Author Topic: Light conversion to watts questions  (Read 4057 times)

2019-09-25, 23:22:51

3dwannab

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Hi all,

1. I'm wanting to clarify what the unit of measurement the following have in respect to real-world values:
CoronaLightMtl
CoronaSun


I believe it's W/(sr.m^2) for both especially the CoronaLightMtl going by the helpdesk page.
Quote
Corona supports Candela (cd), Lumen (lm) and Lux (lx) units. If it's not specified (for example in standard lights, Corona material's self-illumination, or Corona Light material's intensity), the unit used is Watts per steradian per square meter - W/(sr.m^2).

2. Also is W/(sr.m^2) which is Coronas' default for CoronaLights' the same as W/m^2 which is the unit I'm trying to convert to or in laymans terms Watts? If not, how can this get converted to Watts?

3. I want to convert to watts from the following:

W/(sr.m^2)
No conversion required? Is this just watts?

Lumen (lm)
What is Coronas internal value for lumens per watt? 15, 16? These are the two values I've seen knocking about the internet.
https://sciencing.com/compare-led-light-output-incandescent-bulbs-3061.html
https://www.rapidtables.com/calc/light/lumen-to-watt-calculator.html

Lumens / (Lumens per watt) = Watts
P = 900 lm / 15 lm/W = 60 W

Candela (cd)
Candela / 683 (Lumens per watt) = Watts
P = 40980 cd / 683 lm/W = 14.64 W

Lux (lx)
Lux / 2145.708 =
P = 128742.469 lx / 2145.708 = 60 W

I wanted to check this before anything else.

Cheers.

« Last Edit: 2019-09-26, 16:17:40 by 3dwannab »

2019-09-26, 09:39:43
Reply #1

James Vella

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I usually use Lumen (lm) as its easier for the conversion (in my case)



If you scroll down the page I have listed information on the conversion for lights.
https://www.jamesvella.net/tutorial-lighting-color-matching

lumen 250 = 25watts = 0.015 W/(sr.m^2)

Not entirely sure about the logic using W/(sr.m^2) maybe someone with more technical experience can chime in, however my point is most standard lightbulbs are roughly between 25-150 so the chart does a pretty good job for (lm).

To answer your question regarding coronalightmtl/sun i find the 'intensity' value matches W/(sr.m^2) - so in the above example if you put 0.015 into the lightmaterial intensity it will be 25 watts or 250 lumens. My conversion technique is quite simple really i just create a corona light, put the lumen (lm) value in according to the chart, change the type from lumen to W/(sr.m^2) and it gives you the 'intensity' value.
« Last Edit: 2019-09-26, 12:01:11 by James Vella »

2019-09-26, 12:28:20
Reply #2

3dwannab

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I'm not sure I understand.

If I put 0.5 in as the default for W/(sr.m^2) and then switch to lumen the value that is displayed as 908.408 lm which is 60.56 Watts going by the formula below.

Lumens / (Lumens per watt) = Watts
908.408 lm / 15 lm/W = 60.56 W



This confuses me as the default for the sun is 1 W/(sr.m^2) meaning that:

1816.816 lm which is 1 W/(sr.m^2)

Again, Lumens / (Lumens per watt) = Watts
1816.816 lm / 15 lm/W = 121.12 W

So, the sun = 121 Watts

Which seems quite incorrect :)

2019-09-26, 13:05:01
Reply #3

James Vella

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Well its also related to the size of the light.

for example:

20cm corona light sphere @ 900 lumens = 0.841 W/(sr.m^2)

10cm corona light sphere @ 900 lumens = 3.363 W/(sr.m^2)

The sun is quite large, so im sure its been set by the scene units and calculates it via that. Im not sure exactly how large etc but for specifically matching a corona light to a coronalightmtl on a sphere of the same size it should give you the same result. From my testing I find as long as Im sticking within realistic light bulb sizes and matching lumens it looks very close to the reference. For your other questions im afraid I dont know.








2019-09-26, 13:22:31
Reply #4

3dwannab

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The sun is quite confusing.


1 for intensity and 1 as the size are suppose to be real-world representations of it going by the tooltips both.





The program that I'm exporting to has W/m^2 which again, I'm not sure if it's the same or not as W/(sr.m^2).

2019-09-26, 14:37:36
Reply #5

houska

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W/m^2 and W/(sr*m^2) are two distinct quantities. The former (radiosity) defines the average amount of energy leaving the light in all directions per area. The latter (radiance) defines the average amount of energy leaving the light source per direction and area. They are usually convertible between each other by differentiation or integration (depending on the direction of conversion), but in case of simple emission patterns, the conversion usually entails multiplication or division by some fractions/multitudes of PI.

Admittedly the topic is confusing enough for me too, so I have to refresh my knowledge every time I need to do some calculations.

This Wikipedia article usually helps me: https://en.wikipedia.org/wiki/Radiance

Also, do note that the sun's intensity is relative to Earth's sun. So the value of 1 means "one sun"
« Last Edit: 2019-09-26, 14:47:34 by houska »

2019-09-26, 15:30:58
Reply #6

3dwannab

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So my calculations from lumens, lux and candela are fine to Watts in my first post or does the size of the emitter need to be taken into consideration for these?

Thanks, as part of the dev team I feel a little less stupid when you find this difficult to understand. ;)
I did read that Wiki page and after some head-scratching, I came out with nothing.

So, to recap
For the Sun, lights (with intensity unit of W/(sr.m^2) and LightMtls I req. a conversion (taking into consideration the size and strength) from W/m^2  to W/(sr.m^2)

2019-09-26, 15:43:43
Reply #7

houska

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Well, I would say that your calculations are not entirely correct. Maybe let's start with what quantities you have and which ones do you want to get?

Edit: Ah, you mentioned that in your recap, right? You want to go from Radiosity (W/m^2) to Radiance (W/(sr*m^2)), right?

In that case:

To get W/m^2 to W/(sr*m^2), just take the first value and divide it by PI (3.1415...). Area of the light does not matter, because it is not taken into consideration with these quantities (both are "divided by the area" already).
« Last Edit: 2019-09-26, 15:49:28 by houska »

2019-09-26, 16:16:24
Reply #8

3dwannab

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Oops.. sorry. I meant:

Radiance (W/(sr*m^2)) to Radiosity (W/m^2)

If I understand this now, I take the (W/(sr*m^2)) value and multiply it by PI (3.1415...)?



Then I have the lumens, lux and Candelas values that need to be converted to Watts.

Are my calculations correct in the first post?

2019-09-26, 16:39:11
Reply #9

houska

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Yes, the other way around would be multiplication by PI.

Lumens are the same as Watts, basically, you just have to multiply by some conversion factor, which says how much energy is actually visible and how visible it is (e.g. we are more sensitive to green light). There's a special function for that and it depends on spectrum of the light, but Corona uses a standard factor of 683 (see here: https://en.wikipedia.org/wiki/Luminosity_function)

The rest is obvious, based on the quantities (steradians and meters squared), so the equations would be:

[W] = [Lumens] / 683
[W/sr] = [W] / PI
[W/(sr*m^2)] = [W/sr] / S

Where S is the surface area of the light, which of course will depend on the geometry of the light.

Lumens describe the light of the whole light source, Candelas describe light only in one direction, which makes sense only on average or if you know which direction you are talking about. Without going much into the detail, in our case, you can just use PI again:

[Lumens] = [Candelas] * PI

Edit: Lux is similar to Candela in that it describes all directions, but only in one point on the light source, so you don't use PI as the factor, but the surface area S:

[Lumens] = [Lux] * S
« Last Edit: 2019-09-26, 16:43:24 by houska »

2019-09-26, 16:45:04
Reply #10

3dwannab

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Many thanks.


I'll work on this maxscript and see the results once I export the scene and import to the other software.

2022-03-30, 01:57:23
Reply #11

marche3d

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I belive it´s better and easier to add Watts unit to Corona lights and for the emitted material too if it is used to illuminate as well and without having a unit it is more difficult to use correct lighting parameters for lamps!!!

2022-03-30, 08:05:16
Reply #12

James Vella

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I belive it´s better and easier to add Watts unit to Corona lights and for the emitted material too if it is used to illuminate as well and without having a unit it is more difficult to use correct lighting parameters for lamps!!!

Im afraid that doesn't really make sense since Watts is a measurement of electricity and lumens is a measurement of light. Different light bulbs produce the same Lumen value and vary in Watts used.

Its the equivalent of asking the petrol attendant I don't want to know the cost per liter I want to know how many kilometers I will get to drive, well that varies depending on your engine size, how you drive and other variables.

Here is an example of different types of light bulbs and the wattage used

bulbs" border="0
« Last Edit: 2022-03-30, 08:16:47 by James Vella »