DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

See Introduction to radiometry
and photometry, on page 601.
Radiance, intensity
Image science concerns optical power incident upon
the image plane of a sensor device, and optical power
emergent from the image plane of a display device.
Radiometry concerns the measurement of radiant
optical power in the electromagnetic spectrum from
3×10 11 Hz to 3×10 16 Hz, corresponding to wavelengths
from 1 mm down to 10 nm. There are four
fundamental quantities in radiometry:
• Radiant optical power, flux, is expressed in units of
watts (W).
• Radiant flux per unit area is irradiance; its units are
watts per meter squared (W·m -2).
• Radiant flux in a certain direction – that is, radiant flux
per unit of solid angle – is radiant intensity; its units are
watts per steradian (W·sr -1).
• Flux in a certain direction, per unit area, is radiance;
its units are watts per steradian per meter squared
(W·sr -1 ·m -2 ).
Radiance is measured with an instrument called a radiometer.
A spectroradiometer measures spectral
radiance – that is, radiance per unit wavelength. A
spectroradiometer measures incident light; a
spectrophotometer incorporates a light source, and
measures either spectral reflectance or spectral transmittance.
Photometry is essentially radiometry as sensed by
human vision: In photometry, radiometric measurements
are weighted by the spectral response of human
vision (to be described). This involves wavelengths (λ)
between 360 nm to 830 nm, or in practical terms,
400 nm to 700 nm. Each of the four fundamental quantities
of radiometry – flux, irradiance, radiant intensity,
and radiance – has an analog in photometry. The photometric
quantities are luminous flux, illuminance, luminous
intensity, and luminance. In video engineering,
luminance is the most important of these.
204 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES