Can someone please explain Inverse Square Law to me?

Klaus Hustle

Hi, sorry for the question that apparently easy to understand and easy to google.

But i am still struggling, please hear me out;

For my 2D interior-scenes i would like to use this law to "calculate" how i apply the lightning.

But first of all, i need to understand the basics of ISL. I made this drawing, as i dont want to get into trouble with other sites for copying their infographics.

The Rule says: 

"The inverse-square law describes the light received from most sources. If the distance between an object and the light source is doubled, a given area receives only one-fourth as much light; if the distance is tripled, only one-ninth."

Source: http://kids.britannica.com/comptons/art-167423/The-inverse-square-law-describes-the-light-received-from-most



Ok, if the distance between the light-source and the object is doubled -> 1/4 light.

But when is the Light-Intensity at100%? How is the starting-point defined? I need a starting-point, from where i can say "yeah, hear it's 100% light, from now on it has this falloff blabla"

In my drawing i highlighted the distance X in cyan color. How is this distance defined? Is this a vague instance where the light has no notable falloff or something? Because, i could place the first plane anywhere lol.

I don't know why this confuses me so much, probably because i am not the smartest when it comes to physics

I know this is a mess, i find it hard to explain, would be easier if i would get it in the first place.

Cheers!

Eric Chadwick

The way I understand it... at the point of emission (the inside of the lightbulb) the intensity is 100%. At the position titled (1) the light is at a certain intensity, for example maybe 50% of the emission. Then at double that distance (position 2) the intensity is 1/4 of 50%, which would make it 12.5%. Does that make sense?

musashidan

Distance X is going to be determined by the lumens that the light source emits. After that it's a mathematical formula which might be a bit over the top  for your purposes. But if you have a 3d package you could set up a simple test scene with physical lighting and real world dimensions to study the falloff based on lumens and distance.

Klaus Hustle

musashidan said:

Distance X is going to be determined by the lumens that the light source emits. After that it's a mathematical formula which might be a bit over the top  for your purposes. But if you have a 3d package you could set up a simple test scene with physical lighting and real world dimensions to study the falloff based on lumens and distance.

i could also imagine the strength of light-source, the stronger, the bigger is the radius of X where the light is at 100%, and after that just apply ISL, right? Of course im looking for a simplification. 

I am also working with 3DsMax and Vray, but i i never had to set up anything properly in it because im doing the main-thing in Photoshop. I'll give it a shot, good idea. I am just wondering if the outcome is very dependent on right settings, or if physical lightning, 

Klaus Hustle

Eric Chadwick said:

The way I understand it... at the point of emission (the inside of the lightbulb) the intensity is 100%. At the position titled (1) the light is at a certain intensity, for example maybe 50% of the emission. Then at double that distance (position 2) the intensity is 1/4 of 50%, which would make it 12.5%. Does that make sense?

that makes sense, but i imagine if this was ultimately the case the light would have a much too high falloff,  i don't know to be honest :F but in doubt i would go with that. definitely trying it out in 3D. I hope i can set this up right, as i am not a specialist in Vray-Lightning

Klaus Hustle

would you recommend rather the physical camera exposure control or the Vray Exposure Control? I am on 3DsMax 2016 and using Physical Cameras. There is just a Vray Dome Camera, but i don't think that fits my needs.

€: Ok, Vray Exposure Control seems to be the only useful option when set up correctly

musashidan

Well if you're a Vray specialist then you'll already know that every time you double the surface of an area light you increase the multiplier x4 to maintain the same luminence. This is essentially the inverse square law for artists. Test it yourself. Just set up a plane for the ground and 2 plane area lights: one with a size of 500 mm2 and a multiplier of 4, and one 1m2 and multiplier 16. They both will have the same falloff.

A lot of 2d digital artists block out a scent in 3d so they get the light/shadow and perpective for free, and then just do a paintover of the 50% grey render.

Klaus Hustle

well, i am by no means a vray specialist, never mentioned that, wish it would be the case but i am working on it now and then to support my 2D, as you mentioned.

i know what you mean, but you confused the settings, if its vice versa it's correct, small light 4 times as bright as the bigger light, and i get the same falloff as a result, yeah!

Klaus Hustle

in addition, i probably found a better way to understand it know; it's pretty easy to measure the ISL with the size of the Light-Sources spot, as it increases in size with distance it get's less intense. That's basically the Law, but now i got the missing instance for my measurement. but still not sure tho

musashidan

Klaus Hustle said:

well, i am by no means a vray specialist, never mentioned that, wish it would be the case but i am working on it now and then to support my 2D, as you mentioned.

i know what you mean, but you confused the settings, if its vice versa it's correct, small light 4 times as bright as the bigger light, and i get the same falloff as a result, yeah!

Haha! Sorry I mixed up twice in one post I thought you said you are a Vray specialist. And yes, of course you're correct, I meant the other way around. Larger light higher multiplier to account for the lumens having a larger, less concentrated area to cover.

I don't paint in 2d but I can imagine how difficult it would be to represent light falloff physically.