POV-Ray : Newsgroups : povray.advanced-users : fade_distance for intensity specified in Lumens : Re: fade_distance for intensity specified in Lumens Server Time: 29 Oct 2020 23:00:44 GMT
  Re: fade_distance for intensity specified in Lumens  
From: clipka
Date: 4 Oct 2017 14:52:05
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

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