Radiometry and Photometry: LED Fundamentals

LED Fundamentals

Radiometry &Radiometry & Photometry

08-03-2011

Definitions, Units and Symbols

Radiometry – deals with the detection and measurement of electromagnetic radiation across the total spectrum

Photometry – subfield of radiometry; radiometric power scaled by the spectral response of the human eye

LED Fundamentals | Radiometry & Photometry |

Definitions, Units and Symbols

Radiometry – deals with the detection and measurement of electromagnetic radiation across the total spectrum

Photometry – subfield of radiometry; radiometric power scaled by the spectral response of the human eye

Radiometric PhotometricQuantity Symbol Units Quantity Symbol Units

Radiant Power Φe W Luminous Flux Φv lumens (lm)Radiant Intensity Ie W/sr Luminous Intensity Iv lm/srRadiant Intensity e W/sr Luminous Intensity v lm/sr

Irradiance Ee W/m2 Illuminance Ev lm/m2

Radiance Le W/m2-sr Luminance Lv lm/m2-sr

“e” = “energetic”

“v” = “visual”


v = visual

Converting to Photometric Units

Power (Watts) is converted to luminous flux (lumens) via the relation:

780 Φv = flux (lumens)

380

dVPK evPe = Power

V = photopic response function of the human eye1

K = constant (683 lm/W for photopic)

0.7

0.8

0.9

ty

0.4

0.5

0.6

Rel

ativ

e Se

nsiti

vi

0

0.1

0.2

0.3R


0400 450 500 550 600 650 700 750

Wavelength (nm)

Convert Radiometric to Photometric

eP V vP

0 4

0.6

0.8

1

ensi

ty [a

.u.]

0.4

0.6

0.8

1

0.4

0.6

0.8

1

0

0.2

0.4

400 450 500 550 600 650 700 750

Wavelegth [nm]

inte

0

0.2

400 450 500 550 600 650 700 750

W aveleng t h ( nm)

0

0.2

400 450 500 550 600 650 700 750

W aveleng t h ( nm)

780

380

dVPK ev


Solid Angle

Surface Area = A

Sphere radius = R

2Solid Angle Ω = A/R2

Units: Steradians (sr)


Projected Solid Angle

)( area angle Solid d d Area Angle Solid

)(cosanglesolid projected of Aread ) (cos angle solid d

Unit sphere


Unit sphere

Projected Solid Angle

sin ddd

2

cos

proj dd

00

sincos proj ddα

2sin j sin proj


Photometric Units and SymbolsPhotometric Units

Quantity SymbolMetric Units Name

English Units Name

Luminous Flux Φ lumens (lm) lumens

Luminous Intensity I lm/sr

candela (cd) or

candlepower

Illuminance E lm/m2 lux (lx) lm/ft2 Ft-candleLuminance L cd/m2 nits cd/π*ft2 Ft-Lambert

II EL


Lambertian Source

Brightness (luminance) is independent of angle 0LL

Intensity falls off as cosθ cos)( 0II Many LEDs are very nearly

Lambertian sources0I

cos0I


Lambertian Source

Total flux in a 2θ cone:Total flux in a 2θ cone:

''sin'2

dI ''sin'20

dI

''sin'cos2

20

0 dI sin2

0 I

lumens.is cd1ofsourceLambertian a offlux totalThe


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f f OS O S Gproduct and a product of one of OSRAM Opto Semiconductor GmbH's partners.


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Radiometry and Photometry: LED Fundamentals

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