# 46. Sources of Light and how to Measure Light

version from 2016-01-29 19:01

## Section

1. One way to measure light is to see at what intensity it evokes the sensation of sight in a person.
This is LUMINOUS INTENSITY and its unit is the CANDELA.
2. Building from this, you could measure light by the intensity of light in an area when 1cd of it is
emitted into a specific area (1sr) called a solid angle. This would be LUMINOUS FLUX and its unit
is the LUMEN.

PhiLuminous Flux (lm)
ILuminous Intensity (cd)
omegasolid angle (sr)

3. Building from this, you could measure the luminous flux (intensity from 1cd in 1sr) for an area.
This would be the ILLUMINANCE and its unit is LUX.

EIlluminance (lx)
PhiLuminous Flux (lm)
AArea

4. Going back to Luminous Intensity (intensity needed to evoke sight), you can measure the
Luminous Intensity per area. This would be the LUMINANCE and its units are CANDELA/m^2.

IIntensity (cd)
AArea (m^2)

5. You could also measure the time factor of light. This would be the EXPOSURE and its unit is
the LUX SECOND.

HExposure (lux.s)
EmMean Illuminance (lx)
tTime (s)

6. You could also measure the energy transmitted by the electromagnetic waves. This would be
the RADIANT ENERGY and it is measured in JOULES.
7. You could also measure the power transmitted by this radiation. This would be RADIANT
FLUX and its units are WATTS.

tTime (s)

8. Lastly, you could measure the radiant flux in a given area. This would be the IRRADIANCE
and its units are W/M^2.

AArea (m^2)

## And now just a little definition for all of them

MeasurementDescriptionUnit
Luminous IntensityIntensity at which light evokes sight responseCandela, cd
Luminous FluxThe Intensity of light in 1sr when 1 cd is shone into itLumen, lm
IlluminanceLuminous Flux on an areaLux, lx
LuminanceLuminous Intensity on an areaCandela per meter squared, cd/m^2
ExposureTime factor of lightLux Second, lx/s
Radiant EnergyEnergy transmitted by electromagnetic wavesJoules, J