Studio lighting

The following item appeared in rec.arts.fine, in response to a request
for recommendations for lighting for a painter's studio:

    Article: 87 of rec.arts.fine
    From: macrakis [at] gr__osf__org (Stavros Macrakis)
    Newsgroups: rec.arts.fine
    Subject: Light for painting
    Date: 28 Dec 90

    Lighting engineers measure lamp color on two axes: the color
    temperature and the color rendition index (CRI).  The CRI tells you
    how close the spectrum is to a black-body spectrum, and the color
    temperature tells you what temperature (and therefore color) of
    black body it would be if it were one.

    All lamps that work on the incandescence principle, whether
    conventional or halogen-filled, have a black-body (continuous)
    spectrum and therefore a CRI of 100%.  The color temperature depends
    on what regime you run the lamp in.  If you dim a lamp, you will
    move the temperature down and therefore the spectrum towards the
    red.  All practical incandescent bulbs are reddish, the
    lower-wattage ones redder than higher-wattage ones.  They are all
    also relatively inefficient (i.e.  produce more heat for a given
    amount of light). You can also get incandescent bulbs with built-in
    filters to make them bluer (and raise the effective color
    temperature).  These are even more inefficient, since all a filter
    can do is convert some of the (unwanted) light into heat.  Warning:
    if a drop of water falls on a hot halogen bulb, it will likely
    explode.  This is a real danger, so if your studio leaks when it
    rains, beware!

    Fluorescent bulbs have discontinuous (spiky) spectra, and are much
    more efficient than incandescent.  But with appropriate dosing of
    phosphors, lamp engineers can come very close to whatever
    (perceived) spectrum they want.  You can get fluorescent bulbs with
    CRIs anywhere from 40% to 95% (numbers from memory), and with
    effective color temperatures in a wide range.  Top-CRI fluorescents
    are used by photographers, restorers, etc.  They are a specialty
    item, and cost somewhat more than regular fluorescents.  Also,
    beware of incompetent dealers who've never heard of a CRI before.

    Note also that sunlight varies considerably in color, depending on
    the angle (how much atmosphere it's going through) and the
    atmospheric conditions.  What's called `daylight' is not direct
    sunlight, but the famous `north light' of painters.  This is does
    not have a black-body spectrum, and is bluer than direct sunlight.

    I _do not_ recommend mixing light sources to try to get a more even
    color.  The usual result is that you get weird shadows--e.g. shadows
    to the left of an object are bluish, while those to the right are
    reddish.  This even happens if you mix different color fluorescent
    tubes in one fixture.  (Of course, if you diffuse and mix the light
    well enough, this problem goes away.)  Also, your visual perception
    apparatus can get confused as to the `reference white'.

    I also do not recommend using filters, for several reasons: good
    filters (Wratten gels) are not cheap--I doubt that theatrical
    filters are appropriate; selecting the proper one is not trivial;
    they do nasty things like melt and burn; they eat up good light and
    therefore run hot; filter holders which keep in unfiltered light
    while letting out hot air are hard to built and expensive to buy.

    Bottom line:

    If reddish light is OK, get yourself a battery of halogens (all the
    same) if power consumption and heat are not a problem.  I don't know
    the current market in the US, but in France you can get a basic 500W
    halogen floor lamp for about $30, and a garden-type fixture for $10.
    (But be sure you diffuse properly.)

    If you need light closer to skylight, look into high-CRI
    fluorescents. Most lighting dealers will not know the details of
    these things, so you may want to ask for specification sheets from
    the manufacturers (GE, Philips, etc.).  These also have the
    advantage of running cool.

    Stavros Macrakis
    Open Software Foundation Research Institute

    Mail:  2 av de Vignate, 38610 Gieres (Grenoble), France
    Net:   macrakis [at] gr__osf__org
    Phone: +33/76.63.48.82  Fax: +33/76.51.05.32

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                  Conservation DistList Instance 4:35
                 Distributed: Friday, December 28, 1990
                        Message Id: cdl-4-35-005
                                  ***