arc shape of metal halide lamp

[Pawel Paron]

Does anybody know what is the arc shape in a typical 400W metal halide lamp,
commonly used now for big area lighting (streets, etc.)? I'm going to use
such lamp in a diy LCD projector, and I wonder if I can mount it diagonally
to the LCD array (to make the whole box smaller, as this lamp is long). But
I don't want to obtain a non-symmetric differences in screen brightness, if
the arc isn't actually circular, or very close to circular.

Thanks in advance for information

Regards
Pawel

 

[Ioannis]

Does anybody know what is the arc shape in a typical 400W metal halide lamp,
commonly used now for big area lighting (streets, etc.)? I'm going to use
such lamp in a diy LCD projector, and I wonder if I can mount it diagonally
to the LCD array (to make the whole box smaller, as this lamp is long). But
I don't want to obtain a non-symmetric differences in screen brightness, if
the arc isn't actually circular, or very close to circular.

Thanks in advance for information

It depends on many factors. Most important factor, is plasma pressure
and temperature, but other factors come to play, such as tube shape and
discharge constituents.

Usually halides have specific burning positions, but can tolerate some
deviation from the indicated standard, with some reduction of their
output and change of their color rendering.

Most tend to have fixed burning positions as convection currents push
the arc towards the upper part of the tube depending on position, with
higher iodide concentrations towards the bottom of the tube.

For example the American MetalArc halides tend to condense Scandium and
Sodium iodides on the bottom, giving the bottom part of the arc a
slightly warmer color, while the upper part tends to be mercury dominated.

European halides have a different tube design which allows for greater
tolerances in burning position, although Sodium, Thallium and Indium
condensates can be found on their tube bottoms as well, causing the arc
to bend (for horizontal burning ones) and with the lower part looking
sodium-yellow while the upper part looks more Mercury like.

If you are concerned about uniform arc distribution, check the
Super-MetalArc by Sylvania, which have rigidly fixed burning positions
achieved with Position-Oriented Mogul bases and specially designed
discharge tubes to accommodate convection.

For all the rest, you will get a non-uniform arc color, even if you burn
them in their specified positions, because of the condensates.

The only 400W halides I've seen with almost perfectly uniform arc shape,
are the HQI-T/D by OSRAM. All the rest bend or distort their arc somewhat.

Note that the distortion is of course much less on vertical burning
positions, but it still happens: The arc is warmer on the bottom of the
tube and cooler on top, and if memory doesn't fail me, it's also a
little thinner on the bottom.

 

[Pawel Paron]

Usually halides have specific burning positions, but can tolerate some
deviation from the indicated standard, with some reduction of their
output and change of their color rendering.

My light will be always fixed in horizontal position. This projector will
work upright, with all the optics above the lamp, and 45 degree mirror on
top. What I consider is to mount lamp either parallel to the box sides (but
this will enlarge my box), or diagonally across the bottom, 45 degree to
sides, but still in horizontal position, parallel to the floor. So for the
lamp there is no difference, I wonder about arc symmetry looking from top at
lamp put flat on the floor.

The only 400W halides I've seen with almost perfectly uniform arc shape,
are the HQI-T/D by OSRAM. All the rest bend or distort their arc somewhat.

Actually, my budget will be tight, and I will use only cheap, readily
available components. So I choose Venture Lighting 400W HQI with E40 base.

Thanks for responding, that explained me quite a bit.

Regards
Pawel

 

[Ioannis]

Pawel Paron wrote:
[snip]

My light will be always fixed in horizontal position. This projector will
work upright, with all the optics above the lamp, and 45 degree mirror on
top. What I consider is to mount lamp either parallel to the box sides (but
this will enlarge my box), or diagonally across the bottom, 45 degree to
sides, but still in horizontal position, parallel to the floor. So for the
lamp there is no difference, I wonder about arc symmetry looking from top at
lamp put flat on the floor.

Looking at the arc from above on a horizontal burning halide, there is no
visual distortion afair, except that this part of the arc's
color will be slightly Mercury like.

The mechanisms of an Episcope that I've seen in the states, used a BD
1000W MetalArc (burned vertically) with mirrors on the sides to direct
the light upwards.

Also, make sure you provide for adequate ventilation, as these lamps can
get really hot in a closed environment.

Thanks for responding, that explained me quite a bit.

No problem.

 

[Victor Roberts]

Does anybody know what is the arc shape in a typical 400W metal halide lamp,
commonly used now for big area lighting (streets, etc.)? I'm going to use
such lamp in a diy LCD projector, and I wonder if I can mount it diagonally
to the LCD array (to make the whole box smaller, as this lamp is long). But
I don't want to obtain a non-symmetric differences in screen brightness, if
the arc isn't actually circular, or very close to circular.

Thanks in advance for information

Regards
Pawel

Sorry - original response was sent as e-mail by mistake and I'm sure
did not get through.

I'm not sure what you mean by circular. The arc is an elongated
tapered cylinder when the lamp is operated vertically. When the lamp
is not vertical, the arc is bowed by convection currents in the arc
tube, hence the name "arc discharge." The amount of bowing will depend
upon lamp design as has been pointed out. Even when vertical, there is
some color separation along the vertical axis and also outward from
the axis (radially). This may show up when you image the lamp to
illuminate the LCD panel. You may be better off with high pressure
mercury. These lamps have lower efficacy but will probably have more
uniform color. You probably know that the UHP lamps used for small LCD
projectors are very high pressure mercury discharges.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

 

[Pawel Paron]

I'm not sure what you mean by circular. The arc is an elongated
tapered cylinder when the lamp is operated vertically.

Sorry, perharps my question wasn't precise, or my English is crappy (or
both). What I meant, if the lamp is put horizontal, and you look at it from
above, what shape of the arc would you see? Round, or eliptical, or
something else? Light will be focused with a 11cm spherical mirror (part of
the sphere of 124mm diameter), and condenser lens, to 15" LCD array hanging
about 22cm above the bulb. I don't have the whole stuff yet, just started
drawing the box, and I have 40cm wide pieces of wood, that I would make the
box of, but only if I can mount the lamp diagonally across the bottom.
Otherwise it will not fit, and I have to search for wider pieces of wood,
and make this box even bulkier.

Regards
Pawel

 

[Pawel Paron]

Looking at the arc from above on a horizontal burning halide, there is no

visual distortion afair, except that this part of the arc's
color will be slightly Mercury like.

You mean that the arc will be round?

The mechanisms of an Episcope that I've seen in the states, used a BD
1000W MetalArc (burned vertically) with mirrors on the sides to direct
the light upwards.

Also, make sure you provide for adequate ventilation, as these lamps can
get really hot in a closed environment.

This is my primary concern, especially that lamp will be at the bottom of
the box, so heat will travel up to the LCD. I will use two 12cm fans at the
bottom, that will drag the hot air out of the box. Hope it will be enough.

Regards
Pawel

 

[Victor Roberts]

Sorry, perharps my question wasn't precise, or my English is crappy (or
both). What I meant, if the lamp is put horizontal, and you look at it from
above, what shape of the arc would you see? Round, or eliptical, or
something else?

It will still be somewhat elliptical - thinner near the electrodes and
fatter in the center. But now the center will also be closer to your
lens. It is most definitely not round.

Light will be focused with a 11cm spherical mirror (part of
the sphere of 124mm diameter), and condenser lens, to 15" LCD array hanging
about 22cm above the bulb. I don't have the whole stuff yet, just started
drawing the box, and I have 40cm wide pieces of wood, that I would make the
box of, but only if I can mount the lamp diagonally across the bottom.
Otherwise it will not fit, and I have to search for wider pieces of wood,
and make this box even bulkier.

I don't understand why the diagonal mounting will make any difference
with the optics. The optics are circular so mounting the lamp
diagonally in the box doesn't seem to make anything more complicated.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

 

[Pawel Paron]

It will still be somewhat elliptical - thinner near the electrodes and
fatter in the center. But now the center will also be closer to your
lens. It is most definitely not round.

OK, I can now imagine that.

I don't understand why the diagonal mounting will make any difference
with the optics. The optics are circular so mounting the lamp
diagonally in the box doesn't seem to make anything more complicated.

Because light source is not a point, but some other shape (something
elliptical, as you wrote above). So my understanding is that if you don't
have a perfect point source, and not a spherical one (that was my first
question), it makes some difference when you rotate it, you get uneven light
distribution, that will somehow correspond to the shape of the source. And
it would be best to match this light distribution with display dimensions
(to put more light horizontally, as screen is bigger horizontally). But I
might be wrong, as I have very little knowledge of optics.

Regards
Pawel

 

[Victor Roberts]

OK, I can now imagine that.


Because light source is not a point, but some other shape (something
elliptical, as you wrote above). So my understanding is that if you don't
have a perfect point source, and not a spherical one (that was my first
question), it makes some difference when you rotate it, you get uneven light
distribution, that will somehow correspond to the shape of the source. And
it would be best to match this light distribution with display dimensions
(to put more light horizontally, as screen is bigger horizontally). But I
might be wrong, as I have very little knowledge of optics.

OK, I understand. If the source were a rectangle, for example you
would want to align that rectangle with the object you were
illuminating, which is the rectangular LCD screen.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

 

[Ioannis]

You mean that the arc will be round?

[snip]

It's more like elliptical seen from above, like Vic says. If you want to
see its exact shape, you can get a piece of welder's glass and look at
the arc through it. This will allow you to calculate how map it directly
to any optical device you want.

 

[Pawel Paron]

It's more like elliptical seen from above, like Vic says. If you want to
see its exact shape, you can get a piece of welder's glass and look at
the arc through it. This will allow you to calculate how map it directly
to any optical device you want.

That's a good idea. A media from floppy disk should do the same trick, I
used it once to watch sun eclipse.

Regards
Pawel

 

[Ioannis]

That's a good idea. A media from floppy disk should do the same trick, I
used it once to watch sun eclipse.

Yes. The changes in color from the condensates will be visible as well,
if you look at it from the sides.