Brightness of a light bulb: Lux and Lumens conversion

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Brightness of a light bulb: Lux and Lumens conversion




by Christophe » 30/09/06, 17:17

How to convert Lux lighting units to Lumens and vice versa?

Edit: for Lumens to Lux conversion, see https://www.econologie.com/eclairage-cal ... -4359.html et https://www.econologie.com/eclairage-pui ... -4360.html
And see a range of Megaman economical bulbs: economic and LED bulbs


Following a question on this forum ( https://www.econologie.com/forums/ampoules-a ... t2118.html ) on theLUX conversion to Lumens Here is a very simple small measurement method that seems to be accurate enough to determine the actual brightness of a Lumens light source. The opposite method is much easier: just divide the Lumens by the illuminated surface to obtain an approximation of Lux.

The overall path of the measurement protocol has been explained in detail.

There must be a luxmeter to do this. We used an ITC-777 in Luxmeter function.

Calculation of determination of lumens by approximation by a sphere.


Hypotheses :

1) the point source,
2) it is located at 40 cm from the ground
3) the bulb illuminates on a half globle divisible in half sphere by point of measurement.
4) the lighting is evenly distributed over each sphere.
5) the inclination of the sensor is the same in the 2 cases with respect to the radius of the sphere (false it will be corrected later if we have aberrant results).

1) Measuring point at 50 cm.

Radius of the Sphere: 64 cm
Surface of the sphere: 4 * Pi * R² = 5,147 m²
Lux measurement: 86

Power of the source: 86 * 5,147 / 2 = 220 Lumens

2) Measuring point at 100 cm.

Radius of the Sphere: 108 cm
Surface of the sphere: 4 * Pi * R² = 14,657 m²
Lux measurement: 51

Power of the source: 51 * 14,657 / 2 = 373 Lumens

-> difference far too important therefore necessary correction according to the inclination (this is where it may get worse :D )

Calculation of angles:

Point to 50 cm:
tg = 4 / 5, from which an angle of the sensor with respect to a radius of the sphere of: 38,7 °

Point to 100 cm:
tg = 4 / 10, from which an angle of the sensor with respect to a radius of the sphere of: 21,8 °

Here is the explanation ... We have almost twice the inclination on the point at 50 cm compared to the alignment with the radius.

As by "miracle" the ratio 38,7 / 21,8 and 373/220 are very close ... (1,77 and 1,69) ... but it may be a coincidence ... so I will have to redo radial measurements!

Radial measurement for a Sirius LED bulb:

150cm, 33lux
100cm, 48lux
50cm, 243lux
25, 1014lux
10, 5000lux

We therefore assume the angle zero when in reality it is respectively (the diameter of the bulb placed on the ground is 6 cm or 3 cm height if we equate the bulb to a point of light):

150 cm, 1,14 °
100 cm, 1,72 °
50 cm, 3,43 °
25, 6,84 °
10, 16,7 °

It is quite a mistake anyway ... well we will only use the 150 and 100 measurements of this series then.

1) 150 cm

Radius of the Sphere: 150 and dust cm
Surface of the sphere: 4 * Pi * R² = 28,274 m²
Lux measurement: 33

Power of the source: 33 * 28,274 / 2 = 466 Lumens

2) 100 cm

Radius of the Sphere: 100 and dust cm
Surface of the sphere: 4 * Pi * R² = 14,657 m²
Lux measurement: 68

Power of the source: 68 * 14,657 / 2 = 498 Lumens

Conclusion

Rather coherent ... The power in Lumens of this bulb is therefore of the order of 450 500 Lumens .... it should nevertheless, make a true radial measurement on 4 or 5 point to get a more precise measurement!

On the box it is indicated 40 lumens by W ... or it consumes 9,2W to the max ... and therefore all this is coherent (lumens measurement method and technical indication) ... except that it is given as a bulb of ... 5W ...

Edit: see also https://www.econologie.com/eclairage-cal ... -4359.html et https://www.econologie.com/eclairage-pui ... -4360.html
Last edited by Christophe the 04 / 04 / 12, 11: 45, 3 edited once.
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by Christophe » 02/10/06, 17:45

I just applied the same method to a tubular 15 W CFL (given for 910 lumens) but applying a complete sphere for the calculations given the shape of the bulb and it looks like it gives "fairly good" results (taking into account the method and the means of measurement).

Measuring point

30 cm: 1016,72 lm
50 cm: 1168,64 lm
75 cm: 742,18 lm
100 cm: 879,62 lm
150 cm: 1130,94 lm

Average 987,62 lm while the bulb is given for 910 lm.
Measured consumption: 18,7W (233V, cos phi = 1, A = 0,08) ie a "real" efficiency of 52,8 lm / W.

All this is therefore rather coherent! If there are candidates to confirm ...
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by Christophe » 02/10/06, 17:50

ps: the tests with this tube lamp were all the same quite laborious because the luminosity depends on the inclination (or rather the rotation) of the bulb along its main axis ... (because of the placement of the tubes and the no presence of "diffuser")
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by Woodcutter » 02/10/06, 18:16

Thank you for taking the trouble ... : Wink:
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