CSIR-National Physical Laboratory

सीएसआईआर-राष्ट्रीय भौतिक प्रयोगशाला

Optical Radiation Metrology

Optical radiation measurements cover the wavelength region of electro-magnetic spectrum from 200nm to 25µm. The wavelength region from 200 nm to 2500nm is maximally used for measurement of many interlinking disciplines. As Radiometric measurements are used in a wide range of industries including opto-electronics, telecommunications, lighting, space and in health and safety, precision measurements are required for specification for systems, for quality control in industries and in scientific research. The objective of this section is to establish, maintain, and upgrade the existing base unit of optical radiation, i.e., candela and to provide calibration facilities for various photometric parameters namely luminous flux, illuminance, luminance, luminous intensity, detector responsivity, color temperature and radiometric parameters namely spectral radiance and spectral irradiance in the range from 200nm to 2500nm.

In addition, this section also provides measurement facilities for spectroscopic parameters namely spectral reflectance, spectral transmittance, absorbance, and polystyrene film calibration by FTIR. Other calibration facilities include calibration of thermo-vision camera, black-body and NIR related measurements.

Variable temperature blackbody, Goniophotometer, Fourier Transform NIR and IR spectrophotometer, Raman-spectroscope, spectral irradiance measurement setup, spectrophotometers, corretected photon metrology setup, LED measurement setup, various standard lamps and detectors.


Source based primary standard of spectral radiance in the form of a variable temperature blackbody has been established. This blackbody works in the temperature range of 1800K – 3200K with temperature stability of ± 0.2K. Its emissivity is 0.999, and exhibits radiance uniformity within 0.1%, in the wavelength range 0.2 μm-2.5 μm. The uncertainty in spectral radiance measurement using this blackbody is 0.3-0.5% in the wavelength range 0.2 μm-0.4 μm, and 0.1-0.3% in the wavelength range 0.4 μm-2.5 μm, respectively. The established facility is shown in the Fig. 1.

Fig.1: Blackbody Setup

Luminous Intensity, Illuminance and Illuminance Responsivity Measurement

Luminous intensity, illuminance and illuminance responsivity measurements are carried out using 3.0 m optical bench. The optical bench and other measurement instruments are shown in Fig. 2.

Fig. 2(a) illustrates the optical bench while, Figs. 2(b) and 2(c) shows the standard intensity light source and standard detector and illuminance meter respectively. Reference scale for luminous intensity is maintained in the form of tungsten filament lamps of very high quality. These lamps are free from manufacturing defects, stable and their electric and photometric parameters remain unchanged over a length of time. The NPL luminous intensity scale is maintained at 2856 K and 2800 K (nominal) correlated color temperatures. To check the compatibility of the scales, reference standards lamps for luminous intensity are use to calibrated periodically from PTB, Germany. The photometer used is a PTB calibrated V(λ) corrected Si photodetector with its spectral response approximately matching the CIE luminous efficacy function.

Fig. 2. Luminous Intensity and Illuminance Measurement Setup

Luminous Flux Calibration Facilities

Absolute luminous flux measurement is carried out by using Gonio-photometer (see Fig. 3(a)). This automatic system is used to prepare working standards for various lamps. Using these working standards, the luminous flux calibration is performed for lamp and lighting industries. For luminous flux measurements we use 3.0 m (Fig. 3(b)) and 1.0 m (Fig. 3(c)) diameter integrating spheres.

To check the compatibility of the scales, reference standards lamps i.e., Polaron, 110V, 200W for luminous flux are use to calibrated periodically from PTB and BIPM, Germany.

Fig. 3. Luminous Flux Measurement Setup

Color Temperature and Chromaticity Coordinates Measurements

Reference scales for color temperature measurement are of very high quality. These lamps are free from manufacturing defects, stable and their electric and photometric parameters remain unchanged over a length of time. Incandescent lamp standards of color temperature are calibrated on a photometric bench by comparing the ratios of the portion of the visible spectrum in the red and the blue portion of the spectrum for the test and the standard source or by measuring the temperature directly using a tristimulus colorimeter. The measurement setup is shown in Fig. 4. Tristimulus colorimeter head is elaborated in Fig. 4 (b).

Fig. 4. Color Temperature and Color Coordinate Measurement Setup

Luminance Measurement

Luminance standard is created by ab-initio method by using a flux standard. The lamps to be used for deriving luminous scale were calibrated for their total luminous flux F using NPL reference standards. The luminous flux leaving the aperture was measured using a silicon photo-diode photometer provided with a filter. Luminance meter is calibrated against this standard of luminance using luminance standard. In most of the calibrations, the LMT luminance standard is used and to reduce the luminance value spectrally non-selective neutral density filters of different transmissions are used. The calibration setup is shown in Fig. 5.

Fig. 5. Luminance Measurement Setup