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Light is measured with different techniques, and therefore there is a handful ofrelated, but different, units of measurements. This article gives a brief overviewof the most widely used measures and presents a few equationsfor conversion of one measure into another. For convenience, a tablewith conversion calculators is at the bottom of this page.
Candela Tourism: Tripadvisor has 979 reviews of Candela Hotels, Attractions, and Restaurants making it your best Candela resource. Candela is another word for candlepower. Candlepower measurement is from times when the candle was the primary source of illumination. The word candlepower was substituted with the term candela in 1948. One candela is approximately equal to the light that a candle with specific dimensions, produces.
Optical radiation covers a broad spectrum, including infrared and ultravioletlight. For the sake of brevity, this article focuses on visible light(the field of photometry).
Candela
The candela (unit cd) has its origin in the brightness of a'standard candle', but it has received a more precise definition in the InternationalSystem of Units (SI) —and at that time the unit was also renamed from 'candle'to 'candela'.
The candela measures the amount of light emitted in the range of a (three-dimensional)angular span. Since the luminous intensity is described in terms of an angle, thedistance at which you measure this intensity is irrelevant. For ease of illustration,in the picture at the right the three dimensions have been flattened to two. Inthis picture, screen B would catch exactly the same amount of light rays (emittedfrom the light source) as screen A —provided that screen A were removed to notobscure screen B. This is because screen B covers the same angle as screen A.
The angular span for candela is expressed in steradian, a measure without unit(like radian for angles in a two-dimensional space). One steradian on asphere with a radius of one metre gives a surface of one m2.A full sphere measures ( 4pi ) steradians.
See the section on lux for the relation between candela and lux.
Lumen
If you look at LEDs, especially high-brightness LEDs, you may notice that the LEDswith a high luminous intensity (in candela or milli-candela, mcd) typically havea narrow apex angle. Similarly, LEDs with a wideapex angle typically have a relatively low luminousintensity. The same is true for halogen spots with reflector: those with a narrow-beamreflector have a higher rating in candela than the 'floodlight' spots of the samepower.
The cause for this relation is the total energy produced by the LED. LEDs of aspecific class (for example, 'high flux') all produce roughly the same amount ofluminous energy. However, when a LED emits its total energy in a beam with a narrowangle, the intensity will be greater (in the direction of that angle) than whenthe same energy had been emitted over a wide angle.
The lumen (unit lm) gives the total luminous flux of a light source bymultiplying the intensity (in candela) by the angular span over which the lightis emitted. With the symbol ( Phi_v ) for lumen, ( I_v ) for candela and( Omega ) for the angular span in steradian, the relation is:
[ Phi_v = I_v cdot Omega ]If a light source is isotropic (meaning: uniform in all directions),( Phi_v = 4pi I_v ). This is because a sphere measures ( 4pi )steradians. See the topic on apex angles to get thethree-dimensional angular span ( Omega ) from an opening angle.
As a frame of reference, a standard 120V/60W light bulb is rated at 850 lm, andthe equivalent 230V/60W light bulb is rated at 700 lm. A low voltage (12V)tungsten halogen lamp of 20W gives approximately 310 lm.
Lux
Lux (unit lx) is a measure of illumination of a surface. Light meters oftenmeasure lux values (or footcandles, but these are directly related: onefootcandle is 10.764 lx). Formally, lux is a derived unit from lumen, which isa derived unit from candela. Yet, the concept of lux is more easily compared tocandela than to lumen.
The difference between lux and candela is that lux measures the illumination ofa surface, instead of that of an angle. The net result is that the distance ofthat surface from the light source becomes an important factor: the more distantthat the surface is from the light source, the less it will be illuminated by it.In the picture at the right, screen A has the same size as screen B.
One steradian on a sphere with a radius of one metre gives a surface of onem2 (see the section on candela). From this, it follows that at ameasuring distance of 1 metre, the values for candela (lumen per steradian) andlux (lumen per m2) are the same. In general, measurements in lux canbe converted to and from candelas if the measurement distance is known. Notethat when measuring LEDs, the virtual origin of the light source lies a fewmillimetres behind the physical point source because of the lens of the LED—this becomes relevant when measuring LEDs at a short distance.
Luminance
Luminance is a measure for the amount of light emitted from a surface (in aparticular direction). The measure of luminance is most appropriate for flatdiffuse surfaces that emit light evenly over the entire surface, such as a (computer)display. Luminance is a derived measure, expressed in Candela per square metre(( cd / m^2 )).An alias for the unit ( cd / m^2 ) (unofficial, but still commonly used) is 'Nit'.
Luminance and illuminance ('Lux') are related, in the sense that luminance is ameasure of light emitted from a surface (either because of reflection or becauseit's a light-emitting surface), and illuminance is a measure for light hitting asurface. Assuming a perfect diffuse reflecting surface, you can multiply themeasure in 'Nits' by ( pi ) to get the equivalent value in Lux.That is, with ( L_v ) for Luminance and ( E_v ) for Lux:
[ E_v = L_v cdot pi ]As with Lux, there are several older units for luminance, of which thefoot-lambert is probably the most common (because of its 1-to-1 relation withthe footcandle on a Lambertian-reflecting surface). These older units are easilyconverted to candela per square metre by multiplying them with a scale factor. Forfoot-lambert, the scale factor is 3.425.
Apex angle
Since the lumen and the candela measures are related through the viewing angle(or apex angle), it is useful to know how this angle is defined.
One measures the angle between the axis where the light source gives its highestluminous intensity and the axis where that intensity is reduced to 50%. In thepicture at the right, this angle is denoted with ( theta ). The apex angle istwice that angle (meaning ( 2 theta )).
Observe that the reduction of intensity to 50% is based on a linear scale, butthat our perception of brightness is not linear. The CIE has standardizedthe relation between luminous intensity and perceived brightness as a cubic root;other sources claim that a square root better approximates this relation. See alsothe page on colour metric.
The three-dimensional angular span for an apex angle, using ( Omega ) for theangular span (in steradian) and ( 2 theta ) for the apex angle, is:
[ Omega = 2pi left( {1 - cos {2 theta over 2}} right) ]Lighting efficiency
There are ample ways to illuminate a surface or a room: incandescent lamps,fluorescent tubes, LEDs, tungsten-halogen bulbs, electroluminescent sheets,and others. These are often compared in their efficiency of turning electricalenergy to luminous energy.
The official name for lighting efficiency is 'luminous efficacy of a source'. Thisshould not be confused with the 'luminous efficacy of radiation', which disregardslosses due to heat generation and others (and therefore gives significantly highervalues). The lighting efficiency is measured in lm/W (lumen per Watt).
Lighting efficiency is often expressed as a percentage, based on the theoreticalmaximum value of lighting efficiency of 683.002 lm/W (at a wavelength of 555 nm).For example, at the time of this writing, a white 1 Watt 'lumiled' can reachan efficiency of over 100 lm/W, giving an efficiency of 15%. While this mayseem low, LEDs are actually quite efficient in comparison with other lighting methods.
Equations
The equations in this sections are given without further explanation or derivation.For details, please refer to the technical literature, orWikipedia.
Candelabra Aloe
| From | To | Given | Equation |
|---|---|---|---|
| Candela (( I_v )) | Lumen (( Phi_v )) | apex angle α | ( Phi_v = 2pi I_vleft( {1 - cos {alpha over 2}} right) ) |
| Lumen (( Phi_v )) | Candela (( I_v )) | apex angle α | ( I_v = {Phi_v over {2pi left( {1 - cos {½ alpha}} right)}} ) |
| Lumen (( Phi_v )) | Lux (( E_v )) | surface area A (m2) | ( E_v = {Phi_v over A} ) |
| Lux (( E_v )) | Lumen (( Phi_v )) | surface area A (m2) | ( Phi_v = E_v cdot A ) |
| Candela (( I_v )) | Lux (( E_v )) | measuring distance D (m) | ( E_v = {I_v over {D^2}} ) |
| Lux (( E_v )) | Candela (( I_v )) | measuring distance D (m) | ( I_v = E_v cdot D^2 ) |
Candela Boats
As an example, if the data sheet of a high brightness LED mentions that itproduces 1500 mcd (1.5 cd) at an apex angle of 70°, that LED gives:
[eqalign{Phi_v &= 2pi cdot 1.5left( {1 - cos {70^circ over 2}} right) cr&= 3pi left( {1 - cos 35^ circ } right) cr&approx 1.70,{rm lm}}]Conversion calculators
Candela Energy
Based on the equations developed and presented above, the table below allows youto quickly convert one measure into another.
Candelabra
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Candela Gentlemax Pro
Evaluation
The outcome of the equations (and the calculators based on them) may differ fromthe data given by a LED or spotlight manufacturer, or from what you measure witha Lux meter, for several reasons. The manufacturer may specify the luminous intensity(in candela or milli-candela) perpendicular to the lightsource, instead of the average value over the apex angle.Another complexity is that the values for candela, lumen and lux are standardizedfor light with a wave length of 555 nm, or green light. For LEDs of a differentcolour, a weighting function should be applied, using a standardized model ofthe human eye. Standard Lux meters have, at best, only filters for daylight andincandescent light, and may therefore deviate significantly for LEDs (even whiteLEDs, as the spectrum is not the same as for incandescent lamps).
