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'[EE] Superb light measurement explanation'
2004\10\12@200920 by Russell McMahon

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Someone else (Dave VanHorn?) just posted this deep in the body of a post
It is so useful that it deserves more prominent notice.

If you are less than fully informed about the relationship of lumen,
candella, watts, lux, steradian and other related optical terms (and most
people are) then this page deserves studying. Despite its clear presentation
and pretty pictures most will find it extremely hard going. Do spend the
time wading through it if you want to understand the material - I have not
yet seen a more complete explanation. It's probably worth attempting to
summarise its key points in a less explanatory but more usable form. (I'll
be happy if somebody else beats me to it ! :-) ).


       http://www.intl-light.com/handbook/ch07.html

A 70 page PDF or HTML version of the entire "Light Measurement Handbook"
that this is a chapter from can be obtained from

       http://www.intl-light.com/



       Russell McMahon

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2004\10\12@203009 by Dave VanHorn

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At 07:08 PM 10/12/2004, Russell McMahon wrote:

>Someone else (Dave VanHorn?) just posted this deep in the body of a post
>It is so useful that it deserves more prominent notice.
>
>If you are less than fully informed about the relationship of lumen,
>candella, watts, lux, steradian and other related optical terms (and most
>people are) then this page deserves studying. Despite its clear
>presentation and pretty pictures most will find it extremely hard going.

Was me.. Isn't optics and photometry fun! :)



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2004\10\13@151826 by Peter L. Peres

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On Tue, 12 Oct 2004, Dave VanHorn wrote:

> At 07:08 PM 10/12/2004, Russell McMahon wrote:
>
>> Someone else (Dave VanHorn?) just posted this deep in the body of a post
>> It is so useful that it deserves more prominent notice.
>>
>> If you are less than fully informed about the relationship of lumen,
>> candella, watts, lux, steradian and other related optical terms (and most
>> people are) then this page deserves studying. Despite its clear
>> presentation and pretty pictures most will find it extremely hard going.
>
> Was me.. Isn't optics and photometry fun! :)

Yes. For corncob-based units. But if you want SI units (which incidentally
seem to have been the official standard in your country for a while now):

1 lux = 1 Watt = 1 Joule * 1 second (power)

1 lux = 1 candela *  1 steradian solid angle / m^2 (power)

1 lumen = 1 candela * 1 steradian solid angle (radiation density)

1 phot = 10,000 lux (power)

1 steradian is simply a cone with 1 rad tip angle, intersected by the
surface where you measure the lux. So a lamp with about 60 degree light
cone (~1 radian) about 28 cm away from a plane that produces 1 watt
(=1lux) of light on the plane has 1 candela.

So, f.ex. to obtain 200 lux illumination on a work surface at 1 meter from
a desk lamp the lamp should produce about 9*200 (9 = 3^2 * 0.28 meters
from above - remember the inverse square law) candelas at 60 degrees light
cone or 1800 lumens. Which at 22lm/watt for an incandescent light, and
neglecting reflectivity and losses in the lamp shade, works out to a 75 or
100 Watt lightbulb and is about right from my 'real world' experience with
desk lamps. You can also see that halving the distance between desk light
and work will quadruple the light power.

The most confusing part of working with light units is the way people mix
the denominations for various units. E.g. 'intensity', 'power' and
'brightness' are interchangeable for all but physicists apparently.

Peter
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2004\10\13@162336 by Dave VanHorn

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>
>So, f.ex. to obtain 200 lux illumination on a work surface at 1 meter from
>a desk lamp the lamp should produce about 9*200 (9 = 3^2 * 0.28 meters
>from above - remember the inverse square law) candelas at 60 degrees light
>cone or 1800 lumens. Which at 22lm/watt for an incandescent light, and
>neglecting reflectivity and losses in the lamp shade, works out to a 75 or
>100 Watt lightbulb and is about right from my 'real world' experience with
>desk lamps. You can also see that halving the distance between desk light
>and work will quadruple the light power.

Good old inverse square law :)

>The most confusing part of working with light units is the way people mix
>the denominations for various units. E.g. 'intensity', 'power' and
>'brightness' are interchangeable for all but physicists apparently.

Indeed.  reminds me of an argument I had once over "training" and
"education". The other guy was arguing that they are the same..  After much
discussion, I finally said, Ok, should I sign your daughter up for sex
education, or sex training?

That finally made it clear !!

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2004\10\15@161500 by Peter L. Peres
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On Wed, 13 Oct 2004, Peter L. Peres wrote:

> 1 lux = 1 Watt = 1 Joule * 1 second (power)

This posting is totally wrong, and I apologize for it. I do not understand
what happened. The candela is defined by itself, and the lux is not 1
watt.

1 lux = 1 lm / m^2 = 1 cd sr / m^2

Peter
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