LED Lumens to Lux conversion?

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.

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.

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?

for instance, (taking your numbers), 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

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.

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

Thanks matt