Lumens aren't really meaningful in this case. To get the absolute number of lumens the meter reads, you'd just have to know the area of the photodiode in m^2 and multiply by the lux, which is lumens per m^2.

What you can do, however, is figure out how far away the meter is from the light source, figure out the total area that the lumen output is being distributed to, and backcalculate to get lumens.

The problem with such a calculation is that it assumes that the light is radiating equally in a spherical manner, which is certainly not the case unless the bulb is spherical.

Let me give you an example:

When I've got one bulb on at what is about 7", I get the reading of 65000 lumens / m^2 .

At 7", the area the light is being distributed to is:

4 * pi * r^2 = 4 * pi * (7")^2 = 615 in^2 ~ 3971 cm^2 = 0.397 m^2

So for 65000 lux = 65000 lumens / m^2 , we have:

65000 lumens / m^2 * 0.397 m^2 = 25805 lumens

Obviously, the bulb - rated at 2700 lumens, is not putting out ten times that. It is simply that the distribution of light is not spherical. At either end of the bulb, the lux reading is much lower. You can see how important it is to properly place your bulbs.

If instead you assume that the bulb is distributing light roughly in a cylindrical pattern the length of the bulb - approximately 3" - you get the following calculation.

Area of distribution:

2 * pi * r * L = 2 * pi * 7" * 3" ~ 132 in^2 ~ 850 cm^2 = 0.0850 m^2

So for 65000 lux = 65000 lumens / m^2 , we have:

65000 lumens / m^2 * 0.0992 m^2 = 5525 lumens

This is still not quite right, but the order of magnitude is right. The difference can be explained by the rapid falloff in intensity as you move away from the center of the coil. In addition, the panel above the lights is covered in mylar, which reflects rather effectively.

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Lumens, lux, and adding it all up :: 16K HPS vs. CFL :: DIY Drip Irrigation :: Pack water transport