National Physical Laboratory

Spectral Radiance and Irradiance Primary Scales

Spectral radiance and irradiance primary scales at NPL are directly linked to NPL's cryogenic radiometer through the use of calibrated filter radiometers. The filter radiometers are used to determine the temperature of a state-of-the-art high temperature black body which is then used as a primary standard source whose spectral radiance can be determined from Planck's black body equation. The spectral radiance and irradiance scales are transferred to transfer standard sources using the SRIPS facility.

The facility is used to calibrate both radiance and irradiance sources such as integrating spheres, strip lamps, and FEL and Polaron irradiance lamps. The ultra high temperature black body can also been used for the direct calibration of space- and air-borne remote sensing instrumentation such as Earth imagers.

SRIPS Facility Layout

The measurement instrumentation is mounted on a stage such that it can be positioned in front of each source in turn. Light enters the double grating monochromator at the heart of the facility through an integrating sphere (for irradiance measurements) or two off-axis parabolic mirrors (for radiance measurements). After the monochromator there is a set of detectors mounted on a further stage, which is also fully automated. The detectors and grating sets available allow measurements from 250 to 3000 nm. The facility is currently being developed to extend the range to cover the spectral region from 200 nm to 14 µm

SRIPS Facility Layout
Figure 1: Schematic layout of SRIPS 

The facility can calibrate, at any one time, up to four lamps that are held at constant current under PID computer control by the SRIPS control software, or up to six lamps with their own power supplies. Lamps are usually calibrated directly against the UHTBB but can be calibrated against another standard lamp or source if required.

SRIPS Photo Ultra High Temperature Black Body

The lamps are calibrated relative to the ultra high temperature black body (UHTBB), this black body (model BB3500 from VNIIOFI) can operate at temperatures up to 3500 K

The lamps or other light source being calibrated are measured relative to the ultra high temperature black body. The ultra high temperature black body is measured with the facility before and after the test lamp measurements. The black body is the reference source and it effectively recalibrates the SRIPS facility for every measurement sequence, therefore any long-term drifts are unimportant. Given the temperature of the black body, the radiance of the black body is known at any wavelength.

When calibrating the SRIPS facility using the UHTBB, the integrating sphere input is positioned in front of the UHTBB. The light emitted by the UHTBB irradiates the integrating sphere aperture. The total radiative energy transfer between the black body aperture and the integrating sphere aperture is then defined as:


where g is the geometric factor for the transfer of radiation from one circular aperture to another coaxial circular aperture and LBB is the radiance of the black body in air. The geometric factor is a function of the size of the two apertures (here the black body and integrating sphere apertures) and the distance between them. The measured signal is the black body irradiance multiplied by the response of the entire facility, considering the responsivity of the detector, the reflectance of the mirrors, the efficiency of the gratings, the transmittance of the integrating sphere and so on.

When this facility is used to measure lamps, the measured signal is the irradiance of the lamps is multiplied by the same response and divided by the entrance aperture area. Therefore, the irradiance of the lamps is given by:



  • Elamp = Irradiance of lamp / W m-2 nm-1
  • Siglamp = Experimental signal for lamp / V
  • π LBB = Exitance of black body aperture / W m-2 nm-1
  • g = geometric factor, as above / m2
  • SigBB = Experimental signal for black body / V
  • A = Area of integrating sphere / m2

In practice two measurements of the black body are usually made before and after the measurement of the sources under test to take account of any drifts that may have occurred during the calibration sequence.

Measuring Black Body Temperature

The SRIPS facility uses the ultra high temperature black body as a reference source. In order to know the radiance of the UHTBB it is necessary to measure the temperature of the black body, which could be up to 3500 K, depending on the electric current supplied to heat the black body. The measurement of black body temperature is performed using a filter radiometer, which has been calibrated by comparison to a calibrated trap detector. In turn, the trap detector is calibrated by a cryogenic radiometer. This instrument underpins all optical radiation measurements at NPL, linking optical quantities to electrical scales. The measurement of black body temperature therefore links the primary spectral radiance and irradiance scales to primary detector scales.

When the filter radiometer is used to measure the black body temperature the recorded signal is a combination of its spectral responsivity (known from the calibration) and the black body radiance (a function of temperature). It also depends on the geometry and other similar factors. The equation is:


Where, s is size of source effect, G is amplifier gain, τ is lens transmission (if used), g is geometric factor and r is FR spectral responsivity. LBB is the black body radiance in air. This equation is solved numerically to calculate the temperature of the black body for a given filter radiometer signal.

Measurements are made with a number of different filter radiometers, since different filter radiometers have different advantages. A short wavelength filter radiometer is more sensitive to small temperature changes than a longer wavelength filter radiometer. On the other hand, the longer wavelength filter radiometer will have a higher signal and will therefore be less sensitive to stray light and noise. Usually at NPL we use narrow bandwidth filter radiometers that have been characterised using the laser based methods described elsewhere.

UHTBB Stability
Figure 4: Typical stability of black body over 5 hours
(typical measurements take 30-60 minutes)
as monitored by the feedback filter radiometer
and an external filter radiometer 

The Ultra High Temperature Black Body

The radiating cavity is made from a series of pyrolytic graphite rings stacked to form a cylinder. About two-thirds of the way into this cylinder is the cavity bottom, a flat graphite surface with grooved rings. The cavity is heated by directly passing 650-850 A (depending on desired temperature) through these graphite rings. Surrounding the cavity is a heat shield, which is pyrolytic graphite on the inside and ordinary graphite on the outside with graphite fabric between these layers. This in turn is inside the water-cooled chamber. Argon flows continuously through the black body and this allows it to be operated without a window.

The black body source is stabilised using a front mounted UV feedback filter radiometer. In front of the black body there is a ring mirror. Most light goes straight through the central hole without encountering any optics. A small amount of light reflects from the ring onto a diffuser. Light is focussed from this diffuser by a lens onto the UV filter radiometer. Shorter wavelengths are particularly sensitive to small changes in black body temperature. This filter radiometer is in a temperature stabilised water casing to prevent any signal changes due to room temperature fluctuations. The amplifier is immediately after the detector and this reduces electrical pick-up noise. The signal from the amplifier is fed into a digital voltmeter, which is connected to the control computer.

UHTBB Uniformity
Figure 5: Typical uniformity of UHTBB 

This stabilisation system can hold the temperature of the black body steady to within 0.2 K over the course of a typical measurement as shown in Figure 4. The black body is also extremely uniform, considering its size. Typical results obtained when scanning a filter radiometer, with a peak wavelength of 800 nm and a FWHM of 20 nm, in front of the black body at the typical measuring distance, are shown in Figure 5.

Geometric Factor and Black Body Radiance


where g is the geometric factor, which is given by:


r1 is the radius of the first (in this case black body) aperture, r2 is the radius of the second (in this case filter radiometer) aperture and d is the distance between them. The geometric factor is derived from the form factor for radiative energy transfer between two filter radiometers, but note that g contains additionally the area of the first aperture, which is missing in the form factor.

Note the radiance of the black body, as calculated by the Planck equation, is for light emitted into a vacuum. For black bodies in air, the air version is required; this additionally includes the refractive index of air, n:


For further information, please contact: Emma Woolliams

Last Updated: 25 Apr 2012
Created: 23 Jul 2007


Please note that the information will not be divulged to third parties, or used without your permission