Download Report - Academy of Motion Picture Arts and Sciences

to reduce this radiation is to use a filter which cuts
off slightly beyond 7000 Angstrom units.. In the
case oi smaller equipments such as stereoptican proiectors
when some form of heat reducing device is
.r....ru.y water cells are frequently used and are
particularly efiective. They would of course be out
b{ th. qrr..tion for the larger motion picture ljghting
equipments. Clear mica is also a good filter'
It"reducis the light about as much as a piece of
glass but reduces the heat to a considerably greater
.ilegree.
Ir{n. Bar,r,: I think Mr. Arnold has brought
,up a very important point-it is a little ahead of
the time.
Mn. RavroN: In reference to the heat ques-
'tion, it seems obvious that any heat absorbing filter
placed in front of a lamp must re-radiate th,e heat
lit absorbs or else it will become so hot it will melt.
If the lamp is within the set there will be no relief
rdue to the use o{ filters. If water cooling system
'could be used, the hot water could be circulated into
;another compartment for cooling similar to the
withdrawal of heat from a gasoline engine.
Mx. Ber,r-: Incandescents are going to be quite
widely used but that does not say that Arcs are to
be discarded , at any rate in the near future. Mr.
A. C. Downs and Mr. D. B. Joy of the Development
Laboratory o{ the National Carbon Company
have prepared a paper for this occasion on the
subject o{ "Carbon and Flame Arcs in Motion
Picture Photography" which Mr. Downs will now
present.
PAPER READ BY
MR. A. C. DOWNS
Mn. DowNs: There seems to be some Iittle
.misunderstanding or lack of understanding, of the
,several differenf types of Carbon Arcs- It is the
object of this paper to attempt to clear up that
misunderstanding.
First, there is the old-fashioned Carbon Arc which
is used for projection purposes or spot light work"
It is operated in more or less the position as shown
in Figu.re I.
This might be a cored or a solid carbon and the
source of light is always the positive crater. Over
90% of all the light from this particular Arc comes
from the positive, crater on direct current. This
is clearly illustrated in Figure II which is a photograph
oi the low intensity Arc 11 50- ampe-res and
55 taken from a position directly in front of
the "otts arc where the condenser lens would ordinarily
be placed, the very bright oval spot being the incandesient
crater. It can readily be seen that the negative
arc stream and tail flame contribute very little
to the light from this type o{ Arc. The incandescent
cratir is what is focused in the low intensity
spot lights and moving picture projectors.
- In ihe case of the alternating current arc the
carbons are always the same size, the craters are
much smaller, and only one of the craters can be
L42l
brought into focus with the condenser, therefore,
only half of the light of the alternating current arc
is available. The position in which the carbons are
usually burned and the crater formation is shown
in Figure III-4.
Figure III-B is a photograph of the alternating
crrtt.nt arc from 3/4" carbons burning at 80 amperes
and 35 volts'taken, as in Figure II, in front
of ttt. arc where the condenser lens ordinarily
would be placed. The top crater alone would be
in focus with the condenser lens'
The Flame Arc carbons are a later development
and are usually burned in a vertical position and
the source of light is the flame or Arc stream rather
than the carbott crater. In fact, ovet 90/o o{ the
light comes from the flame or arc stream' This
is:illustrated in Figure IV which is a photograph
of a White Flame Carbon Arc at 40 amperes direct
current and 37 volts between two f" carbons'
The highly incandescent flame is caused by the
material used in the core o{ the carbons and is practically
the same whether the carbons are burned
on direct current or alternating current although
the shape of the arc is slightly difierent as shown in
Figure V, which is a photograph o{ White Flame
Cibo.t Arc at 40 amperes alternating current and
37 volts between two f" carbons. The flow of the
flame material from the core into the arc stream
causes the craters to become very dim compared
with the craters of the ordinary Carbon Arc' The
difierence between the brightness of the arc stream
and craters of this flame type of carbon and the
types previously mentioned is most easily seen by
c-o*paring Figures II, III-8, IV and V.
The light source (the flame) of the Flame Carbon
Arc is much larger than the light source (crater
surface) of the old type carbon arc using the same
currenf and has a lower brilliancy per unit of
area but because of its large volume, the luminescent
flame, despite its lower brilliancy per unit area,
gives more light for an equal amount of energy.
The composition of the cores used in flame arc
,carbons can be varied to give difierent colors of
light over a comparatively wide range. The active
material used in the core of the white flame carbon
is a compound of the rare metal cerium I that used
in the core o{ the red flame carbon a compound
,of the metal strontium, and that used in the core
,of the National panchromatic "O" carbon, a combination
of several compounds of difierent metals.
There are in the moving picture studios' the Sun
Arcs and Rotary Spots which are known as "high
intensity" arcs and which are a development of the
White Flame Arc. A carbon with white flame
material in the core is rotated in a horizontal position
and a neutral cored carbon is placed at the
proper angle with this carbon in the same vertical
plane. Direct current is used and the horizontal
carbon is positive. The action of the arc of this
combination at ordinary current densities is illustrated
in Figure VI. This shows a flame of low
velocity coming from the negative carbon and a