In photometry, luminous intensity is a measure of the
wavelength-weighted power emitted by a light source in a particular
direction per unit solid angle, based on the luminosity function, a
standardized model of the sensitivity of the human eye. The
y ¯ ( λ ) displaystyle textstyle overline y (lambda ) , is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten[citation needed] . Contents 1 Relationship to other measures 2 Units 3 Usage 4 See also 5 References 5.1 Curve data Relationship to other measures[edit]
I v = 683 ⋅ y ¯ ( λ ) ⋅ I e , displaystyle I_ mathrm v =683cdot overline y (lambda )cdot I_ mathrm e , where Iv is the luminous intensity in candelas (cd), Ie is the radiant intensity in watts per steradian (W/sr), y ¯ ( λ ) displaystyle textstyle overline y (lambda ) is the standard luminosity function. If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the luminous intensity: I v = 683 ∫ 0 ∞ y ¯ ( λ ) ⋅ d I e ( λ ) d λ d λ . displaystyle I_ mathrm v =683int _ 0 ^ infty overline y (lambda )cdot frac dI_ mathrm e (lambda ) dlambda ,dlambda . See also[edit] Brightness International System of Quantities Radiance References[edit] ^ "Base unit definitions: Candela". The NIST Reference on Constants, Units, and Uncertainty. Retrieved 8 February 2008. ^ "Hefner unit, or Hefner candle". Sizes.com. 30 May 2007. Retrieved 25 February 2009. Curve data[edit] ^ "CIE Scotopic luminosity curve (1951)". Archived from the original on 2008-12-28. ^ "CIE (1931) 2-deg color matching functions". Archived from the original on 2008-12-28. ^ "Judd–Vos modified CIE 2-deg photopic luminosity curve (1978)". Archived from the original on 2008-12-28. ^ "Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function". Archived from the original on 2007-09-27. SI photometry quantities v t e Quantity Unit Dimension Notes Name Symbol[nb 1] Name Symbol Symbol[nb 2] Luminous energy Qv [nb 3] lumen second lm⋅s T⋅J The lumen second is sometimes called the talbot.
Luminous intensity
Iv
candela (= lm/sr)
cd
J
Luminance
Lv
candela per square metre
cd/m2
L−2⋅J
Illuminance
Ev
lux (= lm/m2)
lx
L−2⋅J
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J Time-integrated illuminance
Luminous efficacy η [nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux or power consumption, depending on context
1
See also: SI · Photometry · Radiometry ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967 ^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy. v t e SI base quantities Base quantity Quantity SI unit Name Symbol Dimension symbol Unit name (symbol) Example length l, x, r, (etc.) L metre (m) r = 10 m mass m M kilogram (kg) m = 10 kg time, duration t T second (s) t = 10 s electric current I , i I ampere (A) I = 10 A thermodynamic temperature T Θ kelvin (K) T = 10 K amount of substance n N mole (mol) n = 10 mol luminous intensity Iv J candela (cd) Iv = 10 cd Specification The quantity (not the unit) can have a specification: Tmax = 300 K Derived quantity Definition A quantity Q is expressed in the base quantities: Q = f ( l , m , t , I , T , n , I v ) displaystyle Q=fleft( mathit l,m,t,I,T,n,I mathrm _ v right) Derived dimension dim Q = La · Mb · Tc · Id · Θe · Nf · Jg (Superscripts a–g are algebraic exponents, usually a positive, negative or zero integer.) Example Quantity acceleration = l1 · t−2, dim acceleration = L1 · T−2 possible units: m1 · s−2, km1 · Ms−2, etc. See also History of the metric system International System of Quantities Proposed redefinitions Systems of measurement Book |