Optical Radiation Standards

1. Objective of the section

Optical radiation measurements cover the wavelength region of electro-magnetic spectrum from 200 nm to 25µm. The wavelength region from 200 nm to 2500 nm 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 200 nm to 2500 nm.

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.

2. Calibration capabilities

Photometric, radiometric and colorimetric measurements



Measurement uncertainty at coverage factor k = 2

Luminous intensity 1 cd to 103 cd 0.016 – 0.014
Luminous Flux 1 lm to 2×104 lm 0.02 – 0.018
Illuminance 1 lx to 5×103 lx 0.016 – 0.02
Luminance 1 cd/m2 to 104 cd/m2 0.016 – 0.
Temperature distribution 1800 K to 3400 K 6 K to 10 K
Correlated color temperature 1800 K to 7000 K 20 K
Chromaticity coordinates   0.002 units
Spectral radiance Wavelength range
    350 nm to 400 nm
    400 nm to 800 nm
    800 nm to 2500 nm
  0.04 – 0.02
0.03 – 0.05
Spectral irradiance Wavelength range
    350 nm to 400 nm
    400 nm to 800 nm
    800 nm to 2400 nm
  0.06 – 0.026
0.026 – 0.06
Spectroscopic measurements
Spectral transmittance 192 nm to 900 nm 0.01 – 0.045
Spectral absorbance 192 nm to 900 nm  

3. Existing facilities

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 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() 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. Color temperature and color coordinate measurement setup



Dr. Ranjana Mehrotra
Mr. V.K. Jaiswal
Dr. Parag Sharma
Mr. K.N. Basavaraju
Mr. Sudama
Dr. Bharat K. Yadav
Mr. Prashant Sharma

4. Research

Spectral switching, Free-space optics (FSO) and FSO communication – Dr. B.K. Yadav

The phenomenon of spectral switching has been studied both theoretically and experimentally for different optical setups and it is now a well-known phenomenon. Spectral switching might find potential applications in demanding fields, namely free-space optical (FSO) communication, FSO interconnects, and the design of spectrum selective optical interconnects. Although spectral switching based techniques and applications are quite new in the field of FSO, these techniques might find great scope in the near future. Recently, the application of spectral switching for information processing in free space has been explored. It has been shown that if the redshift and blueshift of diffracted polychromatic light can be associated with information bits ‘1’ and ‘0’ respectively, or vice versa, the spectral flipping of diffracted light from lower frequency (redshift) to higher frequency (blueshift) can be exploited for information encoding. Interestingly, spectral switching can support different encoding schemes, namely frequency shift keying (FSK), on–off keying (OOK), and information encoding with different spectral switches. The encoded information canbe transmitted in free space using phase singularity based FSO links.

We also carried out experimental studies on polychromatic focused dark hollow Gaussian beam (DHGB). On the basis of experimental and numerical analysis, the possibility and significance of DHGB based FSO links for indoor and outdoor optical communications are explored. Experimental setup and DHGB is shown in Fig. 6 (a) and Fig. 6 (b) respectively.

(a) Schematic of experimental setup (b) Dark-hollow beam

Fig. 6.Experimental observation of polychromatic Dark hollow Gaussian beam (DHGB)

Reference : “Spectral anomalies of polychromatic DHGB and its applications in FSO,” Journal of Lightwave Technology, vol. 29, pp. 960 - 966. (2011).

Infrared spectroscopic study for tumor diagnosis – Dr. Ranjana Mehrotra

Infrared spectra of normal and malignant breast tissues are measured in the 600 cm-1 to 4000 cm-1 region. The measured spectroscopic features which are the spectroscopic fingerprints of the tissues contain the vital information about the malignant and normal tissues. The novelty of this study is that from the spectroscopic data we could differentiate malignant tissue from the normal one. We analyzed Fourier Transform Infrared (FTIR) data on twenty five cases of infiterating ductal carcinoma of breast with different grades of malignancy from patients of different age groups. Infrared spectra demonstrate significant spectral differences between the normal and the cancerous breast tissues. In particular changes in frequency and intensity in the spectra of protein, nucleic acid and glycogen vibrational modes as well as the band intensity ratios for lipid/proteins, protein/ nucleic acids, protein/glycogen are observed. This allows us to make a qualitative and semi quantitative evaluation of the changes in proliferation activity from normal to diseased tissue.

Polarization study of optical vortices – V. K. Jaiswal

We have demonstrated new technique for obtaining polarization entangled photons with classical optics which also depicts properties of polarization discrimination. Classical analogue of quantum entanglement is also shown in the form of inseparability between polarization and spatial modulation. Polarization study of Gaussian and vortex beam shows that orbital angular momentum and spin angular momentum of photons are not coupled in the process.

Switching light with light – Dr. Parag Sharma

Theoretical analyses of laser induced nonlinear absorption processes in rhodopsin protein molecules have been performed. The results validate the feasibility of all-optical switching operation ‘Switching light with light’, in these protein molecules in very simple pump-probe geometry. The switching speed has been shown to be enhanced from milliseconds to nanoseconds time scale. The performance of the switch in terms of contrast has also been enhanced by optimizing the concentration of molecules.

Contact Person

Dr. Ranjana Mehrotra
Head, Optical Radiation Metrology
Room No. 109, Main building
National Physical Laboratory, New Delhi-110 012
Email: ranjana@nplindia.org
Phone : +91 1145609210