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**Optical radiation

Generally speaking, the national references are the materializations of the units or the measuring benches that allow accessing the quantities considered. Regarding the quantities characterizing optical radiation, the references are related to the detectors, the sources or the materials.

The missions of the French metrology concerning characterization of the optical radiations have been distributed over two laboratories: the LNE-INM at the CNAM and the LNE/CMSI. The different measuring benches allow the materialization of the units, comparison between the national references, under the aegis of the CIPM by the Consultative Committee for Photometry and Radiometry (CCPR) or within Euromet; the connection of the different units, the characterization of the optical components and calibration of the industrial standards (radiometers, photometers, filters, materials, fibre components and measuring apparatuses specific to fibre optics).

After an overview of the physical quantities used in optical radiations metrology, the related references will be given under four sections.

Quantities concerned:

Concerning this field of metrology, the base unit of the SI is the candela (symbol cd); it is a photometry unit that expresses the luminous intensity. All the other units used are therefore units deriving from the SI. To introduce the other quantities, two fields of measurements should be considered: photometry and radiometry.

Photometry is the field of metrology that allows expressing visual radiation from physical measurements. According to the definition of the candela, the photometric units are related to the radiometry quantities. The most common quantities are as follows:
- The luminous flux expressed in lumen (lm), flux emitted in a solid angle of 1 steradian by a uniform point source located at the top of the solid angle with a luminous intensity of 1 candela (1 lm = 1 cd·sr) ;
- Illuminance expressed in lux (lx), illuminance that reaches a surface per unit area in unit time, a luminous flux of 1 lumen per square meter (1 lx = 1 lm·m-2 = 1 cd·sr·m-2).
As photometric quantities are directly related to human vision, the International Commission on Illumination (CIE: Commission internationale de l’éclairage) has defined, in 1924, a spectral function V(λ) to describe photopic vision. This definition has been taken back and adopted by the CIPM, in 1982, to carry out the photometric measurements involving common references at international level.

Radiometry is the field of the physical measurements of optical radiation. The quantities are therefore expressed in units deriving from the SI and expressing the energetic, spectral and spatial aspects of optical radiation:
- The radiant power (emitted by a source or received by a detector) expressed in watt (W) ;
- The radiant intensity of one source expressed in watt per steradian (W·sr-1) ;
- The radiance of a beam expressed in watt per square meter and per steradian (W·m-2·sr-1) ;
- The irradiance (received by one surface) expressed in watt per square meter (W·m-2) ;
- The spectral responsivity of the detectors expressed in ampere (or volt) per watt (A·W-1).

All these quantities apply to the measurement of the optical radiation in free-field.
But optical radiation can also be guided into optical fibres. This optical field (fibre optics) calls for specific equipment (sources, detectors, fibres, amplifiers and other optical components for modification of beam). Some quantities are also specific but are nevertheless connected to the traditional radiometry quantities and to the dimensional quantities.

References for the detectors:

The LNE-INM holds an electrical substitution radiometer (ESR) operating at the temperature of liquid helium. This instrument underpins the national reference for the measurements of the radiant power.
A thermal detector is placed at the bottom of the vacuum cryostat and closed by a silica porthole. The portholes used allow covering a spectral range extending from 200 nm to 2 000 nm. Without giving details of its working, the principle is to compare the temperature rise in the cavity, generated by Joule effect in a heating resistor, with the rise produced by optical radiation.

To obtain the best flux measurements, it is necessary to use biased laser beams. The measurable radiant power under optimal conditions is located between 100 µW and 2 mW.
The standard measurement uncertainty of the flux for the visible radiation reaches 5·10-5 in relative value. The lasers available are currently the He-Ne lasers that can generate three wavelengths in the visible range (543 nm, 612 nm, 633 nm) and one in the infrared range (1 523 nm), one argon laser that produces eight wavelengths ranging between 454 nm and 514 nm, one He-Cd at 325 nm, one YAG laser quadrupled in frequency that allows obtaining 266 nm and one diode laser emitting at 375 nm.

Cavity of the cryogenic radiometer
Cavity of the cryogenic radiometer

As much as possible, all the radiometrical and photometrical measurements are connected to this reference (see "detectors"):

Spectral responsivity of the detectors
At the LNE-INM, spectral responsivity measurement of the detectors is obtained through two steps:
- The first step is to establish the relative responsivity curve of the detectors, using a bench involving an extended source, a double grating monochromator and a thermal detector, with a relative uncertainty of 10-3.
- The second step is the definition of the absolute values of this curve: absolute values are obtained, at some wavelengths, by comparison of the detector to the cryogenic radiometer, with an uncertainty of some 10-4, and then the definition of the absolute values of the entire spectral responsivity curve is found by the interpolation method.

To obtain the best uncertainties, the LNE-INM is using the transfer detectors of the trap type for the direct connection to the cryogenic radiometer. They are made up of (visible and ultraviolet) silicon photodiodes. In the near-infrared, the detectors most used in metrology are the Si and InGaAs photodiodes, however the metrological characteristics of the InGaAs photodiodes do not yet allow these to be assembled in the trap-type.
On the basis of a silicon detector calibrated by the LNE-INM, the LNE/CMSI is carrying out the calibrations, by comparison method, of the industrial detectors and radiometers in the ultraviolet, visible and infrared (from 180 nm to 14 µm) ranges.

Luminous intensity
For the materialization of the luminous intensity, the LNE-INM has realized in 1997, photometers using silicon detectors, filters and diaphragms. They are connected to the cryogenic radiometer. The measurement uncertainty of the luminous intensity of a lamp including its photometers is of 0,43%. Since 2004, new photometers are being developed not based on plane detectors any more but on trap-type detectors.

Spectral irradiance
Concerning the measurement of spectral irradiance in the ultraviolet range, the LNE-INM has built and characterized filter radiometers. Constituted by trap detectors, they are connected to the cryogenic radiometer. The absolute measurement of irradiance (radiometers or lamps) is thus made at some discrete wavelengths between 220 nm and 380 nm. Irradiance of standard lamps is determined in relative value over the full spectrum of utilization and on the basis of their radiance. These measurements are carried out on one bench that allows moving easily from one geometric configuration to another.

Functional diagram of the filter radiometers.
Functional diagram of the filter radiometers.

Other references, non connected to the cryogenic radiometer

Detectors operating under photon-counting regime
Between 1998 and 2003, the LNE-INM has developed one absolute measuring bench for the quantum efficiency of the detectors. These detectors can then become references in the range of very low lux, i.e. for the flux well below those used with the cryogenic radiometer. The bench is based on the principle of parametric downconversion of photons in a non-linear crystal that generates two correlated photons. An electronic chain for photon-counting allows determining the efficiency of one detector, without neither source nor detector of reference. In 2003, one measurement had been made at 633 nm with one relative standard uncertainty of 1,1 %. This uncertainty is six times lower that the one obtained by making a comparison with a trap-type detector.

Bench for measuring the quantum efficiency of detectors.
Bench for measuring the quantum efficiency of detectors.

Radiometers for the measurements of the high laser radiation power
In the mid-70s, the LNE started to develop the means for radiometric measurements specific to laser radiation and more particularly for the high power. The needs extend from some dozens of milliwatts (medical lasers) to several dozens of kilowatts (welding or nuclear fusion lasers). The laboratory has constructed several radiometers (for reference and work) to cover the entire range of radiant power. Up to 100 W they are cooled by air, and water-cooled beyond this range. These radiometers operate within a range of radiant power much higher that the one of the cryogenic radiometer. The principle of these radiometers is electrical substitution at room temperature: comparison of the temperature rise in an absorbent cavity through the optical radiation or the heat dissipated by Joule effect via a heating resistor surrounding the cavity. These radiometers are therefore similar to the cryogenic radiometer connected to the electrical standards.
In 1994, in order to ensure consistency in the references for traditional radiometry (via the cryogenic radiometer) and for laser radiometry, one comparison was made around 1 mW, common limit operating zone, through the transfer detectors and radiometers. The ratio of the different powers obtained is of 1,012 with an uncertainty of 0,9 % (2σ).

The LNE/CMSI holds 5 working radiometers and 4 reference radiometers. Since the acquisition in 1999 of power supply with voltage-stabilised at 15 kW, the uncertainty attached to the electrical measurements has become acceptable and the working radiometer designed for the measurements of powers in excess of 1 kW is now one reference (CLR30) enabling measurements up to minus 10 kW.
To date, the laboratory keeps a set of lasers which mean power extends from 10 mW to 3 kW: diodes (IR), He-Ne (633 nm and 3,39 µm), argon (UV and visible), continuous YAG and impulse YAG, CO2 (all the lines between 9,4 µm and 10,6 µm - 3 kW). All these lasers also allow realizing energy measurements between 10 mJ and 100 J.
At the LNE, the maximum radiant power is of 3 kW, but measurements were made in 1998 using the CLR30 radiometer a the Soldering Institute of Yutz (France) in front of one CO2 laser up to 40 kW. And this has confirmed the right sizing of the very high power radiometer.

Measurements of the irradiance produced by radiating thermal sources
Additionally, during these last years, the LNE has finalized and characterized two calibrating benches for thermal fluxmeters of Schmidt-Boelter or Gardon type. These sensors are primarily used in the tests involving reaction to fire or fire resistance. The benches are made up of a blackbody closed by the sensor to be calibrated:
- A bench is cylindrical and operating at low pressure; it is the reference bench where the sensors are calibrated in absolute;
- The other one is spherical and operating under atmospheric pressure; it is the bench where the sensors are calibrated by comparison. Heat flow that reaches the sensor if of radiative and convective nature.

Fluxmeter
Fluxmeter

The reference is the irradiance generated by the blackbody source. It is calculated on the basis of the measurements of temperature, geometric data and the nature of the cavity materials. Connection is therefore established mainly with the temperature references.
The measurement capabilities of the LNE/CMSI extend up to 70 kW.m-2 with uncertainties below 2% above 20 kW.m-2 and from 2% to 4% for the lower irradiance, a range where convection becomes important in relative value.

References for the sources:

Lamps: radiance and irradiance
The sources of radiance of blackbody type have long been used as reference sources connected to the temperature thanks to the Planck’s radiation law. The Radiometry-Photometry laboratory of the LNE-INM keeps a blackbody which temperature is variable up to 2 000 K. Monochromatic radiance is defined by comparison against one lamp with temperature calibration using the radiance comparator of the LNE-INM’s radiation thermometry laboratory.
One bench provides the possibility to move from radiance to relative irradiance. The relative irradiance curve is translated into absolute values via two filter radiometers connected to the cryogenic radiometer. The spectral range of measurements of radiance and irradiance of the lamps extends from 300 nm to 2 500 nm.

Measuring bench for sources radiance and irradiance
Measuring bench for sources radiance and irradiance

To summarize, in the visible and infrared ranges, the lamps are calibrated, in general, in intensity, in irradiance or in radiance with the means of radiometers (or of photometers) connected to the cryogenic radiometer.

Deuterium lamp (front) and tungsten-ribbon lamp (back).
Deuterium lamp (front) and tungsten-ribbon lamp (back).

In the UV range, the LNE had argon arcs as references for radiance connected to the source calculated from the radiation of the Orsay’s collision ring (ACO). These arcs are still used as relative reference sources of luminance but since the Orsay’s installation is no more operational, it was decided in 1999 and just before closing the installation, to connect the LNE’s deuterium lamps to the Bessy I synchrotron radiation of the PTB in Germany. And hence the explanation as to why the French metrology is connected to the PTB for spectral irradiance in the ultraviolet range.

Radiation sources
Over these last years, the LNE/CMSI has implemented means for characterization of the detectors and sources of infrared radiation up to 14 µm. This activity utilizes the sources and the means for temperature measurement of the LNE/CMSI’s radiation thermometry laboratory and the different laser sources of the optical laboratory. The spectroradiometers that are available are complementary. They operate as radiance comparator and allow determining the relative responsivities of the detectors. Spectroradiometers are either with monochromator or Fourier transform. Spectral adjustment is done either with spectral lamps or with lines of laser source. A set of detectors is also available: it includes one pyroelectric detector, HgCdTe, InAs, InGaAs and Si detectors. These detectors are connected between themselves in cascade up to silicon which is itself calibrated by the LNE-INM to the cryogenic radiometer.

References for materials:

This area of metrology consists of putting under control the measurement of the radiometric properties of the materials receiving luminous flux. These are either filters (neutral or spectral), or colour or gloss (glasses, ceramics) standard materials. The quantities measured are the regular transmittance (spectral or not) and the spectral reflectance (regular or diffuse). Industrial applications include optics for characterization of the components, chemical analysis via spectrophotometry, manufacturing of cosmetics, paints, papers, inks (colorimetry), luminous display (screens)...

The LNE-INM holds a measuring bench for regular transmission between 0,01% and 90% on a spectral range that extends from 250 nm to 2 500 nm. This bench is used for measuring the transmittance of neutral filters (densities) or spectral filters necessary notably for the realization of radiometers (or photometers) with standard filters.

Ceramic colour standards.
Ceramic colour standards.

The LNE/CMSI has implemented one set of spectrophotometers, fitted with accessories of different geometries as well as one measuring bench for gloss, for the industrial reference calibrations. The spectral range extends from ultraviolet to near-infrared (3,2 µm) range. This equipment allows measuring the spectral and spectral reflection (regular or diffuse) transmissions. The standards used are neutral filters calibrated by the LNE-INM, white standards (BaSO4 powder), colour standards (ceramics), spectral references (filters of holmium or didymium fluids, spectral lamps) or a glass with known refractive index (gloss).

References for fibre optics:

The LNE/CMSI keeps several measurement capabilities for the optical characteristics of the fibre optical components (sources, fibres, detectors) and for the calibration of the measuring instruments used (reflectometers, amplifiers):
- Realization of reflectance standards designed for the calibration of reflectometers in distance and in attenuation;
- Characterization of the fibres (length, linear attenuation, pass band, polarisation dispersion, backscattering);
- Calibration of the radiometers and detectors (radiant power, pass band, linearity);
- Calibration of the fibre laser sources (radiant power, spectrum);
- Calibration of the optical fibre amplifiers (gain, noise factor);
- Characterization of the active and passive components (in wavelength and polarisation);
- Characterization of the Bragg gratings inscribed into the fibres (reflectance, transmittance and pass band).

Measuring bench of OLCR type.
Measuring bench of OLCR type.

Since 1999, the LNE holds a measuring bench based optical low coherence reflectometry (OLCR). This bench allows locating and evaluating the reflectance of the defects on short (intra component for instance) distances or to measure the chromatic dispersion over short lengths. This bench is also utilized for characterizing the Bragg gratings photo-inscribed in the monomode fibres notably to obtain their index profile and the special fibres (with photonic crystals, high order mode).