# Re: LED Lumens to Lux conversion?

On Wed, 14 Nov 2007 16:50:37 -0800, TazaTek wrote:

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.

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.

.