foot candles vs candela
Jeff
that specifies a maximum of 50 foot-candles for a light source. My LEDs
have a candela spec and we usually specify our devices in nits. Is there a
way to convert footcandles to nits?
I also often get the question to convert lumens to nits.
Thanks,
Jeff
Andrew Resnick
From
http://www.unc.edu/~rowlett/units/index.html
footcandle (fc or ftc) a traditional unit of illuminance or
illumination, defined as the illuminance received by a surface at a
distance of one foot from a source of intensity one international candle.
The "international candle" was the predecessor of the candela as the
standard unit of light intensity. Illuminance is now measured in lux;
one footcandle equals 10.764 lux or about 1.0764 milliphot. The unit is
also spelled foot-candle or foot candle.
candela (cd)
the SI base unit for measuring the intensity of light.
<snip some history>
This definition has also been discarded, and the candela is now defined
to be the luminous intensity of a light source producing single-
frequency light at a frequency of 540 terahertz (THz) with a power of 1/
683 watt per steradian, or 18.3988 milliwatts over a complete sphere
centered at the light source. The frequency of 540 THz corresponds to a
wave length of approximately 555.17 nanometers (nm); normal human eyes
are more sensitive to the yellow-green light of this wavelength than to
any other. In order to produce 1 candela of single-frequency light of
wavelength l, a lamp would have to radiate 1/(683V(l)) watts per
steradian, where V(l) is the relative sensitivity of the eye at
wavelength l. Values of V(l), defined by the International Commission on
Illumination (CIE), are available online from the Color and Vision
Research Laboratories of the University of California at San Diego and
the University of Tübingen, Germany.
nit (nt) an MKS unit of luminance, equal to one candela per square meter,
or 10-4 stilb. The nit is not approved as part of the SI, but it has
been approved since 1947 by the International Commission on Illumination (
CIE) and is in wide use. The name of the unit comes from the Latin niteo,
to shine. This unit is sometimes called the meterlambert, by analogy
with the footlambert.
Ok, so fc is a radiometric measure of illuminance (an areal measure (W/
m^2)), cd appears to be a photometric unit of intensity (an angular
measure (lumen/steradian)), and nit appears to be a photometric unit of
luminance (lumen/m^2*steradian). Converting between these is going to
be a real pain in the a--. You need the following information to go
from cd to fc:
Angular distribution of emitted light
Spectral content of source
I think (not being a radiometry expert) that first you convert cd into
the radiometric equivalent- that is, convert lumens into Watts. That's
what the spectral distribution and CIE tables of V(l) are for. Then,
given the angular emittance and distance to the surface, you can convert
the angular flux distribution (W/steradian) into flux incident onto the
surface (W/m^2).
If you can get your hands on "Introduction to Radiometry" by William
Wolfe, I would do so.
--
Andrew Resnick, Ph. D.
National Center for Microgravity Research
NASA Glenn Research Center
Bob May
the light source? A sharp beam with only a foot-candle of light can be
quite bright and they may consider that to be too bright of a light, much
brighter than a 500 foot-candle light spraying the light all over the place.
--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!
Bob Knowlden
You can't directly convert from foot-candles to nits, because foot-candles
are illuminance units (lumens per square foot, equivalent to irradiance in
radiometry), while nits are luminance units (lumens per square meter per
steradian [unit of solid angle], analogous to radiance in radiometry).
(Candelas are units of luminous intensity, lumens per steradian.)
As an example: a source that is 10 cm (0.1 m) square and located 10 ft from
a surface would provide 0.0001 foot-candles of illuminance on that surface
per nit of luminance of the source.
You may find Warren Smith's "Modern Optical Engineering" helpful in
elementary radiometry/photometry. It's still in print (not the same edition
as I bought in 1977). Calculus is helpful in dealing with concepts like
solid angle.
HTH.
Bob Knowlden
Spam dodger may be in use. Replace nkbob with bobkn.
"Jeff" <see...@insightbb.com> wrote in message
news:17fPa.26586$Ph3.2381@sccrnsc04...
Zane
I suspect that their spec is for the foot-candles _on_ the area being lit.
For example, the street if it were for a street light. If so, you could
easily calculate that using the total output and angular characteristics of
your devices.
Are you aware that there is a sci.engr.lighting group? The guys there
probably know more than the ones that hang out here about the general
problem, and you should probably post your question there.
Zane
Jeff
meter that will measure lux. But if I can predict mathmatically the
foot-candles for the LED device that would something I could prove on paper
from the LED specs.
Thanks for the tip on the other group.
Jeff
"Zane" <zane...@sansnetcom.com> wrote in message
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Jeff
them do not understand the specs. They are looking for a statement from me
that I won't exceed their spec and they will take that.
I purchased the IESNA handbook, I think a lot of the ordinances take their
data from there.
Jeff
"Bob May" <bob...@nethere.com> wrote in message
news:10578664...@news-1.nethere.net...
Jeff
Jeff
"Andrew Resnick" <andy.r...@NOSPAM.grc.nasaDOTgov> wrote in message
news:20030710134...@newsread.grc.nasa.gov...
rll
Handbook" by Alex Ryer. You can download it at their web site:
http://www.intl-light.com/handbookthanks.html
It is an excellent primer on Photonics and radiometry. You can also request
a hard copy from International Light.
Best Regards,
- Russ @ LTS
"Jeff" <see...@insightbb.com> wrote in message
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