What are the nits of an AR/VR display (explained with memes and potatoes)
Today I wanted to learn something new. It’s years that I read about the “nits” of AR displays and why they should be meaningful for outdoor AR, but I never truly understood what they meant. I got they were a measure of brightness, but I couldn’t grasp the exact meaning. I was used to saying “WOW ONE MILLION NITS!” without having the minimum idea of what I was talking about when I was speaking with display manufacturers. It was about time to change that and to understand what is a “nit” and how it does relate to the other measurements of lights, like lux, lumens, and candelas. After a few hours studying stuff on the web, I think I got the idea, so let me tell you what I have learned… in a quite original way.
Potatoptics
Wikipedia defines the “nit” as follows
The candela per square metre (symbol: cd/m2) is the derived SI unit of luminance. The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area. The nit (symbol: nt) is a non-SI name also used for this unit (1 nt = 1 cd/m2).[1] The term nit is believed to come from the Latin word nitēre, “to shine”.[2]
https://en.wikipedia.org/wiki/Candela_per_square_metre
Nit is so a unit measure based on the candela, which is one of the fundamental units of SI.
On the page about candelas, a “candela” is defined this way:
The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018.[7] The new definition, which took effect on 20 May 2019, is:
The candela […] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
If this is clear to you, you can stop reading the article here. If at the “cd sr kg–1 m–2 s3” you started having this expression
then you are in the right place. I will try to explain all the above concepts in an easy and funny way. And to do that, I will use the fundamental unit, the base of the metaverse and life in general, that is potatoes.
So let’s start seeing the meaning of all the most important terms that regard measuring lights… using potatoes. But before starting, I just want to say sorry to all people that are truly experts in optics (like Karl Guttag or Jeri Ellsworth) for the simplifications and the errors that there will be in this article. I hope you will forgive me.
Candela (Luminous Intensity)
Let’s start this article about lighting with the main measure unit for what concerns this sector: the “candela”. This is the only base unit in the SI that we’ll see today: all the other ones are derived units from this one.
According to Wikipedia, “The candela is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction“. The term comes from the Latin word “candela”, which means “candle” because this unit was based on the average lighting that a candle was able to provide.
I promised you some potatoes, so let me explain what a candela is introducing you to the “light potato”, which is my unit of luminous power. Light can be considered as a waveform or a particle, but instead, I will represent it as tiny energy potatoes that fly across the air until they reach your eye. I don’t know if this is realistic, but for sure it is cute. So let’s pick a candle, and let’s see how a potatocandela can be represented:
Here we have a candle, that emanates light in all directions. Let’s consider an angle of a radian (that is around 60 circular degrees), and see how much luminous power passes in that cone of light, so “how much light” passes inside it. In the case of the drawing above, we have only one light potato traveling from the candle inside the circular section of 1 radian that I have highlighted. The amount of light (in this case a potato) that flows through that angular section is the luminous intensity. If the source light is a candle, that amount will be close to one candela.
Before going on, make me underline that all these measurements units are related to a 3D world, and I’m simplifying everything by showing you 2D drawings (that is, 2D sections of the 3D world) because they are easier to understand. But everything is in 3D: so for instance, the radian angle above should be a “steradian”, that is a 3D angle, that is something similar to a cone, or a spotlight to make you understand it better. So everything you see in 2D in this post, is actually in 3D: segments are actually squares, circles are spheres, and so on. Keep this in mind: I’m just showing you a projection for the sake of simplicity.
That said, returning to the example above, it is clear that the candela is a measurement unit related to a light source and that tells you “how much light travels in a certain direction” from that light source. Explaining it with our potatoes analogy, where every potato represents a magical particle of light, the luminous intensity, measured in candelas, represents how many potatoes you can find in a cone of a steradian with a certain direction that starts from the light source. That is, how many potatoes you see above in the region of one radian. In my humble opinion, it is a bit of a weird measurement, and not very useful per se.
Lumen (Luminous Flux)
Let’s go deeper, by talking about lumens. Wikipedia defines them this way: “The lumen is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time“. Long story short, if the candela identifies how much light is emitted inside an angle, the lumen helps in defining how much light is generated in general by a light source.
We have got rid of the boring candle, and we have bought a new light source. As you can see in this graph, this light source is more powerful, and in a cone of a single radian, it emits 4 potatoes. This means that our star light source has a luminous intensity of 4 candelas (4x the one of a candela). Across all the 360° dome, it generates 24 potatoes, so we can say that this light has 24 lumens of luminous flux.
Luminous flux is another property of a light source, and it identifies in every second how much light is flowing from the light source. While candelas just consider a direction, a cone, lumens are a property of the light source across all directions. If you like some maths, I can tell you that if a point light source has equal illumination all around it, and it has one candela of luminous intensity, the total luminous flux is 4π lumens, that is the sum of all the intensities across all steradians of the sphere.
Let’s make a game and kill half of our light, putting all our left environment in the dark. The luminous flux emitted by the lamp is now only a half of before, because half of it doesn’t emit light anymore, and we have just 12 emitted potatoes. The luminous intensity is instead zero candelas on the directions going from right to left since we have no potatoes moving in that direction, and is still 4 candelas on the right part, because on the right, the light is still flowing as before.
This is relevant because it shows that the two measurements are only loosely coupled: a lamp that generates a very diffuse light in all directions, and a strong monodirectional light (like a laser ray) may have the same lumens, but the candelas will be very different. Let’s say that lumens tell you how much light power a source can put out, and the candelas how this gets emitted in various directions.
Lumen is the measuring unit usually employed when selling flashlights, light bulbs, or similar items, because it conveys well the idea of “how much light” a lamp can generate in every moment, that is how many potatoes per second it can create for you.
Lux (Illuminance)
Do you think we are going to stop here? No…
Let’s talk about Lux: “The lux (symbol: lx) is the SI derived unit of illuminance, measuring luminous flux per unit area. It is equal to one lumen per square metre“. Long story short, the lux tells you how much a surface can be illuminated by a light source at a certain position in space. It is a property related to objects that are illuminated by a light source (or of how the illumination of the light source changes in the space around it).
Let’s look at the above figure: our light still emits its 4 candelas and 24 lumens (as before), and if we have a plate of 1 square meter in front of it at a certain distance, it gets hit by 4 potatoes, so we can say that its surface is illuminated by 4 lux. If we put the surface more distant from the light source, we can see that only 2 potatoes hit it, so it is lit by only 2 lux. Iluminance is nothing less nothing more than the concept that the closer you are to a light, the more you are illuminated, and the more distant you are, the less light you receive. This is because the closer you are, the more rays hit your surface, that is more lumens arrive onto the surface.
Notice that in the above picture, we have always four candelas (4 potatoes per radian), and always 24 lumens: these are static properties of a light source. The illuminance instead is a property related to lit objects, and it changes depending on where we are around the light source, and especially at what distance we are from it. Lux are good to express how much an object is illuminated by a light source. If we take all the light emitted by a lamp and thanks to a lens we concentrate it into a point of an object, the lumens of the lamp will be always the same as before, but the lux in that point of that object will be a lot, because all the light energy will be concentrated in a little space. The correlation between lumens and lux is a bit the one that there is between force and pressure: lumens identify the total light available, lux how much of this light hits a certain area. The more intense the light for the same area, or the smaller the hit area for the same light intensity, the higher the illuminance, that is the more the surface will be affected by light (and so the more you will see it as bright).
In some contexts (e.g. among photographers), some studio lights are sold by specifying how many lux they generate at a certain distance (e.g. 100 Lux at 2 meters): this way the user knows that if it puts an object at a certain distance, that object gets a certain amount of illumination, and this can be useful to shoot beautiful pictures.
Nits (Luminance)
Here we are, finally at the Nits, our mystery worlds. Now that we have some more knowledge, it’s a bit easier to understand the definition by Wikipedia: “The candela per square metre (symbol: cd/m2) is the derived SI unit of luminance. The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area. The nit (symbol: nt) is a non-SI name also used for this unit (1 nt = 1 cd/m2). The term nit is believed to come from the Latin word nitēre, “to shine””.
Nits are usually explained in a terrible way on many websites, and there is a lot of confusion here around between lux, nits, and lumens. Luckily for you, I have made my research, and thanks to our lightpotatoes, you will get this concept easily too:
Here we are again with our potatoes, but this time they are not emitted by a candle or a star, but by luminous squares (like two monitors). On the left, we have a monitor with one side of 1m, and it emits 4 potatoes. On the right, we have another monitor with a section of 2m, that emits four potatoes as well. You are now pro enough to understand that the candelas emitted by both monitors are the same (always 4 potatoes for radian), and the same goes for the lumens, but the first monitor can emit 4 potatoes with just one square meter of surface, while the second, that we can suppose being of 2m x 1m, is emitting 4 potatoes with two square meters. It means that to emit the same amount of light, it takes double the area. The first monitor will be of 4 nits, because it is 4 potatoes per square meters, while the second of 2 nits, because it is 2 potatoes per square meters (that multiplied for 2 square meters is again 4 potatoes). The monitor on the right is less efficient, and it emits less light per square meter, meaning that it will overall be dimmer.
Luminance is a property of a light source area, and it specifies how much light power that light source can emit per square meter in a certain direction. The “certain direction” is a fundamental property of this definition, because this amount of light may change across various direction: think about LCD screens of laptops that are bright if seen from the front, and are very dim if you see them from the side.
The more the nits, the more the screen is luminous, is bright. The less the nits, the more the display is dim. Luminance expresses how many candelas an area of the display emits in a certain direction, so how much the display surface is good in “generating light” that goes towards your eyes, and this is fundamental for XR, as we will see in a while.
Typical Luminance values
Just to give you some numbers to make you understand the usual luminance values:
- Old CRT TV were about 100 nits
- LCD screens range from 200 to 500 nits
- HDR TVs are usually in the range 500-2000, but some high-end ones arrive at 4000 nits
- The night sky is around 0.001 nits
- The sun has 1.6B nits (yes, it has the power of generating a lot of light!)
Nits and lux
Let me bend your mind for a moment: you can see that the last two images I added to this article, are somewhat similar. I made this on purpose, because lux and nits are both units that regard an area, but the former is about how much light an area receives, and the second one is about how much light an area emits. But the two things may be related. If I have a light source (e.g. a flashlight), and I illuminate an area (e.g. a desk), this area will be illuminated by a certain amount of lux. But this surface will reflect part of that light, and since it is emitting light, this emitted light will be measured in nits. And if this illuminates another surface, this new illumination will be measured in lux. But the reflection of it will be measured in nits again. And so on forever and ever, for all the ricochets of light.
Why is this relevant for AR and VR?
Well, luminance is always cited when talking about AR glasses, and the reason is easy to understand now: it expresses how strong is the AR display when seen from the direction of the eye in front of it. The more the nits, the more the light is strong, the more the image is crisp and bright. And this is especially important for outdoor usage
When you are outdoor with your AR glasses in front of your face, they show the AR images by casting light to your eyes with a certain luminance (measured in nits, of course). But outdoor you have an enormous light source waiting for you, that is the Sun, that is ready to cast its 1.6B nits towards your eyes. If the light that strikes your eyes (measured in lux) from the display is much lower than the one that comes from the sun, your eyes will mostly only perceive the light coming from the sun, and won’t almost see the output of the AR display. If the display is instead strong enough to illuminate your eyes with enough power to compete with the sun rays, then you see augmentations also outdoor. The more then nits of the display, the more it is bright and visible even outdoor, because it emits more light particles that can hit your eyes.
HoloLens for instance has around 500 nits, and it is believed that you must have at least 1000 nits for something to be used outdoor (it is a rule of thumb): that’s why these AR glasses are unusable if you try to go outside. The sun wins the battle against them. For this reason, many display companies are working in creating displays for AR that have high luminance.
Just one last notice: as I’ve told you before, nits are only a part of the game. Headsets have complex optics systems, and even if the original display has a high value of nits, after the light has passed through all the optical systems (lenses, combiners, etc…), the resulting luminance may have been lost, and so the final real number of nits be much lower than expected. Remember that what counts is actually how many lux arrive at your eyes, not how many nits there were in the beginning.
I hope that everything is more clear for you and that you are now a master of measure units for lighting. Let me know if you enjoyed my explanations with potatoes so that I can decide to use them also for other articles. Have a nice day, and enjoy the nits of light all around you 😉
UPDATE (2022.02.08): Youtuber Brad Lynch has commented to this article with this information that is pretty interesting
Disclaimer: this blog contains advertisement and affiliate links to sustain itself. If you click on an affiliate link, I'll be very happy because I'll earn a small commission on your purchase. You can find my boring full disclosure here.
