Brightness measurement

[Alan]

Can any of the clever people on this group help me out?

I'm trying to compare two A3 LED Light Boxes and work out which is the =

brightest.

The first states - "Lumen: 1800 ~ 2100 mcd"

The second - "Brightness: 730cd/m=B2, Luminous Flux: 372LM"

Obviously they are not necessarily comparing like with like, and are =

probably incomplete specs, so it is confusing.

Is there an easy way to tell from those specs?

-- =

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[Theo]

Post by Alan
Can any of the clever people on this group help me out?
I'm trying to compare two A3 LED Light Boxes and work out which is the
brightest.
The first states - "Lumen: 1800 ~ 2100 mcd"
The second - "Brightness: 730cd/m², Luminous Flux: 372LM"

In the absence of other details:

A3 = 420 x 297mm = 0.124 m2

730cd/m2 * 0.124 = 91cd = 91000mcd

which is 43-50 times brighter than the other one.

However the candela is a unit of intensity per (volumetric) angle and the
lumen is a unit of lighting power; if you focus those lumens on a very
narrow beam you get a lot of candela in one spot and not much anywhere else.
Sort of like you can have a 40W bulb emitting a dull light everywhere or the
same 40W concentrated into a much brighter narrow spot.

However this doesn't make much sense for a light box where the light is
diffuse, rather than a headlight with beam.

372lm maps onto slightly less than a 40W tungsten light bulb, maybe more
like 32W. It's hard to tell what the mcd figure means without knowing
anything about angles.

However a wax candle is supposed to emit 1cd, so the first is about twice as
'bright' as a candle. Since a candle is roughly isotropic, there are
4pi * 1cd = 12.6 lumens, so the second is emitting 30 candles worth. That
makes the second 15x brighter than the first if they were both isotropic.

The numbers don't quite line up, but either way the second is the winner.

Theo

[Alan]

On Thu, 03 Oct 2024 18:07:42 +0100, Theo =

Post by Alan
Can any of the clever people on this group help me out?
I'm trying to compare two A3 LED Light Boxes and work out which is th=

e

Post by Alan
brightest.
The first states - "Lumen: 1800 ~ 2100 mcd"
The second - "Brightness: 730cd/m=B2, Luminous Flux: 372LM"

A3 =3D 420 x 297mm =3D 0.124 m2
730cd/m2 * 0.124 =3D 91cd =3D 91000mcd
which is 43-50 times brighter than the other one.
However the candela is a unit of intensity per (volumetric) angle and =

the

lumen is a unit of lighting power; if you focus those lumens on a very=
narrow beam you get a lot of candela in one spot and not much anywhere=

=

else.
Sort of like you can have a 40W bulb emitting a dull light everywhere =

or =

the
same 40W concentrated into a much brighter narrow spot.
However this doesn't make much sense for a light box where the light i=

s

diffuse, rather than a headlight with beam.
372lm maps onto slightly less than a 40W tungsten light bulb, maybe mo=

re

like 32W. It's hard to tell what the mcd figure means without knowing=
anything about angles.
However a wax candle is supposed to emit 1cd, so the first is about =
twice as
'bright' as a candle. Since a candle is roughly isotropic, there are
4pi * 1cd =3D 12.6 lumens, so the second is emitting 30 candles worth.=

=

That
makes the second 15x brighter than the first if they were both isotrop=

ic.

The numbers don't quite line up, but either way the second is the winn=

er.

Theo

Thank you for that.

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