POV-Ray : Newsgroups : povray.advanced-users : Lumens in LightSysIV? Server Time 1 Apr 2023 16:04:58 EDT (-0400)
 Lumens in LightSysIV? (Message 1 to 4 of 4)
 From: Mike Horvath Subject: Lumens in LightSysIV? Date: 11 Jun 2019 21:32:15 Message: <5d00561f@news.povray.org>
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What is the correct method of calculating and specifying Lumens in
LightSysIV?

Light_Color(light_temp, light_lumens)

Do I need to know the distance and diameter of the Sun beforehand? What

Thanks.

Michael
```
 From: Alain Subject: Re: Lumens in LightSysIV? Date: 12 Jun 2019 13:25:46 Message: <5d01359a\$1@news.povray.org>
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Le 19-06-11 à 21:32, Mike Horvath a écrit :
> What is the correct method of calculating and specifying Lumens in
> LightSysIV?
>
>      Light_Color(light_temp, light_lumens)
>
> Do I need to know the distance and diameter of the Sun beforehand? What
>
> Thanks.
>
>
> Michael

For the Sun, don't bother with light fading : Use a light with the
parallel option and you are good. Unless your scene scale is
negligible.

For the extent of the Sun when using an area_light, make it's diameter
about 0.01 of the distance. Not exact, but close enough.

For more normal lights, the fade_power must be 2. No other value is
realistic.

For fade_distance : Make it small. For an area_light, it should be about
the width of the light. For point light, a value of about 0.1 is correct.
```
 From: Mike Horvath Subject: Re: Lumens in LightSysIV? Date: 12 Jun 2019 22:50:48 Message: <5d01ba08\$1@news.povray.org>
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On 6/12/2019 1:25 PM, Alain wrote:
> For the Sun, don't bother with light fading : Use a light with the
> parallel option and you are good. Unless your scene scale is
> interplanetary, the fading over the extent of your scene is totally
> negligible.
>
> For the extent of the Sun when using an area_light, make it's diameter
> about 0.01 of the distance. Not exact, but close enough.
>
> For more normal lights, the fade_power must be 2. No other value is
> realistic.
>
> For fade_distance : Make it small. For an area_light, it should be about
> the width of the light. For point light, a value of about 0.1 is correct.
>

Thanks. What about Lumens? Should Lumens always equal 1 for parallel
lights mimicking the Sun?

Michael
```
 From: Thomas de Groot Subject: Re: Lumens in LightSysIV? Date: 13 Jun 2019 02:48:42 Message: <5d01f1ca\$1@news.povray.org>
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On 13-6-2019 4:50, Mike Horvath wrote:
> On 6/12/2019 1:25 PM, Alain wrote:
>> For the Sun, don't bother with light fading : Use a light with the
>> parallel option and you are good. Unless your scene scale is
>> interplanetary, the fading over the extent of your scene is totally
>> negligible.
>>
>> For the extent of the Sun when using an area_light, make it's diameter
>> about 0.01 of the distance. Not exact, but close enough.
>>
>> For more normal lights, the fade_power must be 2. No other value is
>> realistic.
>>
>> For fade_distance : Make it small. For an area_light, it should be
>> about the width of the light. For point light, a value of about 0.1 is
>> correct.
>>
>
> Thanks. What about Lumens? Should Lumens always equal 1 for parallel
> lights mimicking the Sun?
>

With LightsysIV, I never bother with lumens. I use CIE which is part of
the package:

#include "CIE.inc"
#declare SunCol  = Blackbody(6500)*Intensity; //Sun temperature:
2800=red; 7500=blue white.

For position and diameter of the Sun, I use:

#declare SunPos  = vrotate(<0, 0, -2>*10e5, <Sun_alt, Sun_azm, 0.0>);
#declare SunDia  = vlength(SunPos)*2/215;  //apparent diameter of the Sun

The later is a formula by Cousin Ricky (somewhen in 2007 I think).

--
Thomas
```