How does adding light values work in real life?

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  08 August 2013
How does adding light values work in real life?

I have a question regarding multiple light sources in real life.
I would appreciate if you could reference to any science documents.

Let say I have two lights, aimed at on point with the same value each.
Say 10W each. Would those values add up on the surface the light hits?

1 light:

2 lights with the same value:

In these pictures above you can see that the light values are added. 10+10

Does light in reallife work in the same way?
  08 August 2013
Yes, it does. This is the reason that people talk about linear workflow and insist that it's so important. Light adds in a linear fashion in the real world: 10+10=20, but if you're in a gamma-corrected compositing space, the math doesn't work the same way. If you render linear cg images, or convert your gamma-corrected images, then in compositing, the light acts "correctly." You can simply add your light passes to the diffuse and get something that looks like the beauty pass.

You do need to save to a floating point format, or at least one that has sufficient bit depth not to lose information, though, since the point of gamma correction is to make it possible to squeeze a visually pleasing image into not-quite-enough space.

Sorry I don't have any reference available off the top of my head, but if you've got access to a light meter and a couple of desk lamps, you could test it experimentally.
Bryan Ray
  08 August 2013
Thanks for the reply, I have hard time finding documents on this.
I have light meter in my camera, so I 'm going to test this tonight!

"If two rays of light have the same wavelength, they can interact in a surprising way. When they overlap with each other, the two amplitudes of the waves add up to twice the amplitude and we get a much brighter light. This is called constructive interference."

I could only find this:
  08 August 2013
While that can happen (and it happens in audio, too, where it's easier to control and perceive), that's not really directly applicable to your question. That's describing light in terms of a wave, but when we talk about reflected light, it's more useful to think about it in terms of particles. Yes, the wave-particle theory of light in practical application! Who knew?

Have you read any of the sticky thread at the top of this forum called The Science of CG? It's got a nice description of how light interacts with surfaces. Actually, it's probably got a very thorough answer to your question in there somewhere, but I've only read the first four or five pages.

In essence, what you have is a lot of discrete packets of light (photons, sometimes called quanta) that strike a surface. Depending on the qualities of that surface, some bounce off, some get absorbed, and some get passed through. We're concerned with reflection in this case. The properties of the surface dictate that at all times, a certain percentage of the photons of particular wavelengths bounce off. It's a straight percentage, so the relationship between the number of photons you shoot at the surface and the number of photons that get reflected is linear. It's as if you're shooting water out of a hose—if you shoot more water, more water splashes back. The water can't be destroyed; it can only be reflected or absorbed. If you add a second hose shooting the same amount of water, you'll get twice as much water reflecting and absorbing.

It's the same with the light. Adding another light means you have twice as many photons, and since the surface is the same, they'll be reflected in the same proportion as they were from the first light. Result: Twice as many photons means twice as much light energy.

An introductory physics textbook will cover much of this, too, although most are rather lean on application, or they talk about circumstances the average student is not terribly likely to encounter.
Bryan Ray
  09 September 2013
Thumbs up

Thank you for clearing that up!
I did test with my Olympus E-300 and it did add light values, it might even happen that it exceeded the light values separately.
When I find more time I will read more about the tech behind CG renders.
Yes, I have had allot of photo sessions in high-school that confirmed the same thing.
I how ever got curios if my visual experience would differ in theory.

Thanks for everybody that answered this thread. Case is close! =D
  09 September 2013
To my understanding, our brains do not translate the amount of light that hit our eyes linearly. When there is no light, our brain is very sensitive and can pick up even the dimmest of light. If you shine more light into existing lights, our brain doesn't pick up as much.

Let's say you have a real life light meter.
- You shine 1 light and meter it, you get value of 10
- You shine 2 of that same lights onto the same spot, the meter may read 20 but your eyes/brain will not see it twice as bright.

Think how see mid-tone grey. Our brain make us think it's 50% grey when in real life it's only 16-17%. Photographer will probably be familiar with that number - 17% grey.

sRGB = You add 2 lights with 0.5 intensity. Result, you get pure white of 1.
linear/real life, what you think was 0.5 was actually 17%. So 2 of those lights add up with be equal to 34%

That's just how I understand it from a photographer's point of view. When you leave your shutter opened 2 times longer, does the image brighten up 2 times? Nope.

Last edited by Panupat : 09 September 2013 at 10:36 AM.
  09 September 2013
Thank you,Panupat!
Yes, the eye is a very complex non-static viewing tool adapted for millions of years.
The question of how light works was the question, not how our human eye interprets light.
I got my answer with Midgardsormr answer and some testing with a light meter in my camera.
  09 September 2013
Before we get ahead of ourselves, let's dimiss the wave/particle fudge of light and get it out of the way. If we're going to start with physics, let's return to physical physics instead of guesswork and mysticism. Give the photon mass and spin, and watch how all these confusions and problems unravel, folks:

Real photons.

The wavelength of light is simply how many spins it completes in a certain timeframe or distance. It's not complex at all, despite attempts by mainstream douchebags to make it seem so. One need only use basic high school algebra to solve most physics.



Also, Panupat's wisdom runs deep... Thanks for sharing, keep it going!
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"Like stone we battle the wind... Beat down and strangle the rains..."
  09 September 2013
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