hobbits. Now I
have to do hobbitry myself and am mystified. (BTW - My chief interest is
protists, and live sample microscopy.)
  Today in going thru my purchase criteria comes the mystery of
illumination. Given my Scots heritage pences are the mothers of pounds, so
it would seem that halogens with a lifetime of 200 hours might be unthrifty
creatures to have about and $20 per head, since some sloth on my part in
cutting down on hours will result in early stock mortality. Of course
replacement stock may be available at the local hard goods store, but from
the specifications provided by the microscope vendors there's nay a notion
of alternative supply and a thrifty price.
  Then of course with halogens it would seem using the palm of hand at six
inches tests that IR emissions seem rather high, and so critters close to
such a lamp might seize up and become haggis with some alacrity.
  So cost per hour and specimen survival become and issue and LED
illumination appears attractive. It appears that LED's (around 3W) provide
several times the lumens per watt as compared to halogens, however even
given the low efficiency of halogens it would still seem that an LED at 3W
will provide around 3 photo-stops lower light levels than a halogen. That
would not seem to be a major problem with photography since a few stops
difference can be easily accommodated using most SLR cameras. How it might
affect digital and digital full motion is a question I won't even hazard.
  But there seems to be another problem with LED's. Spectrum. Some kind
folks (I'll dig out the URL on request) has put up a series of photo's
comparing LED and halogen bulbs in color rendition. It appears that the LED
looked reasonable good if a digital camera color temperature was set to 9000
K. That is about in the range of strong sunlight or harsh moonlight. Very
blue. I believe indoor films are cast for an indoor color temperature of
about 3500 K. Rather more red/yellow. Some other kind folks have put up
spectra of various light sources using a spectroscope made from a CD  and a
paper towel tube, and his results show that LED's peak with a strong yellow
and a blue with very narrow spikes, whereas halogen bulbs produce a broad
spectrum from the IR thru the near blue ranges. Hmm.
   Now prior experience in amateur photography had convinced me the Fuji
films produce stunning color and great detail. Some investigation on the net
revealed that others seem to agree. The Fuji films however are balanced to
warm indoor illumination (3500 K range) or a slightly hotter easy daylight
range. So how would a Fuji film function with a hot and narrow LED spectrum?
Now under average circumstances either film types can be shot with either
illumination type, but that is with compensating filters with at exposure or
during printing. But both assumed daylight or indoor spectra have
distributions approximating Boltzman back bodies, and not spike sources.
Hmm. I'm lost. Can you spread a spiked illumination spectrum using filters
or diffraction grids or some similar optical marvels? Got me.
   Reviewing this I've noticed my questions are cast from a film
perspective. I'm sure a fair evaluation of LED vs. halogen should also
consider CCD characterists. I also realize that my questions might really be
stupid - hell millons of folks do micrscopy just fine, of the shelf. Must
work somehow.

Cordially,
Rob