Training for Non-Technical Cinema Employees

Units of Measurement

This lesson about measurements might get a little technical, but we will try to keep it light.

Remember that saying from the baseball player Yogi Berra? He said, “In theory, there is no difference between theory and practice. In practice, there is.” We find that is true all the time. In theory, 10 minutes exists. 10 minutes and I’ll be done with this! Give me 10 minutes, then we’ll go. In practice, it is always 12 or 15 or more minutes. 10 minutes doesn’t really exist.

In theory, you don’t really need to know each little thing involved in the measurement of sound and light, or why the engineers expect (and respect) the terms.

In reality, the little things can leave us in a swimming mystery if a word or concept seems important but we can’t grasp it.

So, let’s do some grasping – just enough so that we get enough information without diving into the confusions of the internet.

There is a group called the International Organization for Standardization, which most people call the ISO. As you can guess, they organize standards. Other groups bring standards to the ISO, standards about fire safety or electrical safety. For our industry, the Society of Motion Pictures and Television Engineers (SMPTE) develop standards and bring them to the ISO for worldwide implementation.

The ISO also organizes the most basic of definitions. It has been decided that there are 7 basic units of measure, and everything else can be derived from them. These International Standard Units are abbreviated as SI Units, and they are:

Symbol Name Quantity
s second time
m meter length
kg kilogram mass
A ampere electric current
K Kelvin temperature
mol mole amount of substance
cd candela luminous intensity

You don’t need to know much more than that for most of these.

You can go your entire life without using the mole unit, even if it is fun to think that there is a single number that scientists use for counting the number of atoms in a box. In the same way, only people in a special part of the science world use the Kelvin scale – although, many people use the Celsius scale which uses the unit of degrees in the same way (except that 0 in Celsius is the temperature that water freezes at, and 100° C is the point that water boils. In the Kelvin scale,  0 Kelvin is the coldest of cold that nothing can go colder, which is called absolute zero.

Most people will also know what a second is, and we should all be glad that a second is accurately defined and everyone uses the same definition around the world.

In the cinema auditorium we will use the second since sound travels so many ‘lengths’ of distance every second. That length is usually called a meter, and sometimes spelled: metre. Some countries – well, one country, uses the length of feet, which is about 1 third of a meter (or one meter is a little more than 3 feet). Sound travels at 383 meters per second. That is pretty fast. Most auditoriums are not that long. So, the sound from the speaker gets from the speaker to the audience in less than a second.

But the speed of sound is nothing compared to the speed of light. Light goes almost 300,000,000 meters per second – 3 hundred million meters every second! For our purposes, that is instant.


Ampere is something that is used every day in the cinema projector room, but we usually call it ‘amps’. The projector bulb uses energy to create light. That energy sends a  The energy that flows throught the cables to get turned into the light of the projector’s bulb can be made higher or lower – we measure the amount of that energy as so many amps. When the bulbs brightness gets weaker as it ages, we have to increase the amperage, or amps.

There are two more Standard Units, kilograms and candela that we are more familiar with. We use kilograms every day, because it is the unit of measure for weight. And, of course, speakers have weight and movie screens have weight, so we have to have floors that are designed to hold these weights safely. But usually these things are decided years ago.

Candela is really the most interesting measurement for us. Up above it is called ‘Luminous Intensity’.

Luminous is a neat word that comes from the word for Light, which is lumens. And candela comes from the word for ‘candle’. But in the world of standards they don’t just use any candle, they specify it so that anyone, anywhere can create the exact same light if you happen to have a wick made out of pure platinum and a certain amount of the exact wax.

Intensity is also an interesting word, because it hides a sophistication that we need in our work. Intensity is used to describe the strength of something, but we always need to use it with “per unit”. Candela deals with the strength of the light we are using, but we need to use it ‘per’ something like second or meter or screen.

The unit we use is ‘per square meter’. The abbreviation is cd/m2.

To understand this better, let’s look at this picture. First, we see that the energy from the burning wick and wax is converted to light, which leaves the candle in every direction.

Description of Luminance

This article is a Work In Progress – more soon.


This lesson is still a work in progress. The following needs to be incorporated:

The numbers that get thrown around are based around standards set by the ISO, which got them from the CIE*.  SMPTE then bases their work on these two group’s work.
A candela is basically the amount of light that comes from a candle, but it the kind of unit that has to be associated with a dimension, since light emirates from the candle in all directions and we are only interested in a slice that comes out of the projector and hits the screen. The dimension chosen is ‘per square meter.’ Then they do a bunch of science stuff to try to come up with a consistent measurement. And, as we found out, that is near impossible.
Candela per square meter is also called ’nits’ in the camera world, and has gotten more popular in exhibition. The old old standard (pre-1990 perhaps) was measured in foot-lamberts. All specs are supposed to be in ISO units, which foot-lambert isn’t. But some people like to use it. 14 foot-lamberts equal 48 nits, which is the standard ±10%.
Bright is important, but in a theater the amount of dark is just as important. So much of what we see is juggled in the human visual system by using comparisons. The comparison of brightest white to darkest black is contrast, one number divided into the other, and given as a ratio. The classic specification for projectors has been 2000:1 for 15 years, although many projectors only actually make it on their spec sheet. The typical 4K projector in the field is thought to be 1600:1.
It isn’t just black and white that is important, of course. But for measuring we can get a lot out of each because of one fact. White is a combination of equal amounts of red, green and blue light. And mid-grey is a combination of equal amounts of red, green and blue light…although, less of each. And light grey and dark grey are a combination of red, green and blue light, just different amounts. So each time they only took one ‘colorimetry’ reading, which measures where the white combo is on a chart that the CIE came up with.
But science stuff aside, looking in shadows to see what is happening to subtle changing fabrics or doors is more and more possible, and a good way to judge as you go forward. The color of bricks goes from deep red to deeper and deeper red and finally black. Blue jeans go to black and back to blue as they are rumpled. …or is it all black, which is how standard equipment handles deep subtleties. What you heard Jerry called Crushed Blacks.
–––––––– Back to our story…
The Dolby Vision system can light up the screen at 108 cd/m2, more that twice the 48 nit standard that relatively few auditoriums consistentlyspeed meet. They are also famous for their technology that allows the projected black to be 1 million times less bright. Really dark darks have been a major goal of digital cinema since the beginning and they spent a lot getting it so good.
IMAX can light up the screen to 75 cd/m2, a touch more than 50% more than the standard 48.  They also say they have a technology that they say creates a true black, but they won’t give a number.
The last comment about this is a completely different chapter, because the LED screens that some theaters are putting in also claim a million to one contrast ratio and brightness that can go up to 500 nits – which everyone says is too bright. I’ve seen 300 and that is far too bright. But, who asked me.
–––––––– One last math exercise…just because it is fun and because I am always confused with this milli-nit stuff that Jerry likes
Let’s pretend that Dolby Vision isn’t 108 nits, but 100 at the brightest. And pretend that we want to figure out what 1,000,000:1 contrast ratio means.
Which means, if the highest level were 100 nits, then 10:1 would be 100 nits to 10 nits,  and 100:1 contrast ratio would be 100 to 1 nits. Let’s do this as a table
Contrast Ratio Vernacular
While checking rooms for light levels, it became very easy to add new words to the nomenclature of hip. Instead of saying, that reading was point zero zero 9, that number became 9 millinits. Point 01 became 10 millinits. You kinda had to be there seeing all these extraordinary numbers.
Contrast Ratio Ratio measurement – part of a nit
10 100 nits to 10 nits
100 100 nits to 1 nits 1 nit is a 1,000 millinits. That makes sense, right?
1,000 100 nits to 0.1 nits 100 millinits
10,000 100 nits to 0.01 nits 10 millinits
100,000 100 nits to 0.001 nits a thousandth of a nit – a millinit!
1,000,000 100 nits to 0.0001 nits a tenth of a millinit!
This science stuff works.

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