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

4. RADIOMETRIC MEASUREMENTS (continued)



4.3 Radiometry at Light Receiving Plane
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Radiated flux (Phi), see para. 4.1.1, of course is the same in the beam, at the source, and at the sink.
But anyway, the various kinds of flux densities and distributions are of different interest in the beam / at the source / at the sink. Let us now approach the sink and look at the irradiance that we get there:



4.3.1 Irradiance  E
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Key words:
SPATIAL FLUX DENSITY | FLUX PER AREA | RECEIVED FLUX | WATT PER SQUARE-METER | RADIANT IRRADIANCE | RADIANT INCIDANCE | DOSE RATE | OVERFILLED APERTURE | SUBSTITUTE INSTRUMENTATION


Description, definition, unit:

Irradiance  E  is quite a simple concept, very similar to radiant emittance  M   (para. 4.2.1):

Like emittance  M , irradiance  E  also is SPATIAL FLUX DENSITY. It is FLUX PER AREA; or, more precise: the total RECEIVED FLUX per unit detector area:

          E = (Phi) / A2

When calling it "total" RECEIVED FLUX I mean flux received from   a l l   directions of space.
That poses no problem, if the detector aperture of your instrument is very mat and dull, for example the soot layer that is used in some bolometers and thermopiles.
But if your detector's reflection characteristics has some gloss component, then cosine correction (para. 4.3.3) will be necessary.

The unit to apply for irradiance is
      WATT PER SQUARE-METER = W / m^2

It is the very same unit like the one used for emittance  M  (para. 4.2.1); so watch out when you meet the unit alone without the quantity name.


Other names:

This same quantity, Irradiance, is sometimes called "RADIANT IRRADIANCE" and "RADIANT INCIDANCE".
In photochemistry, in phototherapy, and in photobiology, this quantity is called "DOSE RATE" (1).


Measurement procedure:

Your detector's sensitive aperture needs to be filled entirely; better   O V E R F I L L   it. Otherwise the irradiance meter presumes homogeneous radiation in a field that is larger than the actually irradiated field. And hence, irradiance reading will be too low. Fig. 4.3.1 shows it in a cookbook manner.

    fig.4.3.1: overfilling detectors (24 kByte)

Do we really need a specialized irradiance meter, or can we use some SUBSTITUTE INSTRUMENTATION? - We can.
If you precisely know the sensitive aperture area size of your radiant flux meter ("radiometer", "power meter"), then take a flux measurement (at OVERFILLED detector APERTURE!) and divide this reading by detector aperture area. Quick and easy.


Link and Literature:
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Subject used in source
name "DOSE RATE" text ref. (1) Publication CIE No. 17.4 (1987)
"International Lighting Vocabulary"
Para. 06-25, page 170
Commission Internationale de l'Éclairage
(= International Commission on Illumination),
Kegelgasse 27, A-1030 Wien, Austria



Continued: 4.3.2 Radiant Exposure H

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Last modified October 17th, 2003 18:52