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


4.0 General
Key words:


Very simple: we directly measure the POWER that the electromagnetic radiation conveyes.

RADIOMETRY is opposed to "PHOTOMETRY" (para. 5), the latter additionally considering the effect on the human eye.

Strictly speaking: in both, RADIOMETRY and PHOTOMETRY, we do not only consider the conveyed radiation POWER. Instead, we also consider POWER DENSITIES and POWER DISTRIBUTIONS across areas and across (solid) angles. Kind of geometrically derived quantities. That's why many different quantities need to be defined and measured. A host of quantities which need not only to be understood, but also to be learnt.

PHOTOMETRY naming difference:

In both RADIOMETRY and PHOTOMETRY, we use the same symbols for the "same" quantities.
Where it seems necessary, you can add indices for discerning radiometric from photometric quantities.
Take INDEX "e" (energetic) for the radiometric quantity and take INDEX "v" (visual) for the photometric quantity.

The quantity names also indicate whether RADIOMETRY or PHOTOMETRY is meant: names containing "...RADI..." belong to radiometric quantities; names containing "...LUMI..." belong to photometric quantities. You can test this principle when reading the table of contents in para. 0 and the comparison table in para. 8.

Bandwidth problem:

Well, "POWER that the electromagnetic radiation conveyes" sounds beautifully simple.
But: within which band of wavelengths? Just 633 nm? Or 380 to 780 nm? Or 1500 to 1870 nm?
A question as simple as important.

If you wanted to measure (radiometrically) light with unknown wavelengths correctly, you'd need a detector without WAVELENGTH LIMITS. Which is not available or at least, impractical. So, is correct radiometric measurement impossible?
No, of course it is possible. But carefully watch wavelengths and BANDWIDTHs:

** Always denote the wavelength (or, if applicable, the band of wavelengths) at which the measurement has been taken.

** For general application, your detector should have a broad BANDWIDTH and within this BANDWIDTH, a FLAT RESPONSE.
Make sure that the light to be measured does not contain radiation outside the detector's FLAT RESPONSE limits.
An example of good wavelength fit is given in fig. 4.0-a.

    fig.4.0-a: broad-band detector, narrow-band light; 16 kByte)

** If the light to be measured has a broader BANDWIDTH than your detector, you would need BANDWIDTH-limiting filter(s).
These filters would have to provide steep stop-band slopes and flat pass-band attenuation characteristics with a specified attenuation amount ... not easy to get.
Fig. 4.0-b shows bad wavelength fit without these filters.
You'd better prefer spectral measurement as shown in para. 4.4.

    fig.4.0-b: flat detector, broad-band light; 21 kByte)

** Measuring monochromatic (NARROW-BAND) light is easier: Your detector does not need a FLAT RESPONSE.
Only spectral characteristics of detector sensitivity has to be known precisely.
Then you can measure monochromatic light of   k n o w n   wavelength and correct the meter reading by the sensitivity that the detector has at   t h i s   wavelength. Fig. 4.0-c shows two measurement examples.
(POWER meters for laser radiation often use this principle. You type in your laser's wavelength and the meter automatically corrects the reading.   N e v e r   use this meter for general broad-band measurements!)

    fig.4.0-c: detector broad-band but not flat, light monochromatic; 20 kByte)

** I recommend to measure BROADBAND light radiometrically one monochromatic component after another. This is dubbed "spectral" measurement. See para. 4.4.

Continued: 4.1 Radiometry Within the Beam / 4.1.1 Radiant Flux (Phi)

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Last modified April 19th 2004 14:49