POV-Ray : Newsgroups : povray.advanced-users : fade_distance for intensity specified in Lumens Server Time: 19 Jun 2019 07:48:26 GMT
  fade_distance for intensity specified in Lumens (Message 1 to 5 of 5)  
From: Mr
Subject: fade_distance for intensity specified in Lumens
Date: 4 Oct 2017 11:55:00
Message: <web.59d4cb71300e5c016086ed00@news.povray.org>
Hi, In order to create good lamp presets for Blender exporter I am trying to set
up a few example light sources and chose to use the lumens value as light
intensity multiplier, so that the relative power of every light has the proper
order of magnitude.
Chosing 1 candle at 12.57 lumens and a 100w tungsten lightbulb around 900/1000
lumens, then 250W halogen is already burnt with at least 5000 lumens, not to
mention the 1000w high pressure sodium... I need to compensate these high values
with a very low fade_distance such as 0.0000025, right? but then candle  or 40w
get blacked out, or is there any other parameter to use?
I don't belive I could use different fade values otherwise all proportions will
be lost, won't they?


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From: clipka
Subject: Re: fade_distance for intensity specified in Lumens
Date: 4 Oct 2017 14:52:05
Message: <59d4f595$1@news.povray.org>
Am 04.10.2017 um 13:53 schrieb Mr:
> Hi, In order to create good lamp presets for Blender exporter I am trying to set
> up a few example light sources and chose to use the lumens value as light
> intensity multiplier, so that the relative power of every light has the proper
> order of magnitude.
> Chosing 1 candle at 12.57 lumens and a 100w tungsten lightbulb around 900/1000
> lumens, then 250W halogen is already burnt with at least 5000 lumens, not to
> mention the 1000w high pressure sodium... I need to compensate these high values
> with a very low fade_distance such as 0.0000025, right? but then candle  or 40w
> get blacked out, or is there any other parameter to use?
> I don't belive I could use different fade values otherwise all proportions will
> be lost, won't they?

Caution -- you're doing it wrong!


First, let's talk about `fade_distance`:

While that parameter does indeed affect brightness, you should /not/
think of it as being invented for that purpose.

Instead, think of it as a parameter specifying something about the light
source's geometry. It accounts for the fact that if your light source
isn't a perfect point, the falloff will deviate from the ideal
inverse-square-law as you get close to the source; and the value should
have the same order of magnitude as the assumed radius of the light source.


Second, let's talk about measures of "brightness":

In POV-Ray, a light source's brightness parameter does /not/ specify
/luminous flux/ (i.e. the total output of the light source, measured in
lumen); this is most apparent in two phenomena:

(1) Spot lights -- If POV-Ray's parameter was luminous flux, reducing a
spot light's opening angle should increase the light source's apparent
intensity, as the same total output would be focused onto a smaller
area. This is not the case.

(2) Fading lights -- If POV-Ray's parameter was luminous flux, changing
the `fade_distance` shouldn't change the brightness at a reasonably
large distance.

Both phenomena rule out the parameter representing luminous flux;
phenomenon (2) also rules out luminous intensity (i.e. the luminous flux
per unit of solid angle).

I think the most useful interpretation of POV-Ray's parameterization is
/luminous exitance/ (i.e. the light emitted per unit of surface), with
`fade_distance` specifying the radius of the light source.

Luminous exitance is measured in lux, i.e. lumen per square meter.

(Beware that the parameterization for light sources may or may not
differ from that for emissive media and emissive surfaces by a constant
factor such as 2, pi, or 2*pi.)


Finally, let's talk about overly bright images:

If your image is too bright (and there's nothing else wrong with it),
/the/ one and /only/ reasonable way to solve the issue is to reduce the
brightness of everything emissive (light sources, emissive media,
emissive surfaces, and either the global `ambient_light` parameter or
all `ambient` parameters).

You can do this essentially two ways:

(1) Specify all values in a different base unit. -- If for instance a
choice of 1 unit = 1 lux turns out too bright, then choose e.g. 1 unit =
10 lux.

(2) Introduce a scale factor. -- Specify a global constant, e.g.
`#declare BrightnessFactor = 0.1`, and use this wherever you specify
brightness.


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From: Mr
Subject: Re: fade_distance for intensity specified in Lumens
Date: 4 Oct 2017 20:45:00
Message: <web.59d547801300f05461ea16670@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:
> Am 04.10.2017 um 13:53 schrieb Mr:
> > Hi, In order to create good lamp presets for Blender exporter I am trying to set
> > up a few example light sources and chose to use the lumens value as light
> > intensity multiplier, so that the relative power of every light has the proper
> > order of magnitude.
> > Chosing 1 candle at 12.57 lumens and a 100w tungsten lightbulb around 900/1000
> > lumens, then 250W halogen is already burnt with at least 5000 lumens, not to
> > mention the 1000w high pressure sodium... I need to compensate these high values
> > with a very low fade_distance such as 0.0000025, right? but then candle  or 40w
> > get blacked out, or is there any other parameter to use?
> > I don't belive I could use different fade values otherwise all proportions will
> > be lost, won't they?
>
> Caution -- you're doing it wrong!
>
>
> First, let's talk about `fade_distance`:
>
> While that parameter does indeed affect brightness, you should /not/
> think of it as being invented for that purpose.
>
> Instead, think of it as a parameter specifying something about the light
> source's geometry. It accounts for the fact that if your light source
> isn't a perfect point, the falloff will deviate from the ideal
> inverse-square-law as you get close to the source; and the value should
> have the same order of magnitude as the assumed radius of the light source.
>
>
> Second, let's talk about measures of "brightness":
>
> In POV-Ray, a light source's brightness parameter does /not/ specify
> /luminous flux/ (i.e. the total output of the light source, measured in
> lumen); this is most apparent in two phenomena:
>
> (1) Spot lights -- If POV-Ray's parameter was luminous flux, reducing a
> spot light's opening angle should increase the light source's apparent
> intensity, as the same total output would be focused onto a smaller
> area. This is not the case.
>
> (2) Fading lights -- If POV-Ray's parameter was luminous flux, changing
> the `fade_distance` shouldn't change the brightness at a reasonably
> large distance.
>
> Both phenomena rule out the parameter representing luminous flux;
> phenomenon (2) also rules out luminous intensity (i.e. the luminous flux
> per unit of solid angle).
>
> I think the most useful interpretation of POV-Ray's parameterization is
> /luminous exitance/ (i.e. the light emitted per unit of surface), with
> `fade_distance` specifying the radius of the light source.
>
> Luminous exitance is measured in lux, i.e. lumen per square meter.
>
> (Beware that the parameterization for light sources may or may not
> differ from that for emissive media and emissive surfaces by a constant
> factor such as 2, pi, or 2*pi.)
>
>
> Finally, let's talk about overly bright images:
>
> If your image is too bright (and there's nothing else wrong with it),
> /the/ one and /only/ reasonable way to solve the issue is to reduce the
> brightness of everything emissive (light sources, emissive media,
> emissive surfaces, and either the global `ambient_light` parameter or
> all `ambient` parameters).
>
> You can do this essentially two ways:
>
> (1) Specify all values in a different base unit. -- If for instance a
> choice of 1 unit = 1 lux turns out too bright, then choose e.g. 1 unit =
> 10 lux.
>
> (2) Introduce a scale factor. -- Specify a global constant, e.g.
> `#declare BrightnessFactor = 0.1`, and use this wherever you specify
> brightness.


Most values I could find from manufacturers were specified in Lumens; do you
think there would be a few ways to make a coherent conversion into lux or lm/m²?
like choosing a 1m distance from light source and considering the section of the
illuminated area cut by a square meter plane, in proportion to surface of that
illuminated volume (sphere for a point light), as a factor by which to divide
the initial lumens value? with a formula for most cylinders, for bulbs, for
cones?


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From: Bald Eagle
Subject: Re: fade_distance for intensity specified in Lumens
Date: 4 Oct 2017 22:50:01
Message: <web.59d564b31300f0545cafe28e0@news.povray.org>
"Mr" <mauriceraybaud [at] hotmail dot fr>> wrote:

> Most values I could find from manufacturers were specified in Lumens; do you
> think there would be a few ways to make a coherent conversion into lux or lm/m²?
> like choosing a 1m distance from light source and considering the section of the
> illuminated area cut by a square meter plane, in proportion to surface of that
> illuminated volume (sphere for a point light), as a factor by which to divide
> the initial lumens value? with a formula for most cylinders, for bulbs, for
> cones?

When I worked at a business that sold a wide variety of products - including
tactical flashlights, I often had to answer this type of question, and looked
into it a fair bit.  I think that research file has gone the way of many things,
but the short answer is no.

The long answer is the usual one:  it depends on a lot of things, and depending
upon how accurate you want or need to be, you can make some simplifying
assumptions and get a good approximation.

Clearly some people do just that:
https://www.bannerengineering.com/us/en/company/expert-insights/lux-lumens-calculator.html
http://www.ledstuff.co.nz/data_calculators.php

based on this:
https://en.wikipedia.org/wiki/Integrating_sphere

you might be able to figure out a way to simulate this and draw some sort of
conclusion.

You might also try to just illuminate a rectangle and based on the image
produced with an orthographic camera, and letting Ansel Adams' Zone System guide
you - you could do a sort of integration of the brightness of the pixels over
the whole surface of the rectangle.

But keep in mind that this is a raytracer for making pictures - it's not a
professional optical / illuminance software package.


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From: Mr
Subject: Re: fade_distance for intensity specified in Lumens
Date: 5 Oct 2017 19:15:01
Message: <web.59d684761300f05461ea16670@news.povray.org>
"Bald Eagle" <cre### [at] netscapenet> wrote:
> Clearly some people do just that:
>
https://www.bannerengineering.com/us/en/company/expert-insights/lux-lumens-calculator.html
> http://www.ledstuff.co.nz/data_calculators.php
>
> based on this:
> https://en.wikipedia.org/wiki/Integrating_sphere
>
Thanks a lot, That's all I needed


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