Best Bulbs for Color Correct Viewing
Best Bulbs for Color Correct Viewing
- Subject: Best Bulbs for Color Correct Viewing
- From: Garth Fletcher <email@hidden>
- Date: Tue, 10 May 2005 22:50:01 -0400
- Organization: JacqCAD International (Fletcher Applied Sciences, Inc.)
Tim Pitts wrote:
I was wondering if any of you work in photo labs, and if so, what bulbs
do you use for light boxes, and overhead lighting where customers would
view prints. I remember reading that Sylvania F40T12 were good, but
there are 3 different versions, and I'm not sure which we need.
F40T12 is just a basic fluorescent tube description - 40 Watt, 12/8" diam.
The key issue concerns the set of phosphors applied to the inside
of the tube. Cheaper and/or more efficient (lumens/watt) phosphors
produce poor quality lighting - strong spikes at some wavelengths, gaps
at others.
Some phosphor mixes are formulated to produce higher quality light -
meaning a smoother spectrum and a specific color temperature - at
the cost of using more expensive phosphors.
The "quality" of light of a fluorescent tube is indicated by a "CRI"
(Color Rendering Index) rating, with 100 meaning that it matches
standard lights.
"Cool White" tubes have CRI as low as 60 or 65 and produce noticeably
undesirable light for color matching.
Many manufacturers produce special tubes with CRI values above 90
which are specifically designed for use where color matters.
So, you want to look for tubes which
a) have a high CRI (>=90)
b) have the desired color temperature (usually 5000°K).
Though high CRI tubes do cost more, it is not as much as one might
expect - less than twice the cost of a junk tube.
Each manufacturer has its own coding scheme, though usually include
a 2 digit color temp code. The 5,000°K tubes are the most common,
intended to match (loosely) the D50 viewing standard. I believe
that the following are all equivalent high CRI 5,000°K tubes:
"Chroma 50" from GE
"Design 50¨ from Osram/Sylvania
"Colortone 50" from Philips
Here is a small listing from a GE catalog for T12 Medium Bipin (G13)
40 Watts 48" length, 20,000 hours life tubes
=lumens==
# descrip new mean °K CRI Additional info
13794 F40/C50 2250 1870 5000 90 Chroma 50
13795 F40/C75 1950 1680 7500 92 Chroma 75
13797 F40/N 2100 1740 3700 90 Natural
12224 F40/SUN 2250 1870 5000 90 Sunshine
Quoting from the same GE catalog:
=======================================
Color Rendering Index (CRI)
An international system used to rate a lamp’s ability to render object colors.
The higher the CRI (based upon a 0-100 scale), the better colors appear.
CRI ratings of various lamps may be compared, but a numerical comparison
is only valid if the lamps are also rated for the same chromaticity. (See
Chromaticity.) CRI differences among lamps are not usually significant
(visible to the eye) unless the difference is more than 3-5 points.
Color Temperature
Originally, a term used to describe the “whiteness” of incandescent lamp
light. Color temperature is directly related to the physical temperature of
the filament in incandescent lamps so the Kelvin (absolute) temperature
scale is used to describe color temperature. For discharge lamps where
no hot filament is involved, the term “correlated color temperature” is
used to indicate that the light appears “as if” the discharge lamp is
operating at a given color temperature. More recently, the term
“chromaticity” has been used in place of color temperature. Chromaticity
is expressed either in Kelvins (K) or as “x” and “y” coordinates on the
CIE Standard Chromaticity Diagram. Although it may not seem sensible,
a higher color temperature (K) describes a visually cooler, bluer light
source. Typical color temperatures are 2800K (incandescent), 3000K
(halogen), 4100K (cool white or SP41 fluorescent), and 5000K
(daylight-simulating fluorescent colors such as Chroma 50 and SPX50).
In measuring CRI, scientists compare how eight specific colors appear
under the source to how these same colors appear under a reference
source. However, there are two reference sources: incandescent lighting
is the reference for warm color lamps and daylight for cool color lamps.
In this system, both incandescent lamps and daylight are considered to
have “perfect” CRI’s of 100 even though, as we know, materials appear
quite different when viewed under these two lights. These sources are
very different in color temperature (see below) although both have CRI’s
close to 100.
CRI is therefore meaningful in comparing lamps that are close in color
temperature. Also, since incandescent is a standard, any warm lamp
rendering colors differently from incandescent lamps—even if the colors
are more vivid—would be judged to have a CRI less than 100. CRI is not
a perfect measure but is still useful as an indicator of the quality of light
from a source.
Typical Color Correlated Color Indicators for Color Rendering
lamp °K CRI
===================== ===== ======
INCANDESCENT LAMPS
Standard 2700K 97-100
HALOGEN LAMPS
Standard 3000K 97-100
FLUORESCENT LAMPS
C75 ÒChroma SeriesÓ 7500K 90 +
Cinema 55 6900K 95
SP65 6500K 70 +
C50 ÒChroma SeriesÓ 5000K 90 +
SPX50 5000K 80 +
SPX41 ( 841) 4100K 80 +
SP41 (741) 4100K 70 +
Cool White 4100K 62
SPX35 (835) 3500K 80 +
SP35 (735) 3500K 70 +
Cinema 32 3200K 89
SPX30 (830) 3000K 80 +
SP30 (730) 3000K 70 +
Warm White 3000K 50
covRfresh¨ 2750K 87
SPX 27 2700K 80 +
HIGH PRESSURE SODIUM LAMPS
Lucalox¨ HPS lamps 2000K 22
Deluxe Lucalox¨ 2200K 65
METAL HALIDE LAMPS
Standard MVR Multi-Vapor¨ 3500-4000K 65-70
MXR Multi-Vapor¨ 3000-4000K 65-75
Arcstreamª 3000-5000K 75-80
Arcstreamª Daylight 6000K 90
CERAMIC METAL HALIDE LAMPS
CMHª 3000-4200K 80-90 +
=======================================
Do be sure to cross-check for latest data and actual CRI ratings!
Hope this helps...
--
Garth Fletcher
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