Accurate Light Intensity & Camera Exposure (pdf tutorial)

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sinesium
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It's a very important and useful tutorial. Thank you very much!
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JQL
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patricks wrote: I am about to finish a tutorial that will cover this ;)

Greetings Patrick
Nice! I love your tutorials. I prefer the video versions though the pdf's are very informative.
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Christina, this is exactly the explanation I need to fix my black rooms. I'm an 'old school' photographer so it makes absolute sense now. Legend!!
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Has anybody good resource of lm/m2 ? I always find only lm values on manufacturers sites :(
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JQL
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D-W wrote:Has anybody good resource of lm/m2 ? I always find only lm values on manufacturers sites :(
That lm value is all you need:

1 - Create the lamp from the manufacturer;
2 - Create a diffuser object pointing where the lamp should point;
3 - Find out it's area;
4 - Divide the lm value for that area and you have your lm/m2 value.

EDIT: This is not a lm you can reuse for any lamp, but that is the lm/m2 you should use instead of the lm value for that diffuser and that lamp, so you can safely multiply the lamp and reuse it in any project and in any quantity without fading the light when you add more lamps.
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JQL wrote:
D-W wrote:Has anybody good resource of lm/m2 ? I always find only lm values on manufacturers sites :(
That lm value is all you need:

1 - Create the lamp from the manufacturer;
2 - Create a diffuser object pointing where the lamp should point;
3 - Find out it's area;
4 - Divide the lm value for that area and you have your lm/m2 value.

EDIT: This is not a lm you can reuse for any lamp, but that is the lm/m2 you should use instead of the lm value for that diffuser and that lamp, so you can safely multiply the lamp and reuse it in any project and in any quantity without fading the light when you add more lamps.
Thanks JQL for respond but i don't get it :D

Step by step:

1. I'm creating a box 5x5x5 meters
2. I'm creating a manufacturers lamp for eg. diam = 16 mm lenght = 1500 mm and adding emmiter mat with proper lm val for this lamp for eg. 4550 lm
3. Diffuser ? What kind of diff ? It should cover lamp around or what ? Or just simple plane which behaves like diff but also with emmiter mat. Anyway i create plane which has 1x1 meter = 1sq meter
4. What area ? :mrgreen: OMG am i really so dull ? :mrgreen:
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ivkovic
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Thanks Patrick and Thea team,
very usefull tuto. I was made some tests with my latest hotel project.
There is a screenshots (Presto MC)
Attachments
1.jpg
2.jpg
3.jpg

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JQL
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Nice looking renders ivkovic!

D-W,

Here it goes.
  • You have your lamp's geometry! Nice!
  • You have your manufacturer's lm! Great!
  • Now that you know how the light acts and what's it's total power you can create an emitter, that will cast light in the directions needed to replicate the way light flows off the lamp.
  • The manufacturer might even supply an IES file for reproducing the exact light effect but let's Forget about that...
  • So now you have to create a geometry for that emitter. To keep it simple, imagine your emitter is a downlight so the light comes out of it in only one direction.
  • If so, a square emitter will be enough. To save on render time you should create an emitter as big as you can and still fit it inside the lamp. Make sure it is not touching any other geometry or you'll get noise, sometimes severe noise.
  • So your square is facing down, and now you have to make it emmit a total light of, for exemple, 1.100 lumens. See this for reference:

    http://en.wikipedia.org/wiki/Lumen_(unit)
  • Now imagine the emitter you designed to simulate that light has an área of 0.1m2.
  • The total light those 0.1m2 emmits is 1.100 lm, so those 1.100 is distributed evenly in that small area.
  • This means that the lumens that material emmits by square meter are exactly. 1,100/0.1m2
Now imagine you have a completely different lamp but it also emmits 1,100 lm.
  • Imagine that in this new lamp, you are able to create a square emitter 10x bigger than the square of the first lamp. This is very good as it will make render times faster...
  • If you use exacly the same material as the lamp above, as the new area is bigger, it will emit 10x the amount of light.
  • However, that lamp should only emit the 1,100 lm, that emitter has 10x the size, but it emmits the same light, then that emitter's material, should emit 10x less light per square meter.
  • And that's why a material emitter wich is using lm/m2. Can only be used on a certain geometry that you know, will never change. If it would change, you would have to change it's power accordingly or it will be stronger/dimmer...
I hope this helps,

João
Last edited by JQL on Fri May 01, 2015 5:32 pm, edited 1 time in total.
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Really nice renders!!
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ivkovic
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Thanks JQL and nigec.
i will check your suggestion :clap:

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