"Mr" <mauriceraybaud [at] hotmail dot fr>> wrote:

> Most values I could find from manufacturers were specified in Lumens; do you

> 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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