what is it we call color there are
physical colors or spectral colors each
with its own wavelength between 400 and
700 nanometers in all those colors
combined form white a prism separates
them but the perceived color is so much
more than the simple physical color and
that’s because we don’t measure
wavelengths we see color by adding
numbers let’s see how it works imagine
I’ll have two lights on a black screen
imagine I have a third light that’s also
white just like the two above but this
one has first gone through a slipped in
a prison so the colors are spread out
they are dispersed at the bottom of the
screen I now switch off one of the
lights and get back to that later so
these are both white lights the top
circular one has all the colors
overlapping to make white the one at the
bottom has all the colors dispersed
imagine now that I put a red filter in
front of both this white light all other
colors are stopped by the red filter and
red is transmitted I now repeat the
experiment with these two white lights
and I use a green filter so only green
is transmitted imagine now that I use
two white lights with two filters one
red one green and slowly move them
towards one another so that they’ll
overlap to form a third color and when I
do that
we’ll see yellow a beautiful bride
yellow but wait a second didn’t I just
say that there’s no yellow being
transmitted by either or the two filters
indeed there isn’t there are no yellow
wavelengths where we see yellow at the
center of the screen so there’s no
yellow but we see yellow that’s quite
remarkable why is that let’s look at it
quantitatively we perceive color with
light-sensitive neurons in our retina
called cone cells or simply cones there
are three types of cones with a highly
imaginative names type 1 2 & 3 these are
the sensitivity curves for these three
types of counts so if for example we
look at a green laser pointer we can
trace it as a vertical line and we see
that it’s green wavelength is there on
the horizontal axis of these curves and
we can look at where that line crosses
the sensitivity curves so we can read a
number on the vertical axis let’s say
between 0 and 20 just to keep the
numbers simple and the units are nerve
pulses per unit time so when we look at
the green laser pointer we see the type
to send type 2 cones send the brain 17
nerve pulses look at the red laser
pointer we see that type 3 send 8 North
pulses to the brain type 2 sends 0 and
type 1 sin 0 in both cases if we look at
both of them green and red laser
pointers pointing at the same spot on a
screen same spot on the retina we will
see a combination of these signals so
that type 2 will send the brain 17 nerve
pulsus type three will see nine from
green eight from red and it will add
them up so we will do nine plus eight
equals 17 it literally does the math
it’s the neurons in the retina that do
the math not the brain this is what I
mean when I say that the eye is additive
so when we look at red and green
simultaneously the brain will receive 3
numbers 0 from type 1 17 and 17 from
type 2 and 3 but there is another color
that will send the brain precisely those
same three numbers 0 17 and 17 and that
color has a single wavelength the brain
has no way of knowing if it’s perceiving
those 3 numbers from a yellow wavelength
or from red and green wavelengths they
have no young Winton and that’s why we
see yellow either spectral or mixed
yellow and that’s great because this
makes it possible for us to mix
different amounts of three primary
colors red green and blue to create any
color and that’s what computer screens
do they emit light from every pixel with
different amounts of red green and blue
those are combined if the eye of the
observer in color is seen for example
let’s look at an abalone shell showing
off a myriad iridescent colors there are
only three colors in this image red
green and blue if I split that image
into three separate images each with one
of these three colors you see that the
amounts vary from place to place in each
of the three images from black to white
if I now color these grayscale images
and make them go from black to red or
green or blue and slowly move them so I
recombine them I can recreate the
original color image where every pixel
has three numbers the amount of red
green and blue which define the color of
that pixel instead of doing another
demonstration on this computer let’s do
an experiment to verify that it’s the
neurons in your retina that are additive
so it’s your own eye
that’s additive no numbers involved
don’t worry look at the X at the center
of the heart keep looking at it so you
become red adjusted this doesn’t work at
all if you move your eyes so please be
steady in a few seconds I’m going to
switch to another slide in which there’s
nothing only that X precisely in the
same location and please look at that X
as well when you become red adapted you
stop perceiving red that’s how the brain
works a repeated stimulus is
uninteresting therefore it’s filtered
out now you should be nicely read
adjusted so I’m going to switch to
another slide with no heart and you
still see the heart
now focus on the color of that heart
it’s turquoise also known as cyan we
suppress your perception of red
therefore this white screen which is in
fact red green and blue looks green and
blue only to you in your eye ads green
plus blue and you see cyan which
demonstrates that your eye is additive
let’s now look at the transmittance
curve for a red filter you see it has a
broad spectrum is not like the single
wavelength laser pointers that I showed
earlier so it will absorb blue and green
it will transmit red light all the broad
range all red like colors it’s
complementary color cyan transmits blue
in green it absorbs red so it does the
opposite what the red filter did in fact
the spectrum for cyan is the mirror
image it’s the flipped upside down image
of the red spectrum imagine sending
lights through these two filters will
have a red light as I am light the cyan
will contribute red sorry
this again will contribute blue and
green the red will contribute red and
together they will add up to form red
green and blue which is white this is
the definition of complementary colors
colors are complementary when added up
they form white the thing goes for two
other complementary colors green and
magenta blue and yellow and you see that
in all cases the bottom spectra are the
mirror image
to the top ones the top colors RGB are
complementary to the bottom colors see
my it’s easy to remember which is
complementary to which if you recall
them in the familiar order RGB years in
RGB monitors or see my is in color
cartridges for color printers let’s do
an experiment in which I have the lights
in the same order red green and blue and
see my at the bottom I now slowly move
them so they get to overlap and where
they overlap we see white the color we
perceive is also influenced by the
surrounding color and specifically by
the brightness with the surrounding
color so here I have an orange disc that
looks bright now I make it dimmer and
dimmer and less bright and darker all of
those are synonyms they all mean
precisely the same thing so darker
dimmer less bright and it looks orange
we all still think it’s Orange
but if I surround it with white it’s
distinctly brown that’s because the
brain interprets the relative brightness
not the absolute one so the same dark
orange which is in fact brown Browns
nothing but dark orange looks different
on a black and a white background looked
bright or dark respectively an even more
striking case in which we see this
effect that the brightness is
interpreted
relative to the surroundings is provided
in these two paintings of made in which
we see a Colin casting its shadow over a
checkered floor if I told you that those
two tiles labeled L and D for light and
dark we’re in fact the same shade of
gray you probably wouldn’t believe me I
assure you
I made them from the same paint but I
can show you if I mask off the rest of
the painting first slowly and then
quickly did they are indeed the same
shade of gray let’s go back to the
original keep looking at those so that
you know that it’s really the same and
yet it doesn’t look the same now we know
it we still think that L is light in
this dark and that’s because the tiles
around L are dark those around dr.light
therefore we interpret them as light and
dark and the effect is striking despite
the fact that the surrounding tiles are
incomplete and the L end details are
very close to one another so clearly
this is another example in which the
perceived color and the physical color
differ so we see color by doing additive
color mixing in our eyes we didn’t
invent additive color mixing
we simply exploit it exploit our eyes
additive nature on all our devices my
group and I once did an experiment in
which we
he measured how many different colors we
can distinguish and the result was one
million colors that may seem like a lot
but we only have three types accounts
other animals have more more than any
other the mantis shrimp it has 16 can
you imagine how colorful the world would
look like if we had 16 types accounts I
can’t but if I could I think that this
RGB image would look almost black and
white thank you [Applause]