Light measurement question

[Guest lightnix]

I have an LED lamp which gives 400 lux at 0.5m and a client who wants to know what it gives in cd.

 

Is there any way in which the latter can be calculated from the former? Initial :unsure: searching suggests maybe not :(

 

EDIT: I asked this question in another forum and just got the following reply:

 

"100 candela. First you need to normalize the lux reading for 1 meter using the inverse square law, so you get 400*(0.5)²/1² = 100 lux. Lux at 1 meter equals candela."

 

Does that ring any bells?

[j_b]

I have an LED lamp which gives 400 lux at 0.5m and a client who wants to know what it gives in cd.

<{POST_SNAPBACK}>

 

You need to know the beam angle of the light source.

 

400 lux is equal to 400 lumens per square metre, that's the amount of luminous flux, and will go down 'inverse square' over distance. Yes, at 1m, it will be 100 lux.

 

The candela indicated the luminous intensity in a particular direction.

 

When a light source has an even light output over a given solid angle of 1 steradian, then it is true to say that that light output in candela, say, 100 candela, will light up a surface of 1m² with 100 lux.

 

So you could say, that for 1 steradian, lumens = candela.

 

Therefore, we now need to know the size of the solid angle of light that your LED emits, in order to calculate back to the luminous intensity, in candela.

[Guest lightnix]

Thanks j_b (and Brian for PM'ing me with an interesting article on the subject).

 

The lamp in question is a customised version of one of our 21 LED lamps and has a beam angle of 50°. Sorry to ask people to "do my homework for me", but Maths has never been my strongest subject, so any additional help at this point will be more than welcome :)

[kalmatthew]

Right My working gives 87.3cd. However I cant guarantee I got it right. My working is enclosed below for scrutiny. This formula was worked out purley through dimensional anlysis.

 

Lx is illuminance in lux

m is Distace in meters

cd is intensity in candela

sr is the angle in radians.

 

http://kalmatthew.org.uk/Blueroom/Nixpost.png

[j_b]

Thanks j_b (and Brian for PM'ing me with an interesting article on the subject).

 

The lamp in question is a customised version of one of our 21 LED lamps and has a beam angle of 50°. Sorry to ask people to "do my homework for me", but Maths has never been my strongest subject, so any additional help at this point will be more than welcome :)

<{POST_SNAPBACK}>

 

Let's see what other offers you got, but of I got the conversion right, 50° is pretty close to 3/4 of a steradian.

 

Best way to check, I rekon seeing as you can, is set up a light in a darkened room, and illuminate 1m² evenly using one of these - that's a 63cm diameter circle. Then 1 lux will equal 1 candela of sorts, see poor explanation why below...

 

The illumination of this, in lux is the same number as the light source luminous intensity, in candela. If I got the sums right, that should be in the order of 166 candela. Of course, the light output won't be even, infact it's lambertian for a single LED and the approprate fiddle factors could be used, but for your cluster, hell, just measure it! Take several readings, and get a feel for the measurement. The candela rating for LEDs is usually quoted for straight on axis.

 

Interested to hear how it goes...

 

Edited for spelling mistake and it was really awkward to read - and I did get the sums wrong, still not exactly 100% convinced...time will tell, it's far too hot for that sort of thing!

[Tom Baldwin]

Looking into this for a different problem, I found the following formula lurking on the bottom of some old VL datasheets:

 

for a given output in candela (cd), the center beam illuminance (I) is given, for a specified distance (D) as:

I = cd / D^2

if D is in metres, then I is in lux

if D is in feet, then I is in footcandles

 

It seems to produce plausible results, and avoids having to deal with steradians!