W4: Photometry photometric quantities and instrument Flashcards
Radiometry
The measurement of light waves in any of the electromagnetic spectrum
All light measurement is considered radiometry
Electromagnetic spectrum
Describes all kinds of lights
Photometry
is the measurement of light, which is defined a electromagnetic radiation which is detectable by the human eye
How do eyes respond to light waves
In human eyes, retina has two types of photoreceptors: Rod and Cone cells.
Rod cells are more sensitive with low level of light (Night vision), and they do not play any role in color vision.
Cone cells respond differently to the light: Red-sensing cones 550 - 690nm, Green-sensing
cones 440 - 670nm, Blue-sensing cones 380 - 540nm (RGB
Phototopic
o Maximum visual effect at 555nm (yellow-
green)
o At 510nm and 610nm the sensitivity reduces
to half
Scotopic
o Maximum effect at 507nm (blue-green)
o Vision of the eye under low-light conditions
Luminous flux
o Luminous flux is the measure of the perceived power (brightness) of light → total amount of light emitted by the source
o Luminous flux is also called as luminous power.
o SI unit (International System of Units) is Lumen (lm)
Luminous intensity
o Luminous intensity is the measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. → the power from light source to certain direction
o SI unit (International System of Units) is Candela (cd) → lm/sr
o Luminous intensity = lm/sr
Steradian, sr (solid angles)
o 3-dimension angular volume defined
analogously to the definition of a plane angle
in two dimensions
Illuminance
o Illuminance is the measure of the luminous flux that impacts on a surface → the
power of light source that reaches on the surface
o SI unit (International System of Units) is lux (lx)
o → luminous flux (lm) / area (𝑚2) = cd · sr · 𝑚−2 (∵ cd=lm / sr)
Inverse square law
o Illuminance is how light impact on surface.
o Distance from light to the surface affect illuminance ? Inverse square law
o The Illuminance of light is inversely proportional to the distance from the light source.
Luminance
o Luminance is the measure of the amount of light emitted, transmitted or reflected
from a surface per solid angle.
o Luminance is the value how much luminous intensity can be perceived by human eye.
o SI unit (International System of Units) → candela per square meter (cd/ 𝑚2)
o The non- SI term for the same unit is nit
What measures luminous flux
Goniophotometer
