Does light multiply?

[ectobuilder]

I had two lights side by side facing the same direction.

LIGHT "A" = 2600 LUX @ 13ft
LIGHT "B" = 150 LUX @ 13ft

But with BOTH lights turned on, I was able to meter 4000 LUX at 13ft.

Therefore does light output multiply? It certainly isn't additive! 2600 + 150 does not equal 4000!

 

[paulears]

Has your light meter got a diffuser on it? You used to get this a lot when using the old selenium meters without the diffuser because the acceptance angle was quite narrow, so while they worked accurately by pointing at the light source, moving them off axis even a small amount dropped the reading considerably.

Are you reading the light being reflected from a white card, or pointing at the lights? I'm guessing you are pointing at the lights, so cannot point at both at the same time. Repeat the test pointing at a white card. My old meter was also not that accurate when light got dim or very bright

 

[ectobuilder]

Has your light meter got a diffuser on it? You used to get this a lot when using the old selenium meters without the diffuser because the acceptance angle was quite narrow, so while they worked accurately by pointing at the light source, moving them off axis even a small amount dropped the reading considerably.

Are you reading the light being reflected from a white card, or pointing at the lights? I'm guessing you are pointing at the lights, so cannot point at both at the same time. Repeat the test pointing at a white card. My old meter was also not that accurate when light got dim or very bright

Pointing at the light. What is wrong with this method?

 

[paulears]

Pointing at the light doesn't take into account the fact that few light sources have a completely flat field. In fact some can even have an adjustment for pesky or flat field. Unscrupulous manufacturers used the phenomenon to make their light output figure look better. Good Fresnels, for example, are pretty even through the proportion then do a gentle drop off, while others have a very hot centre, and start to drop off straight away. If you use a meter that has those little clusters of lenses over the sensor, like a kind of blistered appearance, you will find that taking a reading on a lenses light source changes if you change the angle even slightly. Often they are less bright than they appear to be on the meter. A soft light source tends to be more even.

Sticking on a diffuser that collects light from a wider range of angles produces more reliable readings, and measuring bounce from a piece of white card does pretty much the same thing. The maths of light assumes light travels in straight lines, which of course it does, but the on or off axis strength varies considerably.

if you do your test again, run your meter left to right, through the beam and watch what happens. Then you will scratch your head and see how the maths messes up!

 

[Bruce Watson]

...does light output multiply? It certainly isn't additive!

Luminous flux (measured in lumens) and illuminance (measured in lux, or lumens / m^2) are in fact additive. That is, two instruments, each supplying 100 lumens, aimed at the same 5 square meter target, and exactly filling that target, will fill the target with 200 lumens, resulting in 40 lux.

Note that this would be a PITA to measure, even in a well equipped lab.