I am not sure I understand the need for a full spectrum source for black
and white though I agree one must consider the spectrum with more care for
color work.
If you are not using multi-contrast paper I would think a bluer lamp would
look good and a lot of inexpensive fluorescent lamps ought to fill the
bill. For multi-contrast papers you must be sure the required spectral
components are there but my intuition suggests just a couple (or maybe a
few!) selected lines would be sufficient) ("A few" might come into play if
the emulsions have more than two components to gain wide contrast
range.) More than likely is it that a couple well positioned lines could
increase the available contrast range by avoiding overlap of the spectral
responses of the low-contrast and the high contrast components. You would
have to judge this by looking at the lamp data and the emulsion data.
Any fluorescent lamp for which I have seen data tends to have some sharp
line structure superimposed on a continuum. The phosphors are blended to
yield the required spectrum for the application, almost always to give the
eye the correct impression (Some blends seem to be aimed at machines which
have special requirements and there are green lamps for copy machines, to
give a single example.) The narrow lines come from the mercury discharge
which excited the phosphors and some of this light leaks out with the
extent of the leakage depending greatly on the lamp type. The total amount
of light left in these lines is sometimes a lot and sometimes rather small
(you should notice the width of spectral features and compare area under
the curve in determining importance of the leakage.)
Try to obtain the catalog from Osram for a nice set of graphs of the spectra.
Some of you might also want to look at the "POWER STAR HQI". This gives a
pretty smooth spectrum from a concentrated source (possibly suitable for a
condenser enlarger, although the source could be too big in the higher
power types) There is a major problem; you cannot turn these lamps on and
off very easily. When hot they start with great difficulty and the
lifetime drops dramatically with short cycles. A shutter would be
required. Conversely, maybe they are satisfactory for color work at high
powers. The lamps can deliver massive powers (maybe 3500 watts input) with
efficiency better than a hot filament lamp so the same input power is
delivering more light.
Bob
At 01:01 20.09.02 -0400, you wrote:
>Hello:
>
>I am curious if anyone has ever used 'tri-phosphor' lamps for enlarging.
>
>I'm interested in an 8x10 enlarger for b/w only, on a budget.
>
>I started looking into low pressure pulsed xenon - lotta heat, and nearly
>obsolete, $100 for 750W lamp, $3000 for new commercial ballast/power supply,
>and it's not a friendly prospect - 52 V at about 18 amps plus 10000 volt
>spike to ionize gas...so, I think I'll look into other approaches.
>
>Ansel Adams used massive array of logo-less incandescent lamps - heat again.
>
>I'm pondering either full spectrum fluorescent (linear tubes only as far as
>I know), or tri-phosphor complact fluorescent (3 spectral peaks spread out
>over visible spectrum depending on color temperature, 2700K, 3500 K, 4100K
>and 6500K available (CRI 82, but that may not be relevant to film). I got
>spectral plot of the 6500K one today because I was unfamiliar with that one,
>and the spikey spectrum of the tri-phoshor type does make me nervous...hence
>my request for others' fluorescent experience.
>
>What I'm considering is using seveal GE Biax (folded tube) 18 or 27W lamps
>for an 8 x 10 or 11x 14 head, each driven with a high frequency electronic
>ballast. Driving the lamps above 15kHz eliminates the 120 Hz (100 for 50 Hz
>countries) flicker and produces about 15% more lumens than line frequency. I
>have access to a manufacturer of small electronic ballasts (I used to work
>there). I am considering running them from a DC supply with individual
>regulators so the light output could be adjusted for each lamp.
>
>My main worry is the strong spectral peaks in this type of lamp. The intent
>of this lamp design is that the brain is supposed to 'fill in the spectral
>gap', and THINK they are full spectrum lamps.
>
>Thanks in advance for responses.
>
>Murray
>
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