Chaos Corona Forum

Chaos Corona for 3ds Max => [Max] I need help! => Topic started by: Jpjapers on 2020-02-06, 17:17:21

Title: Can anyone explain the link between the light intensity units?
Post by: Jpjapers on 2020-02-06, 17:17:21
In the intensity drop down when using a corona light you can set what units you want to use.
Are these correct conversions when changing the units?
Title: Re: Can anyone explain the link between the light intensity units?
Post by: mferster on 2020-02-06, 19:11:48
Not really because the conversion doesn't take into account the angle/spread of the light into its conversion equation.

Here's some reading

https://www.compuphase.com/electronics/candela_lumen.htm


Title: Re: Can anyone explain the link between the light intensity units?
Post by: Jpjapers on 2020-02-07, 17:09:08
Quote from: mferster on 2020-02-06, 19:11:48
Not really because the conversion doesn't take into account the angle/spread of the light into its conversion equation.

Here's some reading

https://www.compuphase.com/electronics/candela_lumen.htm

Thanks, This is why i wish we had a corona light meter like the mental ray one. Being able to measure lux at floor level and then replicate it in Max would be so useful!
It would be nice to find the time to build some properly set up lights that just work as they should eventually.
Title: Re: Can anyone explain the link between the light intensity units?
Post by: houska on 2020-02-07, 17:21:57
Quote from: Jpjapers on 2020-02-06, 17:17:21
In the intensity drop down when using a corona light you can set what units you want to use.
Are these correct conversions when changing the units?

Actually, yes, these conversions are correct, but they only take into account the limited data that can be inferred without doing any super-complicated and lengthy calculations.

In particular, if I recall correctly, they calculate with the light object's surface volume, they presume lambertian light emission distribution and they assume that the light has a wavelength of 555nm, where the light sensitivity of human light sensitive cells is the highest.
Title: Re: Can anyone explain the link between the light intensity units?
Post by: houska on 2020-02-07, 17:51:43
And if you'd like me to be more specific, the formulas are following:

[Lumens] = [watts/(sr*m2)] * radioToPhoto * π * SurfaceArea
[Candelas] = [watts/(sr*m2)] * radioToPhoto * SurfaceArea
[Lux] = [watts/(sr*m2)] * radioToPhoto * π

Radio to photo is a conversion factor between radiometric and photometric units (this is where the 555 nm are taken into account)
Title: Re: Can anyone explain the link between the light intensity units?
Post by: Jpjapers on 2020-02-11, 12:31:28
This is a very interesting answer, thanks for sharing!
So is there a physically correct way to set up a light for accurate emission based on the datasheet of a light fitting? Would you need to make a corona light that is the size of a bulb filament?
Title: Re: Can anyone explain the link between the light intensity units?
Post by: houska on 2020-02-11, 18:04:54
Quote from: Jpjapers on 2020-02-11, 12:31:28
So is there a physically correct way to set up a light for accurate emission based on the datasheet of a light fitting? Would you need to make a corona light that is the size of a bulb filament?

I'd say your best bet is probably using a light with .ies profile. Modelling a bulb filament doesn't make much sense unless you're planning to do shots very close up.
Title: Re: Can anyone explain the link between the light intensity units?
Post by: Jpjapers on 2020-02-12, 10:03:18
Quote from: houska on 2020-02-11, 18:04:54

<SNIP>

I'd say your best bet is probably using a light with .ies profile. Modelling a bulb filament doesn't make much sense unless you're planning to do shots very close up.

Ahh I thought as much. Thanks for the insight!