Product Review: Nipple Nonsense

A
great deal of angst, concern, and confusion has been created by
recent reports that the orientation of the “nipple” of a metal
halide can have a significant impact on the performance of a
metal halide bulb. Significant declines in intensity, apparent
color shifts, and even the failure to light have been attributed
to the position of the nipple. Unfortunately, these reports have
been based on anecdotal observation and until now not
investigated through systematic testing. In an effort to resolve
the many questions regarding the relationship between nipple
orientation and metal halide bulb performance, I completed a
series of tests to evaluate several assertions made regarding
bulb orientation.

The assertion perpetuated with greatest frequency is that the
intensity of a metal halide bulb is lowest on the side of the
bulb with the nipple. The nipple is a small disc of glass on the
outside of the inner envelope of a metal halide bulb created when
the metal halide bulb is manufactured (figure 1). It is left as
the inner envelope is sealed off after being filled with gases.
The size and location of the nipple varies with different metal
halide bulbs. It can range from as little as one-sixteenth of an
inch (3mm) with German bulbs, to nearly one-eighth of an inch
(6mm) with Japanese bulbs, and even larger with some American
metal halide bulbs. To put this into perspective, the visible
portion of a typical metal halide inner envelope is about 1.5
inches or 37mm. In other words, the nipple covers less than 10%
of the length of the inner envelope.

Methods

Measuring the impact of nipple location is not a simple task.
At the close distances that hobbyists typically place their bulbs
over a reef tank, a small change in distance can have a
significant impact on intensity. To draw valid conclusions
regarding variations in intensity, distances between the bulb and
light sensor need to maintained within a fraction of an inch.
Maintaining these tolerances while measuring light intensity in
360 degrees cannot be done by hand. Consequently I developed a
experimental design that makes use of equipment normally used in
professional panoramic photography. The key component consists of
a tripod with a rotating head calibrated in degrees with three
geared controls that enable one to make fine adjustments in the
X, Y, and Z axes. Using these controls, I was able to place the
long axis of the metal halide bulb in the exact center of a
rotating turntable enabling me to maintain a constant distance
between the bulb and the sensor as it rotates. With this
arrangement, the distance between bulb and sensor could be held
to less than one-sixteenth of an inch (less than 3mm) while
rotating the bulb through 360 degrees. A Li-Cor 2pi quantum
sensor connected to a Li-Cor L1000 data logger was mounted 24
inches from the bulb to approximate light levels at the bottom of
a typical reef tank and. The center of the sensor was placed in
line with the nipple, which is generally at the center of the
inner envelope. I recorded photosynthetically active radiation
(PAR) every 45 degrees through 360 degrees of rotation with the
data logger averaging 60 separate measurements at each point. To
evaluate color temperature, I used a Gossen Color-Pro color
meter.

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For reasons that will be explained below, the glowing arc of
each metal halide was photographed through a high density “solar
filter” normally used in astronomy to photograph the sun. The
majority of measurements were taken with the bulb in a base down
vertical position. This is because it proved to be the most
reliable replicable set up. However, measurements and arc
photographs were also taken with bulbs in the horizontal
position. To do this the sensor was left in the same position
while the bulb was switched to a horizontal position and rotated
as if on a rotisserie. Since the bulb was continually lighted
during the rotation and measurement, one concern was that
measurements might vary if the bulbs were fired in different
orientations. To test this possibility, a second set of
measurements and photographs were taken with the bulbs switched
off between reorientations. No differences were observed.

A total of four different 400 watt bulbs were evaluated, two
Iwasaki bulbs, a German 10,000 degree Kelvin bulb, and a Radium
20,000 degree Kelvin bulb, also from Germany. I tested the
Iwasaki MT400DL/BUD version as well as the MT400DL/BH version.
The difference between the two versions is that the BUD bulb is
designed for mounting in a vertical or pendant arrangement. The
BH version is designed for horizontal mounting. The ballast used
for PAR measurements was a standard 400 watt core and coil metal
halide ballast, although I also photographed the 10,000 and
20,000 degree bulbs using a 400 watt HQI ballast.

Results

The distribution of light around some bulb circumferences did
vary. However, the variation was not consistent from brand to
brand and the nipple position was not an accurate predictor of
intensity. Some bulbs produced greater intensity on the side with
the nipple while others produced greater intensity on the side
opposite the nipple. The polar graphs reproduced below for each
of the bulbs shows intensity in PAR. The orientation of the graph
is from the top of the bulb looking towards the bulb base with
the nipple at 0 degrees. The line represents the PAR in
uE/m^2/sec.

In the case of the two Iwasaki bulbs, the highest intensity in
the vertical (pendant) orientation was radiated on the side of
the bulb with the nipple. This was true for both the BUD and the
BH versions of the bulb. However, there was a significant
difference in the extent of the asymmetry between the two
versions. While the BUD version only varied 13% from side to
side, the BH version operated in the vertical position varied 39%
from side to side. In other words, the bulb operated improperly
produces a light field that is significantly more asymmetrical
than the Iwasaki, when operated as designed. Measured in a
horizontal orientation, the same Iwasaki BH bulb that differed by
39% from side to side in a vertical orientation varied no more
than 5% at any point.

Similar in light distribution to the Iwasaki bulb, the German
10,000 degree Kelvin bulb produced more light from the side with
the nipple than the opposite side. However, neither orientation
produced as much light as either side at 90 and 270 degrees from
the nipple. The German 20,000 degree Kelvin bulb generated quite
a different light field with the lowest light levels generated at
90 and 270 degree, just the opposite of the 10,000 degree Kelvin
bulb.

Color temperature of the bulbs varied less than intensity. The
Iwasaki bulbs varied from 6450 Kelvin at 0 degrees (towards the
nipple) to 6090K at 180 degrees opposite the nipple. The German
10,000 degree Kelvin bulb ranged from 23,250 degree Kelvin 90
degrees from the nipple to 21,270 degree Kelvin in the direction
of the nipple. The Radium bulb’s color temperature is beyond the
40,000 degree Kelvin range of the color meter.

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Discussion and Recommendations

The results make it clear that one cannot state categorically
that light intensity is lower on the side of the bulb with the
nipple. Quite the contrary, on the whole, light intensity tends
to be greater on the side with the nipple. Much more is at work
here than simply nipple orientation. An examination of the inner
envelope and glowing arc within the envelope explains the
differences. The orientation of the arc within the inner envelope
has a much greater impact on light intensity than the glass
nipple. In a vertical orientation, the Iwasaki bulb arc arches
towards the nipple and consequently produces more light on the
side with the nipple. This is true for both the BUD and BH
versions, but is more pronounced with the BH version burned in a
vertical orientation. In contrast, the German 20,000 degree
Kelvin bulb arc arches away from the nipple and therefore
produces greater light opposite the nipple. The 10,000 degree
Kelvin bulb produces the most uniform and centered arc of the
bulbs evaluated. However, the arc tends to be diffused and
somewhat oblong in shape, not cylindrical like the other bulbs
and produces significantly less light along the narrow portion of
the arc and greater light on the broad sides of the arc.

Of the bulbs tested, the most uniform light distribution
measured is produced by the Iwasaki BH bulb operated
horizontally. This is somewhat misleading, however. The two
photographs of the Iwasaki bulb operated horizontally show that
the bulb arc follows along the top of the envelope regardless of
nipple orientation. Since the sensor was in a fixed position and
the bulb rotated, the position of the arc relative to the sensor
remained constant. This would have been true regardless of the
placement of the sensor. However, had the bulb remained fixed and
the sensor rotated, light levels would have also varied in the
horizontal position, but probably less so than they did in the
vertical orientation.

Mounted horizontally in a typical canopy, metal halide bulbs
produce the highest intensity at the top of the bulb, regardless
of the position of the nipple. This can be seen in the horizontal
photos where the arc can be seen clearly closer to the top of the
envelope regardless of the bulb and regardless of the orientation
of the nipple. One thought is that convection currents distort
the arc. Regardless of the reason, mounted either horizontally or
vertically, the position of the arc is determined by physics and
the design of the bulb, not the placement of the nipple.

The arch of the arc towards the top of the envelope means that
when horizontally mounted over a tank, the greatest intensity of
light will be directed into the fixture’s reflector instead of
directly into the tank. At first glance this may seem to be
inefficient use of the brightest light, but quite the contrary,
this is fortuitous. Most hobbyists want to light as much of the
tank as possible. A metal halide bulb is a quasi-point source of
light and therefore alone creates a great deal of light directly
below the bulb and much less light further away from the bulb.
The reflector effectively spreads the light creating more even
light across the tank than the bulb alone could create.
Projecting the greatest amount of light into the reflector better
evens out the light field.

How does one explain the anecdotal observations such as bulbs
failing to fire if the nipple is mis-positioned? First, a reef
tank is a hostile environment for any electrical device,
particularly one that draws a great deal of current. Corrosion
can build up in the contacts and sockets of metal halide
lighting. In the course of trying to reorient the nipple of a
metal halide bulb, one must remove the bulb and by doing so may
remove the corrosion that was preventing the bulb from firing. I
have found that with an older metal halide bulb, simply
unscrewing and screwing the bulb back in will enable the bulb to
fire. Those who have observed a color shift when changing the
orientation of the nipple may also be inadvertently cleaning the
corrosion from the bulb contacts while attempting to reorient the
bulb. There is a very small color temperature shift at different
points around the bulb. It is possible that the small color
temperature difference is more apparent when the bulb is placed
next to a second bulb with a different color temperature. This
would be most apparent as a bulb ages and shifts in color. In
these experiments I was unable to significantly shift the color
temperature of any bulb by rotating the position, but I have
observed color shifts in older unstable metal halide bulbs where
the arc periodically changes position. One should realize,
however, that the nipple has nothing to do with the position of
the arc within the envelope, and that the nipple has nothing to
do with color shift.

My conclusion is that the concern over nipple orientation is
misplaced. The nipple itself has little to do with the
performance of metal halide bulbs including intensity or color
temperature. Each metal halide design creates a different arc of
light within the envelope and it is this characteristic that
creates asymmetrical light fields. While light intensity varies
somewhat with the orientation of the bulb, the use of efficient
reflectors can minimize any asymmetry. The orientation of a metal
halide nipple has far less impact on light over a reef tank than
the choice of bulb, quality of reflectors, and keeping the bulb
free of salt deposits and dirt. It is also important for Iwasaki
bulb users to choose the proper bulb for their application, the
BUD version for pendant users and the BH version for horizontal
mounting.

Note: When it comes to the apparent difference with the 20K
arc colors these differences might be interpreted as caused by
the glass nipple, but in fact that difference resulted because I
had to photograph the 20K vertical arc through welder’s glass,
because the Solar filter made the photograph too dim.

One final note. One should never attempt to look into a
lighted metal halide bulb without proper eye protection. All the
cautions regarding the viewing of solar eclipses apply to viewing
metal halide bulbs as well. Using welder’s glass is the minimum
protection that one should use and solar filters sold by
astronomy supply stores are an even safer choice.

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 Richard Harker

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