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M E A S U R I N G   L I G H T (continued)


Key words:

What "measurement" is:

Let us go back to the roots for a moment: What does "MEASUREMENT" mean?

Well, you assign a numeric value to a quantity, this quantity describing some property in the "real" world. This numeric value is derived from a comparison:
You compare the property in the real world with the analog property of a known item ... the latter being stored in some safe location.

Pretty abstract.
Let's have a look at an example (fig. 2-a).

    fig.2-a: measuring means comparing to a known item ; 34 kByte)

In the real world, there is a cupboard.
It has got a property "width".
You discovered this fine cupboard in a furniture store and you want to know whether it fits into your living room. So you ask a salesman for a folding rule.
Then the holy act of MEASUREMENT consists of unfolding the yardstick, and counting how many yards (or meters) the width of that cupboard is, and writing down this number.
At home, you take your own yardstick and check whether the gap between two cupboards in your living room is wide enough.

The quantity you measured was "width" of a cupboard.
The numeric value was the number of yards (or, preferably, meters).
The comparison was counting the number of meters by the use of a folding rule.
The known item is some kind of STANDARD METER which is preserved in some laboratory and which assures that the salesman's yardstick and your own yardstick will give the same reading.

Base units and derived units:

In metrology, people don't prefer to have standard items for each and every quantity that you could imagine.
Instead, they use standard items for very few "BASE UNITs" only. And derive other units from these basic ones.
Example (fig. 2-b): you got a STANDARD METER and a standard second. From this, you derive not only lengths and durations but also areas, volumes, speeds, accelerations, and so on.

    fig.2-b: base unit and derived units ; 13 kByte)

In the famous SI system (described for example here), all BASE UNITs and all DERIVED UNITs are defined.
Here, the BASE UNIT for the MEASUREMENT of light amplitudes is called "CANDELA".
Its definition is:
"If, in a given direction, a source emits monochromatic radiation of frequency 540E12 Hz (about 556 nm), and the radiant intensity in that direction is 1/683 WATT per steradian, then the luminous intensity of the source is 1 CANDELA." (1)

CANDELA as a BASE UNIT has certainly been chosen because of historic reasons.
The luminous intensity of a certain candle and, later on, of some special area of melting platinum was relatively easy to reproduce.
So this was chosen as a BASE UNIT.

But for understanding the system of light amplitude units, I recommend that you think of luminous flux, unit LUMEN, as a BASE UNIT. Perhaps because I am an electrical engineer. You see: LUMENs correspond directly to WATTs.

For the same reasons I consider the system of radiometric units to be better understandable, if you choose radiometric flux, unit WATT, as a BASE UNIT.

Link List and Literature

Subject used in source
description of SI-system text below fig.2-b
defining 1 candela text reference (1) Chambers Dictionary of Science and Technology, 1999, p.167
cupboard images fig. 2-a Microsoft Windows XP ClipArt

Continued: 3. What is Light?

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Last modified Nov. 29th, 2003; 10:24