In
nature the range of brightness level is very large, almost infinite from pitch
darkness of a closed cave to the brilliance of sunlight. By adoption of the
iris, the human eye can adapt to a very wide range of luminance levels, the
upper limit being up to about 50,000 nits. The lowest discernible luminance is
about 0.03 nits ordinarily, although in dark-adapted vision, the eye can
discern levels below 10-5 nits.
nature the range of brightness level is very large, almost infinite from pitch
darkness of a closed cave to the brilliance of sunlight. By adoption of the
iris, the human eye can adapt to a very wide range of luminance levels, the
upper limit being up to about 50,000 nits. The lowest discernible luminance is
about 0.03 nits ordinarily, although in dark-adapted vision, the eye can
discern levels below 10-5 nits.
The
visual sensation of brightness to the human eye is a logarithmic function of
the picture luminance over the restricted range adaptable to the iris
adjustment, as expressed by the Weber-Fechner law:
visual sensation of brightness to the human eye is a logarithmic function of
the picture luminance over the restricted range adaptable to the iris
adjustment, as expressed by the Weber-Fechner law:
ΔS
= k log B1/B2
= k log B1/B2
Where
B1 and B2 are two luminance levels and ΔS is the
incremental visual sensation due to them. If a value of ΔB is established for
which the brightness of two adjacent areas are just noticeably different, the
ratio ΔB/B is known as Weber’s fraction. Actually the fraction is dependent on
the brightness level and on the viewer’s state of adaptation to it. It has a
nearly constant value of about 0.02 over a brightness range of about 1 to 300
nits, while at very low levels of brightness; the Weber fraction is relatively
larger.
B1 and B2 are two luminance levels and ΔS is the
incremental visual sensation due to them. If a value of ΔB is established for
which the brightness of two adjacent areas are just noticeably different, the
ratio ΔB/B is known as Weber’s fraction. Actually the fraction is dependent on
the brightness level and on the viewer’s state of adaptation to it. It has a
nearly constant value of about 0.02 over a brightness range of about 1 to 300
nits, while at very low levels of brightness; the Weber fraction is relatively
larger.
The
contrast in a picture or scene is expressed as a ratio of the maximum to the
minimum luminance Bmax/Bmin. The contrast ratio discernible to the eye
depends upon the level of ambient light. In a bright daylight illumination, the
eye can accommodate a contrast range of up to 1000:1, while at lower levels of
illumination it can make out a range of about 10:1. The darkest part in a
picture cannot be darker than the ambient light externally falling on it and
the internal scattered light.
contrast in a picture or scene is expressed as a ratio of the maximum to the
minimum luminance Bmax/Bmin. The contrast ratio discernible to the eye
depends upon the level of ambient light. In a bright daylight illumination, the
eye can accommodate a contrast range of up to 1000:1, while at lower levels of
illumination it can make out a range of about 10:1. The darkest part in a
picture cannot be darker than the ambient light externally falling on it and
the internal scattered light.
The reflectance of the TV screen may be over 25%.
Such ambient light is always present and is in fact recommended to reduce eye
strain. In a TV picture, the contrast ratios are less, in the range of 10 to
40, while in a well designed dark theatre, a contrast range of over 100 can be
obtained. For the normal illuminance of the TV screen, of 1 to 40 nits, the
incremental contrast sensitivity of the eye is around 0.03. With contrast ratio
of 10 to 40, the eye can notice a maximum number of tonal gradation of 80 to
130, 100 on the average.
Such ambient light is always present and is in fact recommended to reduce eye
strain. In a TV picture, the contrast ratios are less, in the range of 10 to
40, while in a well designed dark theatre, a contrast range of over 100 can be
obtained. For the normal illuminance of the TV screen, of 1 to 40 nits, the
incremental contrast sensitivity of the eye is around 0.03. With contrast ratio
of 10 to 40, the eye can notice a maximum number of tonal gradation of 80 to
130, 100 on the average.
