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 flows through the cables from the wall and into the lamp from both ends. Somehow – by magic possibly – the gas inside the lamp gets charged up enough to emit light. Not just any light, but the frequencies of light that can be used to light up some magic mirror chips that bounce the colors that show up on the screen.

You will learn more about frequencies in the lesson called: Audio (Sound Basics): Part 2.

You will learn more about these magic mirror chips in A Look At Light – Part 1.

There are several reasons that these bulbs become less efficient at converting electricity into light. Usually it is that the ends of the sparking parts inside get corroded. In this case, corroded means that they eat themselves up. The pointy tips get pits and the pits make sparks that go different directions instead of strait to the other pointy part. That means they need to consume more energy to make up for the electricity that goes sideways instead of strait.

More energy – 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 that we are more familiar with, even if we don’t use the words all the time; kilograms and candela. We use kilograms every day, because it is the unit of measure for weight. Even in the United States where they still use pounds and ounces, everything from breakfast cereal to computers and TVs also have the weight in kilograms or grams on the box. Of course, speakers have weight and movie screens have weight, so we have  floors that are designed to hold these weights safely. Since these things were decided years ago, and don’t change, we don’t have to think about it much.


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 Latin word for Light, which is lumens. Candela comes from the Latin word for ‘candle’, which creates light. But in the world of standards they don’t just use any candle. The ISO 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. It hides a sophistication that we need in our work. The word Intensity is used to describe the strength of something, but it is best when we use it with “per unit” of something. For example, speed intensity might be described as Kilometers per Hour. In our case, Candelas deal with the strength of the light we are using. The ‘per’ that we will use is part of the screen size. In the old days, they used ‘square feet’. They measured the amount of light that fell on a 1 foot x 1 foot square of the screen. Now, we use meters, so we say Candelas per square meter.

The abbreviation of Candelas is ‘cd’. The abbreviation for ‘per’ is the slash sign: ‘/’. The abbreviation for square meters is an ‘m’ with a little 2 placed behind and above it. So, for 10 candela per square meter we write: 10 cd/m2.

To understand the sophistication of this better, let’s look at this picture.

Hmmm…must find that candle drawing.

To understand this better, let’s look at this picture. First, we see that the energy from the burning wick is leaving the candle in all directions. Instead of energy coming from a wall, the energy is coming from the wax of the candle. The energy of the wax is being converted by the fire, and emitted into the air as heat and light.

Can we ignore the word ‘Flux’? No? OK. Let’s just say that comes from the Latin word for ‘flow’, with an example of a river flowing, or air flowing. In this case, light is flowing, or a science person will say, lumens are flowing. But what is the intensity?

Some of the light hits the ground with enough intensity to see or measure. Notice that, even though the ground is bright, the ground is not creating light. It is receiving light, and if someone wants to measure how much light it is receiving, we call that light ‘illuminance’. Professional photographers do this all the time. They put a meter next to a persons skin and measure Lux. Of course, they are also interested in the light coming into the camera. But before that, when they are measuring the light on the scene, they measure the effect of their lights in lux. Lux is an abbreviation for lumens per square meter.

Lux and Illuminance is not something that we deal with in the cinema. We deal with ‘Luminance’. Luminance is the light being emitted from something that is being received by the eyes (or the camera).

In this modern age, this has two uses. One is the light being reflected from the screen. The other is the light coming from the giant LED walls that some cinema theaters are putting in. That light – the light getting to the audience – is the interesting amount for us. Luminance. Candelas per square meter.


So now, after many definitions, we can sum this up.

In the cinema, first we have the energy from the projector. That energy leaves as light, and differently from the candles light, the projector’s light flows from the lens in only one direction. Because it is flowing from the source, it is called – everybody say it – Luminous Flux.

The amount is called – Luminous Intensity. When intensity was coming from the wall, we said that we measured amps. Sometimes you will see this measurement called Watts. Watts is not really a measurement of light output. It is named after James Watt, a person who studied the power of steam in the 1760s! …no association with light. So, don’t let people confuse you. Watts is power, at the time, measured in horse-power. And watts is a subject for another time.

Luminous Intensity is measured in – everybody, all together – candela per square meter. All the hip cinematographers will never use that many words. They will say: ‘nits’.

If we stand at the screen and point a meter into the projector lens, we are measuring  – everybody? – Illuminance. Illuminance is Lux. We don’t do that. It hurts the eyes!

We cinema people point our meters at the screen. We measure light that is reflecting from the screen or light that is exiting from the screen full of LEDs. We measure this because we want to know how much light is going to be received by the eyes of our customers.

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. That is more that twice the 48 ‘nit’ standard that  auditoriums are supposed to consistently meet. Dolby Vision is also famous for the 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 Dolby 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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