Basics of photometry
Accuracy class according to CIE and DIN 5032-7.
Maximum values of basic errors for luxmeters depending on the accuracy class are included in the table below:
Spectral matching f1' error:
Directional matching f2:
Optical radiation is electromagnetic radiation in the wavelengths range between 1nm and 1mm.
Visible radiation (light) is part of the optical radiation that is received and evaluated (by any system) in the same ways as it is by the human eye. The spectral range of visible radiation is not clearly defined and it depends on the energy value of the flux reaching the eye and on the individual sensitivity of the observer. On the whole, the range between 380 and 750 nm is assumed.
Radiant flux (radiant power) the power that is sent, transferred or received in the form of radiation.
Watt [W] is the unit of the radiant flux.
Φ Luminous flux the quantity derived from the radiant flux by assessing the radiation effect on a normal CIE photometric observer (the definition of a normal observer is described further in the text).
In the photopic vision (during daytime conditions in which the colour impression is perceived) the luminous flux can be described by the following formula:
Φ = Km ∫0∞ (dΦe (λ) / dλ) · V (λ) dλ
The unit of the luminous flux is lumen [lm].
I Luminous intensity: the ratio of luminous flux dΦ, transmitted by the radiation source in the elementary spatial angle dΩ, covering a given direction, to the value of this elementary spatial angle:
I = dΦ / dΩ
The unit of the luminous intensity is candela [cd].
L Luminance: is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle:
L = dΦ / (dA · cosθ dΩ)
Φ - luminous flux
θ - angle between normal to surface A and the specified direction.
Ω - infinitesimal solid angle containing the specified direction.
If surface dA is a source of radiation with luminous intensity I, the equivalent formula is:
L = dI / dA · cosθ
If a beam of light falls on the surface dA causing the illuminance E on it, the equivalent formula is:
L = dE / dΩ · cosθ
This formula is used when it is difficult to determine the surface of the radiation source (e.g. sky, plasma discharge, etc.).
The unit of the luminance is candela per square meter [cd / m2].
E Illuminance: the ratio of luminous flux dΦ, falling on the elemental surface dA, to the value of this surface:
E = dΦ / dA
The unit of the illuminance is lux [lx].
CIE normal photometric observer: an ideal observer whose relative spectral sensitivity curve is matched with the V (λ) function in the photopic vision (daytime) and the V '(λ) function in the scotopic vision (low light).
It is assumed that the light is received by the eye only through the daytime vision apparatus at the luminance levels higher than a few cd / m^2^, while with scotopic vision one deals with luminance levels smaller than a few hundredths of cd / m^2^. Between the "pure" photopic and the scotopic view one distinguishes the range of mesopic vision (mixed), which has no metrological application.
Physiology of the eye
The stimulation of the retina is directly proportional to the illuminance E on its surface and, if varoius various losses are omitted, it can be presented as:
E = L · Ω
L - luminance of the observed object,
Ω - solid angle depending on the diameter of the pupil of the eye and the distance between the lens and the retina.
As follow-up to the above-placed formula, it can be concluded that the stimulation of the retina of the eye is directly proportional to the luminance of the luminous surface. Luminance is thus an appropriate measure of the sensation of brightness.
In the case of workplaces with surfaces emitting luminous flux, e.g. computer workstations, othe luminance meter is the only instrument which can properly determine the light conditions in which an employee stays.