Visible light is the range of the
electromagnetic spectrum of radiant energy that can be perceived by the human
eye. This has a range of wavelengths from 0.4 to 0.7µm. The effect of visible
(luminous) energy on the eye depends upon the wavelength and is expressed as
the relative luminosity of monochrome radiation. This spectral response curve
varies from observer to observer, but for dealing with photometric quantities,
the response has been standardised for an ‘average’ observer in the form of the
luminosity function based on results of test by the Commission Internationale d
l’ Eclairage (CIE).
electromagnetic spectrum of radiant energy that can be perceived by the human
eye. This has a range of wavelengths from 0.4 to 0.7µm. The effect of visible
(luminous) energy on the eye depends upon the wavelength and is expressed as
the relative luminosity of monochrome radiation. This spectral response curve
varies from observer to observer, but for dealing with photometric quantities,
the response has been standardised for an ‘average’ observer in the form of the
luminosity function based on results of test by the Commission Internationale d
l’ Eclairage (CIE).
The relative luminosity function shows the
maximum sensitivity of the human eye in the green and yellow regions of the
spectrum. Some of the basic photometric quantities of significance in TV camera
photography are discussed below:
maximum sensitivity of the human eye in the green and yellow regions of the
spectrum. Some of the basic photometric quantities of significance in TV camera
photography are discussed below:
1.)
Luminosity: Luminosity or brightness
is taken to mean the subjective visual sensation of brightness of light or
colour, evaluated in terms of the luminosity function. This is plotted as a
curve representing relative impression of light as a function of wavelengths,
as assesses by the average observer, for a constant radiant power.
Luminosity: Luminosity or brightness
is taken to mean the subjective visual sensation of brightness of light or
colour, evaluated in terms of the luminosity function. This is plotted as a
curve representing relative impression of light as a function of wavelengths,
as assesses by the average observer, for a constant radiant power.
2.)
Luminous intensity: This is the
measure of light intensity (responsible for stimulating visual sensation) at
which the total area of a source emits light in a particular direction. The
international unit of luminous intensity is the candela (cd) which is equivalent
to 1/60th of the luminous intensity radiated by one centimetre of a
black body (full radiator) at the temperature of solidification of platinum
(1774ºC) at right angles to its surface. This quantity gives no information
about the total amount of light flux emitted by a source in all directions.
Light intensity of a source may be different in different directions depending
upon the area of the source, reflector employed etc.
Luminous intensity: This is the
measure of light intensity (responsible for stimulating visual sensation) at
which the total area of a source emits light in a particular direction. The
international unit of luminous intensity is the candela (cd) which is equivalent
to 1/60th of the luminous intensity radiated by one centimetre of a
black body (full radiator) at the temperature of solidification of platinum
(1774ºC) at right angles to its surface. This quantity gives no information
about the total amount of light flux emitted by a source in all directions.
Light intensity of a source may be different in different directions depending
upon the area of the source, reflector employed etc.
3.)
Luminous flux: This is the radiated
luminous power (power of visible light) expressed in terms of its effect on the
average or normal human eye. The unit of luminous flux is lumen (lm) and it is
the luminous flux emitted per unit solid angle (steradian) by a uniform point
source of one candela.
Luminous flux: This is the radiated
luminous power (power of visible light) expressed in terms of its effect on the
average or normal human eye. The unit of luminous flux is lumen (lm) and it is
the luminous flux emitted per unit solid angle (steradian) by a uniform point
source of one candela.
4.)
Luminance: This is the quantity of
light intensity emitted per square centimetre of an illuminated area. Luminance
is thus an attribute of a surface emitting or reflecting it (in contrast to the
term luminance which expresses light received by the surface). The
international unit of luminance is the stilb (sb) which is the luminance of a
surface having luminous intensity of one candela per square centimetre. A
smaller unit is the nit which is the luminance of a surface emitting one
candela of luminous intensity per square metre. One stilb is thus equal to
10,000 nits. Some countries use the unit of foot-lambert which is equivalent to
3.43 nits. A foot-lambert is the luminance of a surface emitting or reflecting
light at the rate of one lumen per square foot.
Luminance: This is the quantity of
light intensity emitted per square centimetre of an illuminated area. Luminance
is thus an attribute of a surface emitting or reflecting it (in contrast to the
term luminance which expresses light received by the surface). The
international unit of luminance is the stilb (sb) which is the luminance of a
surface having luminous intensity of one candela per square centimetre. A
smaller unit is the nit which is the luminance of a surface emitting one
candela of luminous intensity per square metre. One stilb is thus equal to
10,000 nits. Some countries use the unit of foot-lambert which is equivalent to
3.43 nits. A foot-lambert is the luminance of a surface emitting or reflecting
light at the rate of one lumen per square foot.
5.)
Illumination or illuminance: This is
the average luminous flux incident onto a surface. Its unit is lux (lx) defined
as the illuminance of a surface area of 1 square metre, over which a luminous
flux of 1 lumen is uniformly distributed. It is also expressed in foot-candles
as the illuminance of a surface area of one square foot, over which a luminous
flux of one lumen is uniformly distributed. One foot-candle is equivalent to 10
lx. Illuminance is not directly related to visual sensation due to the
brightness of luminance of the illuminated surface, which depends upon the
reflectivity of the surface. Illumination is the only quantity in light
engineering that can be measured directly by means of photoelectric metres.
Illumination or illuminance: This is
the average luminous flux incident onto a surface. Its unit is lux (lx) defined
as the illuminance of a surface area of 1 square metre, over which a luminous
flux of 1 lumen is uniformly distributed. It is also expressed in foot-candles
as the illuminance of a surface area of one square foot, over which a luminous
flux of one lumen is uniformly distributed. One foot-candle is equivalent to 10
lx. Illuminance is not directly related to visual sensation due to the
brightness of luminance of the illuminated surface, which depends upon the
reflectivity of the surface. Illumination is the only quantity in light
engineering that can be measured directly by means of photoelectric metres.
