> Hi, well it's been a long time since I contributed to sci.optics -
> about 20 years I guess. It's great to see that it's still active.
Well, there are some diehards in the world. You should join up--there
are still a few openings. ;)
> A quick question on photometric units - I'm trying to compare the
> light output from Oleds with that from LEDs. I'd like to compare
> total flux emitted by the two sources. Here's my calculation:
> An LED die has 30 candela output A 1 candela lambertian emits pi
> lumens into a hemisphere. Therefore the LED emits 30pi lumens.
Assuming that it's really Lambertian. Some are reasonably close, mostly
the ones with textured surfaces. LEDs with flat facets aren't
Lambertian because of Snell's law and the Fresnel reflection at the
surface, both of which change the angular distribution of the emission.
> An OLED emits about 400 nits which is 400 candela/m^2 The OLED is
> 18mm x 7mm , so the OLED emits 400 * 0.018 * 0.007 candela = 0.05
> candela As above, assuming the OLED is a lambertian emitter OLED
> emits 0.05*pi lumens.
> So my conclusion here would be that my LED has a flux 600 times that
> of the OLED.
It depends on the surface roughness. The two main problems with LEDs
are: (1) while the emission is almost 100% efficient, it occurs deep
within a highly-absorbing substrate; and (2) the substrate has a
refractive index of 3.3 to 3.5, so that almost all the light gets
internally reflected at the surface. The great strides in LED
efficiency that we've seen in the last few decades mostly come from
fixing those two problems.
A good chunk of the confusion IMO comes from the nomenclature. I mean,
how scientific-sounding are quantities such as "luminous intensity" and
"spectral radiance"? They sound like something out of Edgar Allan Poe,
or maybe a romance novel. (Give me "flux density" and so forth any day.)
Much of the remainder comes from People Who Know Best deciding to
redefine perfectly well-defined pre-existing terms to mean something
completely different. For instance, "intensity" means watts per square
metre to normal people, but watts per steradian to a radiometrist.
> My question is about the nit to candela calculation - I've not found
> any references to this. It seems sensible, but photometrics are a
> very strange beast!
Yup. There's a lot of job security stuff in radiometry, and even more
in photometry. Making the candela a SI base unit is weird as hell, for
a start. The lumen is derivable from the joule, the metre, and the
second, times exactly 683 lumens/W peak and the Anointed International
Photometric Eyeball curve. (As I tend to point out to any vaguely
interested person, 683 appears to be the largest prime number ever used
for unit conversion.)
However, they standardized the _candela_, which imports the idea of
solid angle. Which solid angle--projected or spherical? There's a
factor of cos(theta) difference between the two, and the choice depends
on whether you're illuminating a surface or a volume.
How do you make a traceable measurement of solid angle, when the
transmittance of an optical system depends on both angle and position?
Confused yet? (Me too.) And then there's the nearly impenetrable
thicket of redundant units.
But yeah, OLEDs are pretty dim in general.
Dr Philip C D Hobbs
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510