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From fredrick@well.com Thu Jun 1 22:49:00 1995
To: BobbieF103@aol.com, EDL%ATLANTIC@acgate.acus.org, NBFV09A@mail.prodigy.com,
ahicks@prism.nmt.edu, stephen@spin.ho.att.com
Subject: FAQ on lamps.
There have been 16 requests for the FAQ and I am trying to answer a few
questions that some posed with this brief note.
A couple of requestors wanted me to design their lighting setup. I am not an
illumination engineer(that is what a lighting design engineer is called), my
specialty was power electronics and since much of the time I was the only or
one of two electronic engineers at the new products facility I had to be
something of a universal expert and I got to know a lot about lamps. However
we did have a lot of illumination engineers working for the company and they
did the lighting design for customers (especially big customers). Both GE
and Sylvania ( now Sylvania-Osram since GTE sold the Lighting Division to
the German firm Osram) have big forces of field engineers in the various
market areas around the country or globe. They have computer programs that
they might use on your problems if you but ask.
To rlwalker: Your idea of a single 1000 watt HID in the center of the
basement is not going to work because it is impossible to design a reflector
with such a low head-room to spread the light out with some degree of
uniformity. You might try Hydrofarm's lights on a trolly that moves back
and forth over a span of a few minutes to average out the illumination.
To Tony Johns: It sounds as if you don't have enough light . (Etiolated
plants- I had to look that up!) Incandescents are very inefficient light
sources and the waste power goes into heat. Without getting very fancy with
heat absorbing filters and dichroic reflectors I doubt you can get enough
light to do the job without incinerating the plants. HID does make a little
UV but compared to sunlight it is weak. A sheet of plain glass will absorb
almost all of it. When we wanted UV for some reason we had to use special
glass in the lamps to avoid absorbing it. The glass outer bulb of a regular
HID lamp absorbs most of the UV from the arc tube. Conversely if the outer
bulb is broken the bare lamp puts out dangerous amounts of UV. But not to
worry - I had my cataracts removed last year and I can see better now with
my implants than I ever could , even when I was young!
Les: If the plants are ok why worry. The only way to remove the excess heat
is to vent it. you don't need 80% humidity. Phals grow well at much less.
With a light cart setup you are sort of stuck with fluorescents because of
the form factor..
DISCHARGE LAMPS
(more than you ever wanted to know about them.)
By Fred Paget.
The term Discharge Lamp describes an illumination device that creates light
through an electrical discharge through a gaseous medium. Fluorescent lights
are a low pressure discharge lamp. The fill gas is at a low pressure and
consists of argon and mercury vapor. The amount of mercury is tiny and the
light given off is largely in the ultra-violet. The inside of the glass tube
is coated with a highly developed phosphor which converts the UV to visible
light. The glass of the tube stops almost all of the remaining UV.
A High Pressure Discharge lamp has a very small discharge tube generally
made of quartz or alumina contained in an outer bulb of special glass. The
inner tube is operated at a high enough current to heat it very hot. The
fill gas and solid fill compounds exist in a gaseous plasma state while
conducting the lamp current. They are called HID or High Intensity Discharge
lamps. The term lamp is used in the lighting industry to describe the part
most people call a light bulb. A fixture is the device that holds the lamp
and in the case of HID lamps frequently contains a reflector and a ballast.
Ballasts can be located remotely . Lamps requiring starters should have the
starter in the fixture since very high voltages are involved. Ballasts for
large HID lamps are heavy and produce voltages that can be lethal so they
should be treated with respect and installed in a safe manner by qualified
personnel
There are a number of discharge lamps that are available and almost all have
been used for plant growth. Every genus and maybe every species reacts a
little differently under lights so no general type of light or spectral
distribution is best. Fruiting plants like tomatoes seem to need more red.
Roses are grown commercially under high pressure sodium lights. Orchids
which are of such diverse parentage are a puzzle but experience is that they
are not too fussy about special requirements. If experiments were done under
controlled conditions it would probably be possible to optimize for each
type. Your Phals might require a different regimen than your Catts or Paphs,
etc. People I have talked to who sell the systems for hydroponics and
orchids advise the metal halide lamps. These are available in several
different spectral distributions, because the rare earth mixture used inside
the arc tube is different. GE used a different fill than Sylvania. There is
a new light bulb company that started about 4 years ago in this field
(VENTURE) and some of their mixes are probably a little different too. Also
phosphors are used on the inside surface of the outer bulb to give a redder
light mixture. Since we don't know exactly what is optimum anyway it comes
down to using what is available and cheapest.
High intensity discharge (HID) lamps are made in a great number of different
types. They all require a ballast to start and operate - it is usually a big
heavy iron and wire component with associated AC capacitor and possibly a
starter module. Some mercury lamps even use a tungsten filament built right
into the lamp as a resistive ballast and will operate off an unballasted AC
line.
The first and simplest to make was the mercury arc lamp. The fill was an
inert gas and a little mercury which vaporizes into the arc plasma very
easily and is present in small amounts in many of the lamp types since it
makes for easy starting. There is also a starter electrode in one end of the
pure quartz arc tube as well as a main electrode. This simple lamp has a
horrid blue violet color and a lot of the output is in the UV. So nowadays
they all have some red phosphor on the inside of the bulb to correct the
color a bit. These lamps are not the best for plant growth but they are the
cheapest.
Low pressure sodium lamps are the most efficient producer of light of all
discharge lamps. They consist of a rather large lamp bulb with a u shaped
arc tube of considerable length. They come in several wattages and are used
mainly for street lighting. The light is monochromatic yellow. The sodium
fill produces the two sodium "D" spectral lines and very little else. (There
is some neon and other gas in the tube that make a tiny amount of other
colors.) The lamp is operated rather like a fluorescent in that it has a low
pressure discharge but there is no phosphor. I have never heard of these
being used forplant growth.
High pressure sodium lamps (HPS) are filled with a dose of straight sodium
metal and an inert gas like argon plus mercury. Due to the difficulty of
building in a starter electrode as well as a main electrode in one end of
the arc tube (which must be made of alumina and niobium metal to resist the
corrosive atmosphere inside) they are hard to start and require a starter
that pulses the lamp with about 4,000 volts to initiate an arc. There have
been some easy start lamps made that have Penning mixtures (rare gas
mixtures containing neon) so that they may be started on mercury ballasts;
however they are less efficient. After starting the mix heats up and the
sodium vaporizes and the light output comes up to the familiar golden color.
Because of the high pressure in the lamp arc tube the light is a continuous
spectrum with a peak in the yellow. These lamps have a very high visual
efficiency since the human eye peaks in the yellow. They also seem to make
good plant growth lamps. Phillips is advertising one with enhanced blue
output -probably a dose of some other metal as well as the sodium.
HPS lamps have a longer life than Metal Halide HID lamps. In the order of
20,000 hours as compared to half that for the useful life of Metal Halide.
When HPS lamps get too old their internal voltage drop gets higher and they
get to the point where the ballast cannot put out enough voltage to sustain
the arc. When they are not up to full operating temperature during the
warm-up period the internal voltage drop is lower so that the result is that
they start up and then as they warm up for a short time they seem to be OK,
but they then go out. This cycling on and off is a sign you need to buy a
new lamp.
Metal halide lamps are another category and employ a fill of rare earth
metal halide salts plus sodium and inert gas. Each company has their own
proprietary mix. These lamps are also harder to start than the mercury lamps
and usually require a different ballast with a higher voltage output and
designed to meet some of the special conditions that occur during warm-up.
They give a very nice white light and the flowers look pretty good under
some of them. (If you examine the light through a diffraction grating
spectral lines can be seen as well as a continuum. Depending on the mix the
plants may or may not look good to the human eye.) Photographs taken under
this light are usually a disaster and special filtration on the camera
should be used if there is no escape. These lamps are available from about
40 watts up to several thousand watts. The bigger the lamp the higher the
efficiency. Also the horizontal lamp position is more efficient and it was
discovered that in this position the arc wants to bow upward at the middle
so an "arch" lamp is available that has a bowed arc tube to accommodate the
bowing and these have the highest efficiency of all, but must be operated in
exactly the right position. HID lamps are three to six times as efficient as
an ordinary fluorescent lamp. Lamps are sold in vertical base up or base
down versions and either can be used horizontal. The reason for this is to
keep the starter electrode at the top of the arc tube in a vertical lamp.
Condensed fill chemicals gather at the bottom of the tube when it cools off
and could short out the start electrode.
The big advantage that the HID lamps have over fluorescent is their high
wattage, high efficiency, and concentrated nearly point source of light
which allows reflectors to be designed to put the light exactly where it is
needed. Reflectors are available on the best fixtures that spread the light
out uniformly over a wide area without a hot spot under the fixture. Fixture
design is a complex art and science and a simple fold of reflective metal is
not going to do it. Look for complex curves and peened surfaces on the
reflector.
HID lamps operate at high temperature so they typically take a minute or so
to heat up. During this period the light output goes through all kinds of
strange colors and changes constantly, depending on the fill used. If an
operating HID lamp has its supply voltage interrupted for more than a
fraction of a second the internal plasma will deionize and the lamp goes out
and is incapable of conducting current . The hot fill gas would require
perhaps as much as 50,000 volts to reestablish conduction, far more than the
socket and wiring could stand. So we must wait until the lamp cools off
enough so that the internal pressure drops to a point where the ballast
voltage can start it again. This can take anywhere from one to ten minutes
depending on a lot of variables. Then the lamp will restrike and start up
again. This is why when they lose the lights at a night game where there is
HID lighting, it takes so long to get them on again. Special bulbs were once
designed in Germany with a metal cap on top so that a 50,000 volt starter
could be employed to eliminate this so-called Hot Restrike Period but they
did not become popular.
Fluorescent lamps are made in many different types. The big runner in the
factory is the cool white 40 watt 48 inch lamp (F40). These are made on
automatic machines as big as a football field and sell at discount for less
than a dollar. They make a pretty good plant growth lamp for the broad run
of plants especially if they are supplemented with some tungsten
illumination or warm white bulbs to bring up the red a little. As some of
the contributors have said they work pretty well for orchids. The light is
not very intense and it is advisable to get them as close to the plants as
possible without burning the leaves.
Gro-lux and its competitors were a specialty product to try to optimize the
spectrum for a broad range of plants, There are two different Gro-lux types
with different spectral outputs. Because they are not made in such great
numbers as the F40 they cost a great deal more and they have a shorter
useful life too. The phosphor starts to change after a year and the lamps
should be replaced even though they are still burning.
There exist two other types of fluorescent lamps - for commercial
applications mainly. These are the HO (High output) and SHO (Super high
output) types. They require different ballasts since they have higher lamp
current than the F40. They are a lot brighter than the F40 but an analysis
of cost I once made convinced me that you could use a larger number of F40
lamps to get the same level of light at lower cost. This is mainly because
the F40 lamp is so much cheaper than the HO and SHO types. The HO is about
twice as bright as the F40 and the SHO (or VHO as some call it) is about
four times as bright, so you have to use four F40's to replace one if you
have room.
A word about ballasts for fluorescent lamps. The old fashioned magnetic
ballast for 2 F40s is a commodity product selling for only 3 or 4 dollars in
large quantity or about 15 dollars at retail. It has been hard to beat it
using electronic technology. You can buy electronic ballasts that are more
efficient and do a pretty good job of preserving lamp life. These are your
more expensive types. There are also some cheap electronic ballasts that do
not operate the lamps at full specified lamp current and/or give a much
shorter lamp life. They are usually to be avoided. Often they are present
in the loss leader "shop lights" sold in discount stores. If you don't mind
changing lamps more often, then these ballasts do operate pretty cool which
may be a factor in a flask house. Lamp life in fluorescent lamps is
adversely influenced by the peak currents that occur during certain parts of
the AC line cycle in most electronic ballasts. Light output is proportional
to lamp current.
So what is the best light source for orchids? I recommend a simple HID
fixture using a 400 watt metal halide suspended about 3 feet above the
plants. I have seen commercial products that use this setup and appear to be
growing very well.
Change your metal halide lamps at the end of rated life. They will
sometimes keep burning for years but start to give a very poor light.
Fred Paget 155 Elm Ave, Mill Valley, CA 94941 USA
Lighting Addition:
How to tell your lighting level, with a SLR camera! WOW!
from: saul@dkrz.de
I have compiled the tables from the informations given by:
Mark A. Dimmitt <markdim@azstarnet.com>
Robin Garrell <garrell@chem.ucla.edu>
Hilary and Frank Stendal <gblastn@teleport.com>
In order to facilitate the use, I created the table which
includes practically all the exposure time and f-stop range
found in modern cameras. I only calculated it for one film
speed as it is trivial to either set the film speed to
100 ASA/ISO or to correct the measured values by a constant
factor.
I am glad that you want to use the table for the FAQs.
Cheers, Joachim
Here is how to do the job:
Set the film speed of your camera to 100 ASA/ISO. Put a sheet of
white paper in the place where you want to perform the measurement
so that the paper gets the highest light intensitiy. Point your
camera to the paper and get close enough to it so that it fills
out all the finder view. You dont need to focuss your camera. Read
the values that the light meter of your camera gives and transform
them into the corresponding light intensity in foot candles
according to the following table:
| f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
-------+------------------------------------------------------
1/4 | 0.5 1 2 4 8 16 32 64 125
1/8 | 1 2 4 8 16 32 64 125 250
1/15 | 2 4 8 16 32 64 125 250 500
1/30 | 4 8 16 32 64 125 250 500 1000
1/60 | 8 16 32 64 125 250 500 1000 2000
1/125 | 16 32 64 125 250 500 1000 2000 4000
1/250 | 32 64 125 250 500 1000 2000 4000 8000
1/500 | 64 125 250 500 1000 2000 4000 8000 16000
1/1000 | 125 250 500 1000 2000 4000 8000 16000 32000
1/2000 | 250 500 1000 2000 4000 8000 16000 32000 64000
If, for example, you get an exposure time of 1/125 sec and the
f-stop is f/11 this would mean a light intensity of 1000 fc.
Happy light measuring!
kirby.a.smith@lmco.com
Subject: [17469] Re: [17357] Lumens to Foot Candles
RADREX2 <RADREX2@aol.com> wrote:
I have a question regarding the conversion of lumens to
footcandles. My light meter registers in lumens. I have my old Physics
book that says 1 f.c. = 10.76 lumens but have read some literature
lately that states they are equal. What is the hang up? Could it be the
distance from the source is being factored in? Anyone who can shed
some light, please do so. Steven in "wish I were shoveling sand, not
snow" Pittsburgh radrex2@aol.com
A candela (cd) is a lumen per steradian. The illumination a foot away
is a lumen per square foot or a footcandle (fc). The illumination a
meter away is a lumen per square meter or a lux (lx). The later is
less as the observer is farther from the candle. Thus a fc equals
10.764 lx. Note that a footcandle does not equal 10.76 lumens (lm).
FAQ component on light measurement
by kirby (kirby.a.smith@lmco.com)
_______________________________________________
Light measurement and its relation to orchid growing can be complex.
First, we need to clarify the units. There are Systeme Internationale
(SI) units of measure and English units of measure (and other units
irrelevant to this discussion). SI units are built upon the
meter-kilogram-second and English units upon the foot-pound-second.
Second, we need to clarify the difference between radiometric quantities
and photometric quantities. Photometric quantities relate to the
spectral response of human vision. The unit of photometric luminous
intensity is the candela. The candela emits one lumen per steradian
(sr), a solid angle. Of interest to the orchid is the level of
illuminance on its leaves. The English unit is foot-candle (fc), or
lumen per square foot. The SI unit is lux, or lumen per square meter.
One lux equals 0.0929 fc.
The radiometric equivalents of the above are radiant intensity in watts
per steradian and irradiance in watts per square foot or BTU's per
square foot in English units, and watts per square meter in SI units.
Measurements of illuminance and irradiance are made with light sensitive
detectors that are calibrated to yield the desired result under specific
conditions. For example, a spectrally uniform sensor would measure
irradiance independent of the light's spectral distribution. Cameras and
related illuminence meters use detectors which are not spectrally
uniform. Unless specifically filtered to effect a flat response, or a
human-vision-matching luminous response, the signal generated versus
truth will vary depending upon the light's spectral distribution versus
the detectors spectral response. A camera meter, for example, is usually
calibrated such that it will cause daylight film to be properly exposed
when subjected to light of the same spectral distribution as the sun.
When subjected to tungsten light it may read incorectly.
What has all this to do with orchids? The orchids' spectral response
(defined as the spectrum that optimizes growth) is not identical to the
human eye's sensitivity function, and thus photometric units, whether SI
or English, are not directly applicable. Neither are radiometric units,
because the orchids' specral response is not uniform. Measurement of the
photosynthetically active radiation (PAR) irradiance levels is desired.
Call them ft-PARs, or PARems per square meter for SI affectionados. What
we need is a detector that is filtered to match the orchids' spectral
response. Then habitat measurements, greenhouse measurements, and
artificial light measurements would all relate to the same scale.
This author has suspected for some time that the suggested light levels
for various species are those which are reported by photographic light
meter measurements in situ or in greenhouses that are successfully
growing the plants. Such values are applicable to growing under daylight
conditions. When growing under fluorescent, tungsten halogen, high
pressure sodium, or metal halide lights, the reported fc values become
less meaningful. First, few affordable meters will measure such lamps'
illumination correctly, and second, as described above, the photometric
illumination (fc) is not what is actually important to the orchids. For
example, a low pressure sodium lamp can produce many lumens and thereby
lux at some distance, but if the plant is not sensitive to the orange
color, all that illuminance is for naught.
In my limited experience with growing under a metal halide lamp, the
illuminance measured with my Minolta Flashmeter IV does roughly
correspond to my visual impression of the illumination at different
distances compared to that of the sun. That suggests the meter is
providing an approximately true illuminance value. However, the plants
tell a different story. Many act as if the _effective_ illumination is
stronger. This may be due to the lamp's output being better matched to
the plants' needs than to human visual response. The bottom line is
that, for growing orchids, photometric measurement needs to be tempered
by experience.
This culture sheet last modified:
v1.2 4/12/98
-AJHicks
Editor
Wonk
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