# LED Lumens to Lux conversion?

Discussion in 'Electronic Basics' started by TazaTek, Nov 15, 2007.

1. ### TazaTekGuest

Hi,

I'm trying to pick out some LED's for to make a 10,000 Lux @ 36 inches
(or 1 meter) LED lamp. My problem is that I'm not quite sure of how
to convert the lumens spec on the LED to the Lux to figure out how
many LED's I need to buy.

I know that Lux = Lumens/ m2 , but I'm not exactly sure of how that
applies to something that is 1 meter away, and would be, say the size
of a small book.

I'm currently looking at the Phillips' Luxeon 5 watt stars. Any feed
back on that as a high output LED?

Any help would be appreciated.

Thanks

Matt

2. ### NobodyGuest

The lux depends upon the size of the spot, and thus the illumination
angle. A wide-angle LED will produce a lower lux figure for the same
lumens.

To determine a rough lux value, divide the lumens figure by the area of
the LED's "spot". E.g. for even illumination over a 90-degree (+/-
45-degree) cone at 1 metre, the spot radius will be 1m*45*pi/180 ~=
785mm, and the area will be ~1.94 square metres, so lux ~= lumens/1.94.

If the "size of a small book" refers to focusing the entire output of
the LED on a small area, then the only factor is the area on which you're
focusing it, not the distance (obviously, you'll need to narrow the angle
as the distance increases to keep the smaller area).

E.g. if you're focusing on a 10cm square, lux = lumens / 0.01, so you
would need 100 lumens to get 10000 lux.

3. ### Bob MyersGuest

At a bare minimum - we should also note that this would assume
that every photon made by the light source winds up on that
10 cm square, which is hardly ever the case. In other words,
as usual - everything's lossy, and if you don't design in some
margin you should expect to come up short.

Bob M.

4. ### TazaTekGuest

OK. I think I understand. This would apply to the case where we are
slicing the cone for the whole output angle.
but how do I calculate the lux for an object that is less than the
output area?

how would I calculate the lux on an object that has a spot radius of
only 500mm at 1 meter where the LED has a width pattern of 785mm? This
would have a portion of the light going past the object on all sides

Can I *assume* that the light is uniform and therefore just take the
difference ie. lux = lumens/(1.94 - .785) (if its within a fair
margin, I'll call it good)

Thanks

Matt

5. ### NobodyGuest

The lux depends upon the area over which the light is spread, regardless
of what it falls on.

If you have 10000 lux falling on a sheet of paper, and you replace it with
a smaller sheet, it's still 10000 lux.

OTOH, if you narrow the beam to focus the same amount of light on a
smaller area, the lux goes up.
No, lux is independent of the size of the target.

The lumens figure is roughly analogous to watts, except that it's weighted
according to human visual perception. Similarly, lux = lumens/m^2 is
analogous to W/m^2. It is an indication of the brightness at a particular
point. E.g. the brightness of the light from a 40W bulb falling on a sheet
of paper 1m away is independent of the size of the piece of paper.

6. ### TazaTekGuest

Great, I think I get it now....

Thanks
matt