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I'm a big fan of critical illumination, but I can only second to Craig: a
big LED won't help you. You need a bright LED!
There is a pretty strict thermodynamic rule saying that you can never
increase the brightness of your light source (a solid light-emitting
surface) with just lenses and mirrors. More generally it has to do with the
etendue mentioned by Craig. (The well known ways to get around this
limitation are luminescent rod concentrators and all optically pumped
lasers.)
As an example, wit 1mm x 1mm LED chip the best you can do is to image it
into your sample with a (hypothetical, ideal) 1:1 relay with NA=1. If you
make your image larger, it's brightness will naturally go down. If you make
the image smaller, then the LED-side NA of the (ideal) relay will
necessarily be less than 1 and you will fail to collect all the light from
the LED chip, and as a result the image brightness will NOT increase.
Of course, bigger LED will make the optical design easier, but there is
little benefit in sticking a 25-mm diameter LED chip right in the conjugate
image plane - most of the light will be blocked by the objective back focal
plane aperture (that is, miss the objective lens, and even the tube lens
completely), an you would have to drive the LED at 500 Amps to get to the
decent value of 1 A / mm^2 (state of the art LEDs work at 2 A / mm^2).
So, 5 mm diameter LED is not a huge overkill, especially if you want to use
low-magnification high-NA lenses, but with a typical 60x lens less than 2%
of the total light output of the LED will be delivered to the sample.
Note also, that many LEDs have a structure of electrodes on the top
surface, so you may need a diffuser of some sort to blur their shadows.
Best, zdenek
On Fri, Jan 11, 2019 at 1:04 PM Antonio Jose Pereira <[log in to unmask]>
wrote:
> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> Thank you for the directions.
> Craig, it's a critical-illumination configuration that we'll have to be
> using. We're considering some possibilities such as a laser-illuminated
> dynamic diffuser as an (effective) source. Still, there seem to be also
> some interesting large surface LEDs around ... Let's see.
>
> Thanks!
>
> Antonio
>
> ----- Original Message -----
> From: "Craig Brideau" <[log in to unmask]>
> To: "CONFOCALMICROSCOPY" <[log in to unmask]>
> Sent: Friday, 11 January, 2019 16:11:14
> Subject: Re: large surface LEDs
>
> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> Large n x n emitters are good for flood filing a large area with light, but
> if you are trying to focus the light it will not be easy due to etendue
> issues. If you are trying to focus the light you are better off with a
> single emitter of the highest possible power. On the other hand, if you
> *are* trying to flood fill an area with light, the n x n emitters will work
> well, although you may need an additional diffuser to have even energy
> distribution across the sample. Some engineered types of diffusers are
> pretty good at this.
>
> Craig
>
> On Fri, Jan 11, 2019 at 8:10 AM Diego Pizzagalli <[log in to unmask]>
> wrote:
>
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Hello,
> >
> > I think that the search tool provided by the component provider RS-ONLINE
> > is powerful for this.
> > If you go in the section visible LEDs, their website allows to filter
> them
> > by size.
> > For instance the Lumex TitanBrite with code 888-6237 are 12mmx12mm
> > (effective 5mmx5mm) flat SMD LEDs with 800lm.
> > There are even larger.
> > Here the link for reference
> > https://it.rs-online.com/web/p/led-visibili/8886237/
> > Regards,
> > Diego Ulisse Pizzagalli
> >
> >
> >
> > Diego Ulisse Pizzagalli
> > PhD Candidate and Teaching Assistant
> > Institute for Research in Biomedicine - Bellinzona (CH)
> > Institute of Computational Science, Universita' della Svizzera Italiana -
> > Lugano (CH)
> >
> > ________________________________
> > Da: Confocal Microscopy List <[log in to unmask]> per
> conto
> > di Lukas Kontenis <[log in to unmask]>
> > Inviato: venerdì 11 gennaio 2019 14:32:03
> > A: [log in to unmask]
> > Oggetto: Re: large surface LEDs
> >
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Have you considered Cree XLamp XR-E or MX-6 LEDs? They are physically
> > large and have brightness in the 250-350 lm range, not sure if their
> > emitter area is quite that big.
> >
> > Regards,
> > Lukas Kontenis
> > Light Conversion
> >
> > -----Original Message-----
> > From: Confocal Microscopy List [mailto:[log in to unmask]]
> > On Behalf Of Antonio Jose Pereira
> > Sent: Friday, January 11, 2019 2:13 PM
> > To: [log in to unmask]
> > Subject: large surface LEDs
> >
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> > http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> > Post images on http://www.imgur.com and include the link in your
> posting.
> > *****
> >
> > Dear all,
> >
> > I'm looking for LED chips with high power (as those suitable for
> widefield
> > fluorescence) but with a quite large emitting area (>5x5mm)?
> >
> > Any suggestions? Thanks!
> >
> > Best,
> >
> > Antonio Pereira
> > i3S/IBMC, Universidade do Porto
> >
>
--
--
Zdenek Svindrych, Ph.D.
Research Associate - Imaging Specialist
Department of Biochemistry and Cell Biology
Geisel School of Medicine at Dartmouth
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