The video you are watching now is gradually increasing in brightness.
Brightness is a subjective term.
We “perceive” it.
You could pause this video, examine it frame by frame, and tell me if one frame appears
brighter than the previous frame to you.
The human eye perceives differences in brightness a lot better than it does differences in color.
This is an important fact behind how we’ve elected to store and transmit video.
There have been numerous ways to hook up video output to a television over the years.
What signals do they provide?
This video focuses on “Luma” – the brightness or “black and white” component of video
that is transported in several of the cables seen here.
But what IS luma?
How does brightness relate to color?
How is Luma created from some amount of red green and blue?
Why did it stick around after the move to color television in the mid-20th century?
In order to understand what luma is, I believe it is a good idea to understand the journey
of our understanding of light.
In order to do that, we need to go back in time.
1672.
Isaac Newton proves that light is responsible for color.
Using a prism, Newton split clear white light into seven visible colors – the same colors
we see in a rainbow – and then recombined them into white light.
Essentially, he scientifically established our visible spectrum.
Color exists only in the eye and the brain.
It is our reaction to detecting different wavelengths of light.
Our journey isn’t only about understanding and measuring light – but also understanding
how we perceive it.
19th Century – Discoveries are made that prove the existence of light that we cannot
see including gamma-rays and x-rays.
Our visible spectrum occupies a very small portion of the light that is out there.
The value of the wavelengths we see between ultra violet and infrared is about 400 to
700 nanometers.
We also discover that the human eye has three kinds of cone cells that sense light, and
they have peaks of their spectral sensitivity in short (420-440nm), middle (530-550 nm)
and long (560-580) wavelengths.
This knowledge gives us a useful advantage when it comes to intentionally tricking our
eyes into seeing the image we want to reproduce.
1913.
The International Commission on Illumination (CIE) is established.
They help develop basic standards and procedures of metrology in the fields of light and lighting
among many other things.
You have a difficult question about light?
They have an answer.
1924.
The CIE publishes the Luminous Efficiency Function.
This function is subjective by nature as it considers human perception of light.
Observe two lights – One blue, and one green.
If the lights have equal radiance, how bright would they appear to you?
Would they appear the same?
In reality, the green light is brighter than the blue light because we are more sensitive
to green.
If we are to recreate a scene using light on a television, this is an important aspect
to an accurate recreation.
Late 1920s – William David Wright and John Guild independently conduct color matching
experiments using human sight.
Test subjects have a screen placed in front of them that has been split in half.
A color is projected on one side.
The subject would make adjustments to the intensity of three light sources - red, green,
and blue - that are projected to the other side of the screen in attempt to match the
target color.
These experiments showed us that we can break down white light into many colors, generate
those colors using three primary wavelengths, and also manage to mathematically put them
all back together into white light.
People knew this was possible, but now we had a mathematical relationship built on recorded
data to help with our understanding.
Instead of requiring a large number of different wavelengths of light to replicate our perception
of color, we could focus on the combination of three primary wavelengths to achieve an
appropriate response from our eyes.
1931 – The CIE uses the information gathered by Wright and Guild to create the CIE color spaces.
The combination of primaries such as red Green
And blue Can be used in combination to represent human
color perception.
This is the CIE Chromaticity [x,y] diagram.
Our visible spectrum forms the long curve.
All three primaries reach their white level at the circle “E” near the middle.
We have a relationship established between three primaries as well as a mathematical
way to create a given color.
The little x and little y values along the axes help specify a color in the absence of
brightness.
How do we indicate the brightness of a color?
It comes from a third coordinate, Y, called “Luminance.”
Yes, we are actually viewing a three dimensional space.
Watch.
This… might be a lot to take in.
Feel free to pause the video and examine the color space.
You can now see the Luminance axis stretching from the bottom to the top of your screen.
The very bottom – and absence of color - is black, and the very top is white.
The colors appear to increase in brightness to us as our eyes move up the Luminance scale.
If you imagined this as an oddly shaped tent of sorts, the top of the tent pole is the
white point.
If you were to look at just the tent pole inside of all of these colors, you would see
shades of gray stretching from black at the bottom to white at the top.
Putting some intensity of red, green, and blue wavelengths together not only yields
a certain color, but also provides us with a way to determine how bright that color appears.
A weighted sum of this combination of red, green, and blue values yields luminance.
This color science-based methodology of using two coordinates little x and little y to indicate
a color, plus a Luminance value (Y) to help indicate its brightness,
represents a starting point for understanding video.
With the birth of electronic television, we only needed a brightness component – Luma
- as TV started in black and white.
What may surprise a few of you is that the luma signal is still around today.
How do we get from the linear light component of luminance – a term from color science
and physics to the non-linear light component of luma – a term used with video?
That’s right.
There are differences between the terms luminance and luma.
If you have ever experienced the use of the word “luminance” in relation to a video
signal, it is a misuse of the term.
You’re about to find out why.
Luminance is measured in candela per square meter – written like this (cd/m2).
Don’t worry about the unit of measurement.
The numbers will tell the story.
The luminance of an incandescent light bulb is about 130,000.
The luminance of the sun at noon is about 1.6 BILLION.
Retinal damage occurs at about 100 million.
So I ask you – do you want a camera and a television that perfectly replicate luminance?
Probably not.
OK.
All things considered, the casual use of the term luminance generally refers to
“relative luminance” – a much less… dramatic term.
Instead of trying to replicate the absolute or pure luminance of a scene, we normalize
the luminance to a much smaller scale such as from 0 to 1 relative to a reference white.
Photography, movies, video and print use this.
The brightness of the sun would basically be 1 using this scale.
Don’t think of this concept as a rigid assignment of number values to indicate different levels
of brightness.
Given that any analog scale is infinite, we can map it into any range we want – such
as arbitrary voltages or numbers.
Simply from here to here.
Now we just need to get this relative luminance scale to the television.
How do we make luma?
With that question comes the arrival of our final term – Gamma.
Gamma or gamma correction takes the linear ramp of luminance from dark to bright and
raises it to a power.
If you aren’t a math person, don’t worry!
I’ll spare you numbers.
Just watch the graph.
What you see is here is that rather than outputting a nice, even ramp of dark to bright, a gamma
corrected signal is going to take the darker parts and accentuate their differences.
Meanwhile, the differences in the brighter part of the picture are relaxed.
Why do we do this?
Why not just transfer a linear ramp to the television?
A nice, linear increase in brightness.
There are a couple of reasons.
Number 1 is bandwidth.
Our eyes have greater sensitivity to differences in darker tones than lighter tones.
It makes more sense to use a majority of our scale to communicate changes in darker tones.
With gamma correction in place, look how much of the scale is used from black to mid-gray
vs mid-gray to white!
If we can’t see changes in lighter tones as well as darker ones, why should we bother
to allocate the same amount of bandwidth to them?
Let’s let the darker tones take up the majority of the scale!
The second reason we do this is because of CRT physics.
The lightgun in a CRT TV has a non-linear response to voltage levels.
If we were to graph that response, it would look like this.
Rather interesting, huh?
Somewhat opposite of our gamma-corrected signal?
Close to a mathematical “inverse” of sorts?
The CRT’s non-linear response to the voltages of the video signal means it takes a bit more
voltage before you see the brightness start to take off.
What does this all mean?
It means that we can gamma correct linear changes in light to help reduce noise and
bandwidth while optimizing it for our eyes, transmit it to a CRT screen – a device with
an inverse response, and receive a result that appears linear.
Provided the television is decently calibrated, of course.
With our knowledge of relative luminance and gamma correction, we can answer the question
of how to compute relative luminance and luma from Red, Green, and Blue.
I’ll borrow a few numbers from a standard to complete the example.
Luminance is denoted Y.
As mentioned earlier, it can be computed as the weighted sum of linear-light tristimulus values.
Let’s simplify these numbers a bit.
Now you can see that the luminance of a color can be found using roughly 21% power from
Red plus 72% power from Green plus 7% power from Blue.
The numbers you see have changed over the years with different standards, but what is
the key component used to determine what these coefficients should be?
The human element – us.
How we see them.
Now what about luma?
What about video?
Remember the graph I showed you that explained gamma correction?
It comes into play like this: Luma is denoted Y "prime."
The prime symbol is used to indicate nonlinearity.
Luma is the weighted sum of gamma-corrected RGB components.
It is important to note that the value for each of the three primaries is gamma corrected
prior to the calculation.
To summarize so far: 1: The human eye has a non-linear response
to light and is more sensitive at different parts of our visible spectrum.
2: Colors come from our reaction to different wavelengths, and can be replicated using a
combination of three wavelengths of light.
3: Luminance, Y, can be computed as a weighted sum of linear-light Red, Green, and Blue values.
4: Luma, Y’, is used in video and can be computed as a weighted sum of gamma-corrected
Red, Green, and Blue components.
This is an oscilloscope.
If you’ve never seen one before, now you have.
This lets us see an electrical signal over a period of time.
Recall that we talked about signals and time in our 525-line analog video episode.
One of the items we focused on was horizontal sync.
Well guess what – here he is!
Yeah!
You’re viewing an actual horizontal sync pulse for a single line of video on an oscilloscope.
To be specific, this is the composite video signal output from a Sega CDX.
We are viewing a black screen in the 240p test suite, and this is horizontal line #142.
You can also see the horizontal sync pulse to start line 143 right here!
Since there isn’t really anything on the screen right now, let’s switch to the all-white
screen in the 240p test suite.
Ahh ha.
Now we are talking.
So what can you tell me from this signal?
Well, it represents a white line drawn from the left side of the screen to the right side of the screen.
Since the signal is at the brightest white and it goes this high, then white is right here.
And earlier when we looked at the black screen the entire signal just… stayed at the bottom.
So the bottom here is black.
So that means… everything between the bottom and the top must be a shade of gray.
Pretty simple huh?
Let’s take a look at something that isn’t solid white or solid black.
How about this pattern?
What would it look like on the scope?
Of course it depends on the line number you are on!
Let’s split it into sections and work from the bottom to the top.
I’ll add letters to each section for reference.
Section A at the bottom is black.
We already know that that looks like.
If we move up to section B, a single line from this section would look like this.
Do you see the relationship between the scope’s signal and the pattern?
There is a connection, isn’t there?
You can see the brightness increase appropriately for the width of the two bars on the left,
reach this point for the box in the middle, and then show the brightness for the two bars
on the right.
The higher the signal goes, the brighter the signal goes.
If we move up to Section C, the middle box is a bit brighter and the signal a bit higher.
Now Section D.
Now Section E! Wow!
The signal is noticeably higher in that center white box.
And then, of course, the section above that returns to black.
Did this make sense?
Remember each example I have shown you represents the brightness for just a single line from
each section.
OK.
Your turn!
What about this screen?
What would a single line from the top half look like?
Did you guess… stairs leading up?
Yeah!
What about the bottom half?
Stairs leading down.
Look - We can even find the point where we switch from the top half to the bottom.
Looks like line 142 is going up and line 143 is going down.
Finally, let’s look at one example that is a bit more fun.
This software lets us take the raw video signal from a laserdisc and decode it.
It is part of the Domesday Duplicator project.
We are examining a frame of video containing a squirrel.
Underneath the video, you can see a software-based oscilloscope containing the luma signal for
a given line.
After our examination of some simple examples, now we have something much more complex!
Let’s click on a few lines and observe the scope at the bottom.
If I click near the top of the screen, the scope signal is up high as we move from left
to right and dips just for a little bit as we move across the area containing the squirrel’s
head and ear.
The area is mostly in the sunlight.
How about a line from just above the squirrel’s eye?
You can see a peak right here as we chose a line containing that bright white patch
above the eye.
If I select a line containing the eye itself, we are going to move from a previous line
containing white to a new line containing black in the same horizontal location.
Watch the signal.
It dropped!
And if I click under the head down here, you can see the bright area on the left that is
in the sun, and the darker area on the right that is in the shadow.
That is all there is too it.
The higher the voltage, the brighter the image appears.
Now…let’s return to our timeline and see the evolution of television, the addition
of color, and what happened to luma.
1940 –The National Television System Committee is established in the United States.
March 1941 – The NTSC adopts RS-170 as a standard for broadcast.
Television in the US begins operation at 525-lines per frame at a rate of 30 frames per second.
The image is interlaced.
Each frame is made up of two fields.
One field making up half a frame, and a second field making up the remainder.
The TV signal is an early form of luma.
January 1950 – The NTSC reassembles.
Their task is to standardize color television.
They are faced with a simple question – create a new color TV signal for broadcast and invalidate
all TV sets sold over the last decade, or use the signal they already have and find
a way to add color while maintaining backward compatibility with existing sets.
Backward compatibility wins.
Luma remains.
December 1953 - The FCC adopts NTSC Color – named after the committee.
Along with this came a change to the original luma signal in order to accommodate this addition
of color.
The framerate shifted from 30 to 29.97 in order to weave in the color signal.
And thus, composite video was born.
A topic for a separate video, to be sure.
January 1963 – After observing NTSC for several years, a new color system is developed
and presented to the European Broadcasting Union.
It is called PAL.
June 1967 – The first PAL broadcasts begin in the United Kingdom.
The PAL color system is typically applied to 625-line analog video running at 25 frames
per second.
No framerate shift was required for the addition of color, but like NTSC, it consists of a
combination of a monochrome luma signal with a color signal added to it.
October 1967 – France has its first broadcast of a third color television system.
It is called SECAM, and also consists of a monochrome luma signal with an added color signal.
And now we have three color television systems.
They are all based off the combination of a black and white luma signal mixed with a
signal containing color information or “chroma.”
In a matter of speaking, you could refer to color television as “colorized television”
as it continued to use black and white as a primary signal and found a way to add color to it.
And now we understand a bit more about luma – an important piece of the video puzzle.
We know more about light, how our eyes respond to it, and the concept behind how it is presented
to a display.
And while RGB uses a three primaries approach to presenting color video, the remainder of
these analog signals is built upon luma – either mixed with the color signal – or – as
a separate component.
As for how we add color to luma, well… we’ll get there.
I hope you enjoyed this journey of light from early understanding to an analog display.
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