This article deals with the general light measurements, which can be performed with the spectroradiometer JETI specbos 2501 and its accessories integrating sphere and CIE 127 cond. A/B set up.

The following four basic quantities exist in light measurement and can be measured with a complete specbos 2501 system:

Quantity	Symbol	Unit	Simplified definition	Used to characterise ...
Luminous flux		lm	"optical power" into all directions	Halogen lamps, LEDs
Luminous intensity		cd (basic unit)		Point like sources as LEDs and lamps with reflector
Luminance		Unit  cd/m2		Homogenious sources as monitors and video walls
Illuminance		lx		Brightness in a certain plane

Quantity: Luminous flux	Symbol	Unit  lm	Simplified definition  "optical power" into all directions	Used to characterise ...  Halogen lamps, LEDs
Quantity: Luminous intensity	Symbol	Unit  cd (basic unit)	Simplified definition	Used to characterise ... Point like sources as LEDs and lamps with reflector
Quantity: Luminance	Symbol	Unit  cd/m2	Simplified definition	Used to characterise ... Homogenious sources as monitors and video walls
Quantity: Illuminance	Symbol	Unit  lx	Simplified definition	Used to characterise ... Brightness in a certain plane

A street lamp emits a Luminous Flux Φ_v and a part of this flux will be radiated into a certain angle Ω. The quotient is the Luminous Intensity I_v. This intensity creates the Illuminance/ Irradiance E_v on a certain area on the street and this value generates, in combination with the reflectivity of the street, the Luminance L_v, which is the visual impression for the observer of the street.

These four quantities can also be considered without connection to the human eye, but as physical quantities, then they are called radiometric quantities. Furthermore, each of them can be considered as spectral quantity (quantity per nm) or as total quantity (sum across the whole wavelength range).

An extended scheme with photometric and radiometric quantities can be found in Technical Note 20.

A spectroradiometer measures the spectral radiometric quantities in its operational wavelength range. This radiometric spectrum will be used afterwards to calculate all photometric, total radiometric and other quantities.

The relation between these four (photometric) quantities is shown in the following drawing

 

The basic unit specbos 2501 spectroradiometer can measure spectral radiance. It can be combined with an integrating sphere for flux measurements and with a CIE 127 set up to measure the radiant intensity of LEDs. Alternatively, the spectroscopy instrument can measure in this mode using its irradiance data and applying the inverse square law (see the application radiant intensity calculation using irradiance measurement).

Hence this specbos system covers all spectral spot measurement applications in the lab and is therefore best suited as a basic laboratory equipment.

Recommended product and accessory

specbos 2501 Integrating sphere