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

5. Photometric Measurements (continued)

5.3 Photometry At Light Receiving Plane

5.3.1 Illuminance  E

Key words:

Description, definition, unit, other names:

This photometric quantity is very closely related to its radiometric counterpart, which is the irradiance  Ee , see para. 4.3.1 .

Illuminance  Ev  is quite a simple concept, very similar to luminous exitance  M  (para. 5.2.1):
Like luminous exitance  M , illuminance  E  also is SPATIAL FLUX DENSITY. It is FLUX PER AREA; more precise: total RECEIVED FLUX per unit detector area:

              E = (Phi) / A2

And you certainly know that in photometry, this equation is to be read as
 " Illuminance  =  (luminous flux)  per  (receiver area) "

For the term "total" RECEIVED FLUX please see para. 4.3.1 .

The unit to apply for illuminance is
      Lumen  per  square-meter  =  LUX
                      lm / m^2  =  lx

You might also come across different names for the quantity "illuminance": "LUMINOUS INCIDANCE" or even "illumination".

Measurement procedure:

Though very simple, let me add a few words about the procedure:

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

If the illuminance on some surface (e.g. a table) is to be measured, just place the illuminance meter detector on this surface and let it "look" towards the light source. (If "horizontal" illuminance is asked, let the detector look vertically upwards.) Then read the LUX value from the meter. Did you notice a change? - Bingo. Moving your head over the illuminance meter display will almost always throw some kind of shadow onto the detector. So try and keep your body's influence as small as possible. Good illuminance meters have a cable between detector and display unit. Use it!

Importance, meter examples, usage and "abuse":

During most of our every-day life, we deal with objects that are not self-radiating ("optically passive"). Visibility of such objects is governed by illuminance (and, of course, governed by surface color contrast).

So, everywhere in architecture, in lighting technology, and in ergonomics, illuminance meters (or "luxmeters") are in WIDE-SPREAD USE. Look at six ILLUMINANCE METER EXAMPLES in the next figures, which I have chosen by chance from the ample market.

fig.5.3.1-a: Illum.-meter by Gigahertz-Optik(60 kByte)

fig.5.3.1-b: Illum.-meter by Gossen (34 kByte)

fig.5.3.1-c: Illum.-meter by International Light (73 kByte)

fig.5.3.1-d: Illum.-meter by PRC Krochmann (74 kByte)

fig.5.3.1-e: Illum.-meter by Minolta (30 kByte)

fig.5.3.1-f: Illum.-meter by UDT INSTRUMENTS (21 kByte)

And this kind of meters became the most INEXPENSIVE means of measuring light. It's worth while thinking whether they can be used for other light-measurement tasks as well. You'll find some hints on this topic in the other paragraphs that deal with photometric measurements. Especially in 5.1.1 Luminous Flux  (Phi)  and in 5.2.2 Luminous Intensity  I .

Though, in radiometry, the illuminance meter is hard to apply. This is because of the very special spectral sensitivity characteristics of the illuminance meter ("V(lambda)", shown in fig. 5.0-b of para. 5.0). The device under test (d.u.t.) has to fit very well into these characteristics (see para. 4.0). And only if you know the spectral characteristics of your d.u.t. for certain, and only if you perform the 4-step spectral calculus described at the end of para. 5.0, then you can transform a "LUX" reading into its radiometric counterpart. This counterpart, of course, is irradiance . (A listing of radiometric/photometric counterparts is to be found in table 8.1 in para. 8.)

One exception, though, can do without this "4-step calculus": monochromatic light. Here you simply use the  V[(lambda)]-value  at this light wavelength together with the maximum luminous efficacy of radiation
     Km  = 683 lm / W
for a (compound) transformation factor. But please don't use this method outside the range of  450 nm  up to  680 nm ! Only within this range there will be  V[(lambda)]-values  big enough so as to supply exact "radiometric" values.

While the "ABUSE" of illuminance meters for the measurement of other quantities can be quite an elegant method, please don't forget the ACCURACY problem. An error band of 5 to 10 % is quite normal with any light measurement and this will get even worse if you need subsequent transformations.


As I mentioned above, illuminance measurements are in widespread use. That's why not only a host of example measurements are published, but also various regulations cover this subject. Here are some LUX EXAMPLES that I found and that might give you a bit of a feeling for LUX values:

Subject Illuminance in Lux source
clear sky with sun, summer noon up to 1E5 (1)
clear sky with sun, winter noon up to 1E4 (1)
diffuse daylight 2E4       (3)
clear night sky with full moon about 0.2 (1)
clear night sky without moon about 1E-3 (1)
artificial indoor illumination, older type up to 300 (1)
artificial indoor illumination, newer type up to 2E3 (1)
artificial arena illumination (for color TV recording) up to 1500 (1)
platform at railway station 10    (2)
staircases 100    (2)
manufacturing, office, school rooms 300...2000    (2)
demanding visual tasks (microelectronics, surgery) 3E3...2E4    (2)
incandescent bulb, 100W, distance 1 m 100       (3)
artificial indoor illumination 200...600       (3)
office rooms 500          (4)
rooms for technical drawing 750          (4)
rooms for assembling smallest parts 1500          (4)

Summer noon with  1E5 lx  and full moon with  0.2 lx  show the phantastically wide adaptation range of our eyes: nearly factor  1E6 !

With the knowledges we gathered so far, the statement
 " 100 W  incandescent bulb at  1 m  distance delivers  100 lx " (3)
can easily be checked. And that's what we'll do as an exercise:

100 lx  means  100 lm/m^2 .
A sphere with a radius of  1 m  has a surface of  4 * (pi) m^2 .

fig.5.3.1-g: Lux-to-lumen sphere (19 kByte)

We assume that the bulb is nearly an isotropically radiating point source. And hence, it would have to emit in total
   100 lm/m^2 * 4 * (pi) m^2  =  1257 lm
This would give a bulb efficiency of
   1257 lm / 100 W  =  13 lm/W
Which is a reasonable value for old incandescent bulbs; lamp manufacturer Osram claims  13.6 lm/W  for his 100-W bulb "CLAS A CL 100" (5).

Link List and Literature

Subject used in source
illuminance example values text ref. (1) AEG Lichttechnik "Lichttechnische Erlaeuterungen" Booklet L 1/L 07.06/0373, Leuchten-Vertrieb der AEG-Fabriken Hameln page 5
illuminance example values text ref. (2) Harry Paul, "Lexikon der Optik",
Band 1, Berlin 2003, page 67
illuminance example values text ref. (3) Hans-Georg Buschendorf:
"Lexikon Licht- und Beleuchtungstechnik"
Berlin 1989, page 28
illuminance example values text ref. (4) R.Baer,
Berlin, 1996, page 58 f.
citing DIN 5035 (artificial indoor illumination)
and CIE-Publ. No. 29.2 "Guide on interior lighting", second edition 1986
efficiency of 100-W lamp text ref. (5) Osram CD: "Light Programme 2000/2001",
--> Lichtprogramm --> Allgebrauchs-Gluehlampen
--> Classic A Standard -->
Classic A-Kolben/Standardform, klar
Illuminance-meter by Gigahertz-Optik fig. 5.3.1-a Gigahertz-Optik (photometer)
Gigahertz-Optik (detector head)
Illuminance-meter by Gossen fig. 5.3.1-b Gossen
Illuminance-meter by International Light fig. 5.3.1-c International Light
Illuminance-meter by PRC Krochmann fig. 5.3.1-d PRC Krochmann
Illuminance-meter by Minolta fig. 5.3.1-e Minolta
Illuminance-meter by UDT INSTRUMENTS fig. 5.3.1-f UDT INSTRUMENTS

Continued: 5.3.2 Luminous Exposure  H

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Last modified May 2nd 2004 08:06