Page 7 - pm_1952_01

Your Laboratory Manager says-

In response to the numerous requests to our Editor, I intend to write upon
the subject of filters and their functions. A table of filter uses and their
appropriate aperture variations, or filter factors, was .printed in the June/ July
issue of the Pathescope Monthly this year. This subject is also c~sely connected
with Film Emulsions, upon which I wrote in the October 1951 Pathescope Monthly.
Bearing in mind that "white" light consists of a combination of various colours,
called a spectrum, and that Pathescope S.S. and V.F. stocks are sensitive to the
majority of these colours, particularly the blue band, -the function of filters is
to control the amount of light, or degree of colour, falling on the emulsion.
Although I am only discussing our monochrome, or black and white films, the
colours of objects photographed, and also the colour of the light which illumin-
ates these objects, controls completely the graduation of light and shade of the
photographic image. This complete control is only operative when the exposure
in the camera is correct and there is no necessity for compensation. Any large
deviation of exposure does tend to destroy correct tone rendering.
Returning to the question of white light, this consists of a number of colours
which are measured in wave lengths as mu (millimicrons). A millimicron is the
millionth part of a millimetre. Wave length of light is sometimes measured in
Angstrom Units (A.U.), one unit being a ten millionth part of a millimetre.
Below is a graphical representation of the spectrum from 300 to 700 mu.
,300 400 sao looo 700
U .VI~T VIOI.£7 SI.U£
The second graph shows the spectrum of visible light which the human eye
is capable of receiving, showing the relative amount of each colour and the
response of the same light to our panchromatic stock.

i ...
p I'ANCHROM~TIG',
0 5TOc.t< ~.., ...
N
~ '
' '

300 4-oo soo boo 100
\IIOJ..ET BLUE GREEN VE.U.o W R£0
It can be seen that Panchromatic stock is very sensitive to the 400 to 500 mu
band, i.e., the violet blue region. Thus an ordinary skyscape will lose its cloud
contrast because of the predominence of blue. The first function of the filter
is to cut down the amount of blue light, or change the 'pan' curve to one similar
to that of the human eye response.
To achieve a partial cut in blue, an amber filter (ref. C298} is used, which
partially increases contrast. The next filter is the Yellow / Green (ref. C90 I C906)
which is virtually of 'stronger' density than the amber. While Red/ Orange (ref.
C900 C905) is denser it still gives a sharper contrast to the image. This greatly
cuts down the blue and is useful for counteracting haze through a diminuation
of the powerful blue band.
Page Seven