8 THE PHOTOGRAPHIC IMAGE, BROCK
wards. The T.V. camera being only the first link in a long chain of various kinds of
equipment had to fit into the general electronic scheme for expressing performance.
Moreover the scanning mechanism of the T.V. camera compels its user to recognise the
relation between spatial and temporal frequency. (Even to the extent that lenses may
be graded in “megacycles per second”.)
The performance of an electronic amplifier, landline etc., is normally specified in
terms, of its response to sinusoidal signals over a range of frequencies. The sine-wave is
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Fig.2. Sine wave, cosine? wave, and definition of modulation.
used partly because it is the simplest wave-form; all other wave-forms, of whatever
shape, can be analysed into a series of sine-waves, or synthesised from the same series.
A sine-wave signal remains sinusoidal, however changed in amplitude or phase, when put
through a linear filter (“filter” here is used in the general sense, meaning any system
which receives a signal and passes it on). Any complication generated in the wave-form
indicates non-linearity in the filter. The sin x function periodically changes sign, corre-
sponding in electronics to the reversals of an A.C. voltage, but optical targets cannot
simulate negative luminances. À more appropriate function for the electro-optical case
is therefore cos? x, which has the same shape as sin x, but never becomes negative.
However, the term “sine-wave” is loosely used in the literature to describe simply the
sinusoidal shape (Fig. 2). The amplitude of modulation, m has to be specified independ-
ently of the mean target luminance which may be quite different in object and image.
max | — in
me xt T i
target) when I , represents complete blackness, m = 1.0, and when there is no modula-
tion (I, = I.) M = 0. The response at a given frequency, or the contrast transmission
at that frequency, is the ratio of the modulations in object and image. The chart in Fig. 3
enables modulation to be converted to luminance ratio or log luminance ratio. It will be
seen that modulation and log luminance ratio are numerically approximately equal up
to 0.7.
Suppose now we have a television system with 400 lines in the frame: we require a
corresponding resolution along the lines. At 25 frames per sec the line-scan frequency is
10,000 per sec and there should be 400 changes from white to black along each line.
Reckoning the frequency of the light modulation from the instants when the intensity is
the same and changing in the same direction, and ignoring the necessity for synchronising
pulses, the rate is 2 megacycles per sec, or, as usually expressed, the bandwidth is 2
megacycles. Each link in the chain must therefore be capable of passing 2 megacycle
signals without attenuation. If the frame of the camera is 25 mm across, then since each
line is scanned in 10—*4 sec, 2 megacycles corresponds to “8 lines per mm” in the image
plane. The lens must therefore give no attenuation at 8 lines/mm. For smaller frames
It is defined as (Fig. 2). Thus at maximum modulation (high contrast
the line
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