Date: Tue, 1 Feb 2000 09:24:14 -0500 (EST) From: "Aaron M. Ellison" <aellison@mtholyoke.edu> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg311$foo@default> Subject: foot candles, lumens, and light
In response to:
>Date: Mon, 31 Jan 2000 15:17:17 -0400
>From: osito@intraweb.cl (Jose Gengler L.)
>To: cp@opus.hpl.hp.com
>Subject: foot candles to lumens
>Message-ID: <20000131192523614.AAB338@osito>
>
>Hi!
>
>I have been looking at James and Patricia Pietropaolo's book entitled
>"Carnivorous Plants of the World". It is a very technical, sinthetic,
>and full of wise advises. Units come both in metric and american
>systems. I am very glad I bought that book.
>
>But one thing has got my attention. They measure luminosity in foot
>candles, a unit they say is widespread. All the literature I have been
>reviewing measures luminosity in lumens. I don't know the relationship
>between these two units. I supose it is not a linear relatinship. I would
>greatly appreciate if someone shares a conversion formula.
A foot candle is the amount of light given off by a 'standard candle'
(whatever that is) at 1 foot. A foot candle = 1 lumen/square foot, so
there is a linear conversion. However, neither lumens nor foot candles
measure what a plant sees, which is photosynthetically active radiation
(light in the 400-700 nm [nanometers] wavelength range), and is measured
as photosynthetic photon flux density, or PPFD, in moles of photons per
meter squared per second. There is no conversion between lumens (or foot
candles) and PPFD, because lumens/foot candles are what your eye sees.
As an example, one lumen measured at 562 nm = 1.49 milliWatts of
radiation, and one lumen measured at 430 nm 1 126.7 milliWatts of
radiation.
It's much more useful how much PPFD your artificial lights are putting
out. Cheap radiation sensors for PPFD are undoubtedly available, but the
light manufacturers should be able to provide the information directly.
Aaron Ellison
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