Light
There are physical quantities that are measured and others that are calculated from measurements. Three well known base quantities used in measurement are mass, length and time. The International System of Units (SI), popularly known as the metric system, assigns units of kilogram, metre and second respectively. SI comprises a coherent system formed from the seven base units of measurement, each corresponding with a base quantity. Derived units are formed by combining base units, the dimension of a derived quantity is expressed as combinations of the base.
Luminous intensity is the base for all derived photometric quantities, the corresponding SI unit is the candela. Luminous intensity is a perceptual quantity, differing from all of the other base quantities which are defined objectively. It relates the power and frequency content of electromagnetic radiation with the perception of brightness in a human observer. The stimulus-response function peaks near a wavelength of 550 nm and tails off either side; outside the visible region the response is zero.
The science of the measurement of Light is photometry, derived from the Greek “photo” light, and “metry” to measure. In addition to the candela, common photometric quantities are tabulated below
Luminous intensity is the base for all derived photometric quantities, the corresponding SI unit is the candela. Luminous intensity is a perceptual quantity, differing from all of the other base quantities which are defined objectively. It relates the power and frequency content of electromagnetic radiation with the perception of brightness in a human observer. The stimulus-response function peaks near a wavelength of 550 nm and tails off either side; outside the visible region the response is zero.
The science of the measurement of Light is photometry, derived from the Greek “photo” light, and “metry” to measure. In addition to the candela, common photometric quantities are tabulated below
Quantity | Symbol | SI units | Base units |
---|---|---|---|
Luminous intensity | I | candela | cd |
Luminous flux | phi | lumen | cd.sr-1 |
Illuminance | E | lux | cd.sr-1.m-2 |
Luminance | L | candela / metre2 | cd.m-2 |
The chemistry of perception
Light is perceived in the brain through a a series of chemical reactions that begin with the rods and cones in the eye through the activation of the neurons in the visual cortex. In the eye are cells that are broadly divided into rods and cones, chemical reactions in these cells are driven by energy from incoming radiation. The magnitude of the response from a cell is dependent on the intensity of the radiation and the spectral absorbance of the receptors in the cell. All rods have similar absorbance characteristics described by the scotopic spectral curve, the three different types of cones have responses concentrated in the red, green and blue parts of the visible spectrum, the photopic curve describes the combined response.
The receptors in rods are more sensitive than cones so at low levels of incoming radiation (think moonlit night) scotopic vision dominates. They are more sensitive to rapid modulation of the radiation intensity so are the major contributors to our perception of flicker. Cones allow us to perceive colour and fine detail. |
To account for the varying sensitivity of the eye with the ambient illuminance, three visual states have been defined.
• Photopic or cone vision, daytime adaptation of the eye dominated by the response of cones
• Mesopic vision as a combination of the response of cones and rods, predominantly greyscale and washed-out colours
• Scotopic vision as full darkness adaptation where only the rods respond to the stimulation of light resulting in a monochromatic perception.
• Photopic or cone vision, daytime adaptation of the eye dominated by the response of cones
• Mesopic vision as a combination of the response of cones and rods, predominantly greyscale and washed-out colours
• Scotopic vision as full darkness adaptation where only the rods respond to the stimulation of light resulting in a monochromatic perception.
The response of the eye in the photopic state yields a peak sensitivity at 555 nm, scotopic vision peaks near 507 nm. The mesopic state does not have a defined peak sensitivity, it is a linear combination of the photopic and spectral responses factored by the illuminance level.
In addition to the above, perceived brightness is related to the current state of adaptation of the eye. Adaptation occurs over significantly longer intervals than perception, typically many seconds or even minutes. For example, once the eye has become fully dark adapted, objects in the very low light condition may now be discerned. |