The
image formed on the retina is always retained on it for a short period. This is
because the brightness is sensed by the rods in the retina of the eye by a
photochemical process which has its own lag. Hence, the sensation on the eye resulting from
single short flash is a function of duration as well as the intensity of the flash,
and continue for around 20ms. Actually, the eye acts somewhat like a cumulative
storage device. An object of low brightness viewed for a given time produces
the same sensation in the eye as an object of greater brightness viewed for a
shorter period of time.
image formed on the retina is always retained on it for a short period. This is
because the brightness is sensed by the rods in the retina of the eye by a
photochemical process which has its own lag. Hence, the sensation on the eye resulting from
single short flash is a function of duration as well as the intensity of the flash,
and continue for around 20ms. Actually, the eye acts somewhat like a cumulative
storage device. An object of low brightness viewed for a given time produces
the same sensation in the eye as an object of greater brightness viewed for a
shorter period of time.
For
intermittent flashes of light incident on the retina, a vigorous photochemical
process continues for a fraction of a second (around 20ms) even after the
stimulus has disappeared. The continuation of the photochemical process means
combination of brightness impression in the visual centre of the brain and is
called ‘persistence of vision’. This is used in cinema and television in
obtaining the illusion of continuity by means of rapidly flashing picture
frames. If the flashing is fast enough, the flicker is not observed and the
flashes appear continuous. The repetition rate of the flashes at and above
which the flicker effect disappears is called the ‘Critical Flicker Frequency’
(CFF). This is dependent on the brightness level and the colour spectrum of the
light source. Because the human eye has the greatest sensation to yellow-green
light, the flicker effect is maximum in that region. In cinema, the classical
example of this effect, a film speed of 16 frames per second was used in
earlier films to obtained the illusion of movement. Lack of smooth movement was
noticeable in these films. The present day standard for movie film speed is 24
frames per second and at this speed, these effects are very much reduced. The
flicker problem is further reduced in modern projectors by causing each frame
to be illuminated twice during the interval it is shown, by means of fan
blades. The resulting flicker rate is quite acceptable for cine-screen
projection, because it is viewed in subdued light and a wide angle display
area.
intermittent flashes of light incident on the retina, a vigorous photochemical
process continues for a fraction of a second (around 20ms) even after the
stimulus has disappeared. The continuation of the photochemical process means
combination of brightness impression in the visual centre of the brain and is
called ‘persistence of vision’. This is used in cinema and television in
obtaining the illusion of continuity by means of rapidly flashing picture
frames. If the flashing is fast enough, the flicker is not observed and the
flashes appear continuous. The repetition rate of the flashes at and above
which the flicker effect disappears is called the ‘Critical Flicker Frequency’
(CFF). This is dependent on the brightness level and the colour spectrum of the
light source. Because the human eye has the greatest sensation to yellow-green
light, the flicker effect is maximum in that region. In cinema, the classical
example of this effect, a film speed of 16 frames per second was used in
earlier films to obtained the illusion of movement. Lack of smooth movement was
noticeable in these films. The present day standard for movie film speed is 24
frames per second and at this speed, these effects are very much reduced. The
flicker problem is further reduced in modern projectors by causing each frame
to be illuminated twice during the interval it is shown, by means of fan
blades. The resulting flicker rate is quite acceptable for cine-screen
projection, because it is viewed in subdued light and a wide angle display
area.
In
television, the field rate is concerned with (i)large area flicker
(ii)smoothness of motion and (iii)motion blur in the reproduced picture. As the
field race is increased, the parameters show improvements but tend to saturate
beyond 60Hz. Further increase does not pay off, and increases the bandwidth.
Hence the picture field scanning is generally done at the rate as the as the
mains power supply frequency, which conventionally happens to be 50 or 60Hz for
the same reasons of reducing illumination flicker from electric lamps. At 60Hz,
the flicker is practically absent, while at 50Hz, a certain amount of
borderline flicker may be noticed at high brightness levels used to overcome
surrounding ambient light conditions. Use of frequency of the power mains for
vertical scanning reduces possible effect like supply ripple and 50Hz magnetic
field, in the reproduced picture.
television, the field rate is concerned with (i)large area flicker
(ii)smoothness of motion and (iii)motion blur in the reproduced picture. As the
field race is increased, the parameters show improvements but tend to saturate
beyond 60Hz. Further increase does not pay off, and increases the bandwidth.
Hence the picture field scanning is generally done at the rate as the as the
mains power supply frequency, which conventionally happens to be 50 or 60Hz for
the same reasons of reducing illumination flicker from electric lamps. At 60Hz,
the flicker is practically absent, while at 50Hz, a certain amount of
borderline flicker may be noticed at high brightness levels used to overcome
surrounding ambient light conditions. Use of frequency of the power mains for
vertical scanning reduces possible effect like supply ripple and 50Hz magnetic
field, in the reproduced picture.
