The Color of White Light
The biggest lie in color photography is that you can
represent the colors of objects by simply recording the amount of red,
green and blue in them. This technique - the only one in current
mainstream use - gives good results only
when the spectral sensitivity curves of the camera precisely match
those of the human eye, and when the spectrum of the light used to make
the photo is perfectly smooth, and no different kind of light will ever
It's clear that the first of these requirements is hard enough to meet,
and that the second one is, simply and plainly, never true.
And that's why photographers are always battling to get the right
colors - and never do get them!
Not only in photography is this matter an important one. In daily life
it is, too. Lots of electronic technicians hate that stupid problem of
not being able to correctly read resistors. It happens that old-style
resistors, and some other parts too, are labelled with color bands or
dots, instead of numbers. Under some lighting conditions it can be hard
to tell a red from an orange, or a green from a blue. This leads to the
wrong resistors being installed in equipment, and thus more
Housewifes know the same kind of trouble, for example when trying to
color-match a button to a shirt. Indoors under the electrical light
they find the exact right button, that matches the others, sew it on,
and when the dear hubby goes outdoors next day, !BANG!, that button
sticks out like a sore thumb!
Well, I have to admit that many housewifes these days don't know how to
sew a button to a shirt, but they tend to have the same kind of
trouble when getting their make-up just right, only that outdoors it
doesn't look right any longer! And that's a big
get a better grip on the problems of light color, I built myself a
spectrograph a few days ago. It has been a lot of fun so far, so I'm
making this colorful web page, both to bring my results into an orderly
shape, and to let other people learn from them.
The spectrograph is a simple attachment for my DSLR camera. Using my
lathe, I made a two-part piece of plastic tubing, that assembles in an
angle of 146.6 degrees. At the junction of the two parts, a diffraction
grating is installed. It's an inexpensive foil-type grating that has
1000 lines per mm, which I bought on eBay.
Each end of the angled tube screws into the filter thread of a lens. I
used 50mm lenses on both sides, but other arrangements are workable
too. The camera's lens stays focused at infinity, while the additional
lens, uses as a collimator, has a narrow slit installed in the center
of its focal plane. I made that slit by hot-gluing two pieces of hobby
knife blade over a central hole in the cap, on its inside. The blue
pipe is simply PVC water pipe, machined on the lathe to press-fit the
lense's bayonet, and to have 42mm length. This allows using
lense's focusing ring to bring the slit into the exact position, and
thus focus the entire spectrograph.
Looks cool, eh?
There are several web sites describing the construction of such
spectrometers in greater detail. Many of them don't use the second
lens, and instead use simply a long tube, placing the slit at a long
distance from the diffraction grating, and focusing the camera's lens
at that distance.
The work being done, let's go and play.
Let's start by
looking at a full,
pretty smooth spectrum of light. It's 1500 pixels wide, so it would be
best to set your browser window wide enough. The left border must be
roughly at 700nm wavelength, maybe a little lower, while the right one
is slightly into the UV range, a little bit shorter than 400nm. I got
this spectrum from a blue-tinted "daylight" incandescent bulb.
The stage being set, let's look at some single-color LEDs. Here is a
red LED dating from roughly 1988. It's pretty far down in the deep red
range. It looks very dim to the eye, just enough to use it as an
indicator light on a front panel.
The following one is a modern, high efficiency red LED. It works much
higher in the spectrum.
Next in the spectrum comes an old (1988) yellow LED. The spectral range
that looks yellow to the eye is pretty narrow, and those old LEDs had a
fairly broad bandwidth. It spans from the red over the orange and
yellow, well into the green. It looks a dirty yellow to the eye.
This instead is a modern "orange" LED. It has a much narrower
bandwidth, allowing a more precise color definition. It looks a
beautiful golden color to the eye.
Then comes an old green LED. As you can see, it's not cleanly green,
falling into the lower half of the green range, and
into the yellow and orange. It looks green enough to the eye, partly
thanks to a green tinted housing! Without that tint, I guess this LED
would look rather yellow.
Instead a modern green LED has a far narrower bandwidth, and is better
settled in the green range. I'm not sure where the Moiré interference
in this spectrum came from. I don't think it's an actual ripple in the
What you can see here is a historical device, straight out of my
museum: The very first blue LED I bought! It was very expensive, and
gives a very dim glow, but with some goodwill it looks blue
indeed! I bought it just to show off with it, in a time when
people still thought that blue LEDs were impossible to make! I
understand it's a silicon carbide LED.
As you can see, like all old LEDs it's pretty broadbanded, and is
centered in the cyan range rather than the blue.
This instead is a modern, true blue LED.
And what follows is a LED that was sold to me as "ultraviolet". I can't
tell how much actual UV output it has, because my camera and my lenses
will cut off any higher UV. But it looks promising that I had to
lengthen the crop of my spectrum image, to fit the far violet part of
the spectrum, which I had cut off before because I though my camera
wouldn't see anything there!
Note that it also has some faint green and red output.
As you can see, there are LEDs for pretty much any part of the visible
spectrum. In my collection I don't have any clean, nice, narrowband
cyan LED, but it surely exists too!
Let's go to actual lamps now. The above LEDs are intended mostly as
indicators, having strong colors. Lamps instead are normally designed
to produce the whitest possible light, or let me say, a light as close
as possible to daylight.This is because the standard for color vision
and reproduction is how it looks in nature, under the open sky. This
can be either a sunny day, with the proper mix of yellowish sun and
blue sky shining down, or a smooth layer of white clouds, which mixes
together the light. Of course, clouds do absorb some wavelengths more
than others, and so the color of cloud light is _not_ the same
as well mixed light of a sunny day. Matters ain't
The smoothest spectrum comes from blackbody radiators. Plain
incandescent glow bulbs are such blackbody radiators - only they aren't
hot enough! To produce the holy grail in perfect lighting, we need a
blackbody heated to about 5000 kelvin, which is pretty close to 4700
degrees Celsius. The problem is: What material can be used, that
survives such temperature while remaining solid?
So we use tungsten, the metal with the highest melting point of all,
and heat it up as much as we possibly can, without
it vaporizing too fast. That's the humble glow bulb. What we get is a
smooth spectrum, but poorly balanced, with intense red and
blue, and almost no violet:
The above is a 25W bulb, while the one below is a 100W one. The spectra
of these two are almost identical. Both look very yellow to the eye.
Engineers searched desperately for ways to increase the filament
temperature, to improve the blue end output. This led to the
development of halogen glow bulbs, which use a high pressure halogen
gas to recover tungsten that went off the filament, and re-deposit it
where it belongs. This allows running the filament of a halogen bulb
slightly hotter than that of a common bulb, leading to slightly
stronger output in the blue, and thus an overall better balance:
And there was also a brute-force approach to getting whiter light from
incandescent bulbs, which was simply to add a blue filter, by tinting
the bulb blue. This works not by increasing blue, but by
_decreasing_red! So it does achieve the goal of producing a well
balanced light spectrum, but the efficiency, already very low for any
glow bulb, simply gets ridiculous! This spectrum, the same I displayed
for reference at the beginning, is of a Philips 60W
"daylight" glow bulb:
As you can see, it has a little bit more blue light than the halogen
bulb, making it the smoothest light source in my collection, with the
best color rendering and visual impression - but also the least
For comparison, here is the spectrum of true daylight - the sun, plus
blue sky, at mid afternoon, so it should have an average daylight
balance. It can be seen that the "daylight" lightbulb, even with its
blue glass, is significantly redder than real daylight.
The dark stripes in the sun spectrum are the Fraunhofer
Now that we have seen the spectrally smoothest light
let's see the most "adventurous" ones: Gas discharge lamps! They are
essentially pure spectral line radiators.
This is the spectrum of neon, obtained from one of those
neon indicator bulbs, that look - can you guess it? - red/orange:
And here comes the spectrum of of a discharge through mercury vapor,
which is used in many lamps. It looks green/blueish:
The spectrum above was obtained from a metal halide lamp, right after
switching it on, while it was still cold. The other metals in it don't
get excited by the electrical arc, so they don't glow yet. But a few
minutes later the lamp has warmed up, and the other metals in it get
thermally excited, adding a lot of spectral lines of their own. That's
when the light of a metal halide lamp turns white to the eye, and very
It's no wonder that such lamps just cannot have a very good color
rendering, right? Most of the visible spectrum gets no light at all! An
object reflecting just narrow ranges of the spectrum might
appear completely black in this light, while it would have bright
colors when seen in daylight, or under a glow bulb. And another object
that looks red in daylight might look green under a metal halide lamp,
simply because it has a strong reflectance in the deep red, like
650-700nm, and a weak one in the center of the green range. Under pure
smooth white light, the strong red reflection will dominate over the
weak green one, and the object will look bright red, maybe with a
slight orange tint. Instead in metal halide light it will look green,
because the lamp has no output at all in the deep red range, so that
the weak green reflection of the object makes it look perfectly green
when lit by the lamp's intense green spectral line!
It's a philosophical question whether such incorrect and even
unpredictable color rendering is the fault of the lamps, or of any
objects, dyes, and pigments that reflect only narrow bandwidths. But my
view is that we have far more such objects and dyes and pigments, than
we have lamp types, so lamps are easier to control! That's why we
should not use any lamps with strong line spectra, whenever we want
decent color rendering.
There is a special kind of gas discharge lamp, very well known to the
photographer: Xenon flash tubes. I made a spectrum of my Pentax AF-280T
flash, and once more, I was surprised. Although there are well defined
lines, they almost blend into a very smooth, even broadband radiation!
For many years, the fluorescent lamp has been the mainstay of lighting.
It's efficient, cheap, long lived, and very versatile. Can one ask for
more? Sure! One could ask for good color rendering, which fluorescent
lamps just don't do.
A fluorescent lamp is a long glass tube , either straight or bent in
various ways, in which mercury vapor is electrically excited. The
mercury emits its characteristic spectrum, and a coating of special
materials covering the inside of the tube walls converts some of this
radiation into other colors. These special materials
"phosphors" for purely historical reasons, given that modern
"phosphors" don't contain any phosphor (chemical element) at all!
These lamps come as straight long tubes, or as bent tubes, to be used
with external control circuits (ballasts), or they come as very
compactly bent tubes with integrated ballasts, as "compact fluorescent
lamps", CFLs, or "energy saving lightbulbs" - even if they are no bulbs
but tubes, and they don't save any energy (in absolute terms)
like any lamp does!
Old fluorescent lamps were said (on many websites!) to be "two-color
lamps", combining the blue mercury glow with a yellow phosphor. That
would produce brutally poor color rendering! Like so many things one
can read online, and in books, this is only a half truth. It turns out
that this "yellow phosphor" emits a smooth spectrum over a wide
bandwidth, spanning from pretty deep in the red, all the way into the
cyan and even blue range! The spectrum below, taken from an Oryom 18
watt "warm white" CFL, illustrates this very clearly: The
characteristic spectral lines of mercury ride on a very broadband glow
centered in the yellow range.
Let's compare this to a more modern, "three-color" CFL, having
same "warm white" color temperature of 2700K. This is from an Osram
Dulux Star 8W CFL, that claims a color rendering index better than 80%,
which is pretty good - and hard to believe! Although the intense
blue/violet mercury line is largely absorbed by that lamp's phosphor,
there are many broad areas in the spectrum that get essentially no
light. This is very
far from a smooth, even spectral distribution!
Let's compare this to a Westinghouse 8W CFL, which is of the "daylight"
type, specifiying a color temperature of 6500K. It's
obvious that it uses the same phosphors as the lamp above, but has
quantities of them, so that the red is weaker and the blue is much
And this is from a Philips Ecohome 23W CFL, also having a color
temperature of 6500K. The spectrum is for all practical purposes
identical to that of the lamp above. These lamps were all made in
and even if they weren't made by the same factory, surely the
phosphors are made and mixed by the same company...
The last CFL on this page is a General Electric 20W one, which I bought
because it is of the rather rare neutral white color, having 4000K
color temperature. It's the color I like best, by far. The spectrogram
shows nothing unexpected: It also uses the same kinds of phosphors, in
a mix that is in between that of the 2700K and the 6500K lamps:
I would like to test the spectrum of any fluorescent lamp with
color rendering specs, specially the ones rated for color 940, which
means 4000K and above 90% color rendering index. Unfortunately there is
no place whithin my reach where I can buy any! I have searched all
electrical and hardware stores in the wider area, and there are none!
But the catalogs of big manufacturers, such as Osram, do list them. I
would expect such high-CRI lamps to fill out the dark areas in the
spectrum, and further dampen down the bright ones. Perhaps they even
their range into the deep red! That would mean less efficiency, of
course, because the human eye is less sensitive at the extremes.
Nothing comes for free in this world.
Until a few days ago, I believed what I had read on several web sites:
That white LEDs are basically blue LEDs coated with a phosphor that
adds either yellow light in the cheap ones, or red and green light in
the better ones. Now that I'm the proud owner of a spectroscope, I was
able to see the truth myself. And here it is, my dear readers! I will
start by showing you the spectra of an assortment of white ("daylight")
Cheap 3mm 20mA LED bought on eBay:
8mm half-watt LED for general lighting, 160 degrees
dispersion, from eBay:
10mm half-watt narrow focused LED, from eBay:
These three seem to use the same kind of phosphor. You could call it
"yellow" phosphor, but it's so nicely broadbanded that we could almost
call it "white minus blue" phosphor! The exact wavelength of the main
blue radiation varies with manufacturer, LED type, and maybe even with
the batch. The biggest spectral defect I find with these LEDs is that
they have a depression in the cyan range, which is deeper in those LEDs
that have their blue output higher up in te spectrum, and narrower. But
they all make pretty decent all-color lamps - very much
better than CFLs, in any case, let alone metal halide lamps!
Now see this 5mm, 20mA, focused LED, bought locally. The blue is
shifted down, it has nothing at the upper edge, and is weak in the mid
red - of course it has no deep red output at all. This one has poorer
color rendering, but looks extremely bright, thanks to concentrating
its output where the human eye is most sensitive:
Let's see the "warm white" LEDs now, those rated generally for color
temperatures around 2700K. First, a 3mm 20mA cheapy from eBay:
The 8mm half-watt high dispersion one from eBay:
And the 10mm focused one from eBay. These three are the "warm white"
companions to the three eBay LEDs further up.
Clearly these three warm white LEDs follow the same pattern as the
three cold ones, in terms of shifting the blue center around depending
on the exact model, and having a hole in the cyan. And as is logical to
get the low color temperature, they have a lot of red output. But wait
- the red looks more extended too! Could it be that these warm white
LEDs use two phosphors, one being the same wideband yellow the others
use, and the second being an extra deep red phosphor? Or maybe they
use a wideband red and a wideband green phosphor? I don't
But they are pretty close to being a full-spectrum source, if it were
not for the hole in the cyan range!
Let's look at bigger LEDs. This is a 1 watt, warm white LED that comes
mounted on a star board. It seems to have a similar blue
spectrum, and similar phosphor, as the 5mm LED shown higher in the
page, but with more phosphor to give the lower color temperature. It's
not as widebanded as one would want:
And this is my most powerful single LED: A Cree X-lamp rated for 5
watts, in a tiny SMD format! It has the color temperature I like so
much, 4000K. It has a pretty wideband and very smooth response, almost
without a hole in the cyan range! But in the deep red it's weak, and
the cheapest LED of all in my collection, the 3mm one from China, is
better in that regard...
Finally, there is also another kind of white LED, that uses no
phosphor, but instead has a red, a green and a blue LED chip in the
same package. Here is the spectrum of one of those. Three narrow bands,
and nothing else. These might be quite efficient, but I wouldn't expect
great color rendering from them:
Now this is a bit hard to understand for me: I see lots of fluorescent
lamps advertised to have better than 80% color rendering index, and in
catalogs at least I see some rated at better than 90%, while at the
same time white LEDs are typically rated at less than 80%. Judging from
my spectrograms, instead, LEDs seem to be far better than CFLs in this
regard! Is this a case of incorrect hearsay being propagated so much
that it has become an established pseudo-truth, or am I missing
something important enough to turn around the whole
It seems to me that among the light sources shown on this page, some of
the white LEDs are the best. Only the blue-tinted incandescent bulb can
top the LEDs in terms of spectral smoothness - but there we would be
comparing the most efficient to the least efficient light source!
Between a modern white LED, and a blue-tinted incandescent, there is
roughly a 20:1 ratio in efficiency! With LEDs being among the most
efficient lamps in existence, and also among the ones with the
spectral distribution, and apparently being the only
lamps that combine both, they are the way to go!
Clearly CFLs have very
irregular spectra, and metal halide lamps are even worse.
My home is currently lit almost exlusively by fluorescent lamps of
different types. It seems time now to replace them by LEDs. But not
just by any LEDs. It's probably a good idea to buy a few samples of
several different types and brands, test them all, and then decide
which to buy in quantity for the home.
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