Hi there, I just finished a rambling essay about plant growth and spectrum. Rather than repeat it you can read it, or not, at the link.

The simple answer is that it is possible to grow many vegetables from start to finish under only HPS lights. Growth under a metal halide may in some cases be faster some growers claim a better quality crop from HPS light. In any case, HPS lighting is generally used for flowering, fruiting, and general finishing of commercial crops. HPS is about 10% more efficient. HPS light is a weird colour. There are HPS lamps available now that have a bulb coating to produce some blue light, and this may turn out to be the best of all worlds. Look for word like "enhanced coating". There is an enhanced HPS lamp called Son Agro (I think) but of course with the name comes more dollars.

The simplest answer to etiolation is always a higher light intensity. Move the lamp a little closer. Or maybe you could put in a shop light with a couple of 40W cool lamps, which would give a little more light and a good boost to the blue. Then you'd actually be able to see what colour your plants are :)

One last hint, using the lights for more hours each day generally produces more growth, but there is a limit to this. In tomatoes, there is nothing to be gained from more than about 14 hours of light each day. Too many hours of light can actually damage the plants.

Here is a link that might be useful: Photosynthesis and photomorphogenesis

Thanks again shrubs n bulbs, I will read your link.

You can get a Metal Halide bulb for your HPS set-up. It is called a Metal Halide conversion bulb and meant to be used with the HPS Ballast. Make sure that you get the same 400 watt size and that the base of the bulb matches. Also be aware that the reverse MH to HPS conversion bulbs are also available.

Paul

The Ceramic Metal Halide lamp will fit into your 400w
HPS ballast. This is called "retro-white" and is way
more efficient than other retrofits that use the old-
style quartz arc tube technology.

HPS is about 33% more efficient than standard Metal Halide
for plant growth, particularly if you use enhanced HPS
like Hortilux. The Hortilux is more efficient, producing
more light in general. The blue spectrum is not
signficantly enhanced, proportionately, and HPS produces
very little besides yellow-orange.

I read the article "Shrubs" self-references. Unfortunately,
he does not provide links to the data he describes, so
there is no easy way to verify his interpretations.
I googled an article title, and received only a summary of
another article. Perhaps the journals he references are
available by subscription only. Thanks for the references,
perhaps I will get the public library of Seattle to mail
me xerox copies.

Here are a couple of Shrubs' statements:
"In particular the relative levels of red and far red
(or near infrared) light are most important for
controlling etiolation, but by far the most important
factor is the intensity of the light."

(Lermer) Perhaps here, "Shrubs" is referring to the
excessive stem elongation that typically occurs with
insufficient light. In nature, this typically occurs at
the bottom of a forest canopy; and light toward the red
end of the visible spectrum penetrates the canopy much
better because it bends easier. This can be observed when
the earth is in between the sun and the moon; the moon
will appear red, because sunlight passing through the
earth's atmosphere will bend, some hitting the moon. So in
nature, low light levels are associated particularly with
low levels of blue spectrum.

Although high intensity of even red light can counter this
excessive stem elongation, his article talks about his
experiences with fluorescent lights, not a good idea
beyond the seedling stage. The low intensity is good for
only a few inches (assuming you don't have a whole bank of
them). The leaves grow differently and maybe shocked when
later placed under more intense light. (The flourescent
reflector often blocks more natural sunlight than it
artificially produces, so not recommended generally for
greenhouses.) While it is true that infra-red and near
infra-red can cause stretching, typically grow-lights do
not produce excessive quantities in this region.

What is more relevant, is the presence of blue light,
which does reduce stem elongation. Ceramic Metal Halide
produces more blue than a normal MH, and more red than an
HPS. Blue induces growth hormones, red induces flowering
hormones; each has other functions as well. See:
Artificial Lighting in Horticulture by Philips.

(Shrubs) "A few experiments comparing gro-lux lamps, that
concentrate most of the light emission in narrow red and
blue bands, with standard cool white tubes, which have a
spread of wavelengths in the red and blue and also some in
the green, have shown no worthwhile difference."

(Lermer) Dr. Norton of the Mount Vernon Ag station found
that the sylvania gro-lux did worse than the standard cool
white. The grow-lux spectrum was good, but obtained by
filtering out the less efficient parts of the spectrum,
resulting in low over-all intensity. But why even talk
about "Gro-Lux"; I thought this bulb was discontinued.

What you want generally is full spectrum light,
efficiently produced to achieve sufficiently high
intensity (i.e. about 4300 foot candles). Different stages
prefer different parts of the spectrum, this can be fine
tuned by using PSMH bulbs by Life Lights (that come in
four different average color temperatures).