Primary Standard Cryogenic Radiometer
The basis for the traceability of all optical radiation standards, assuring accuracy into the hands of the end user – the customer.
NPL's cryogenic radiometer is the primary standard for the measurement of optical radiant power. It uses the electrical substitution technique, whereby the optical power incident on an absorbing cavity is compared with the electrical power required to heat the cavity to the same temperature. High Tc superconducting leads to the cavity heater ensure true equivalence of electrical and optical power.
The cavity is made of electroformed copper to reduce its mass and is coated internally with NPL super black. The design of the cavity in combination with operation at helium temperatures ensures an optimal response to incoming radiation. The cavity is isolated from fluctuations in the cold head of the cooler by a reference block maintained at a constant temperature. The accuracy of the system is further enhanced by operating the cavity within an isothermal shield.
The thermal link between the cavity and reference block is designed to optimise the temperature rise for a specific power level. A balance is achieved between the need for a high impedance to maximise the sensitivity and the need for a short time constant. This link is adjustable but is set at the time of order.
Purchasing a cryogenic radiometer allows a laboratory to establish their own scales traceable to a primary standard. These scales are usually disseminated using trap detectors as a transfer standard and solid state working standards. The cryogenic radiometer is used with a variety of laser sources.
Specifications
| Operating Details | |
| Cavity operating temperature | 12 K |
| Absolute accuracy | ± 0.005 % |
| Power Requirements | 220 V - 240 V, 50 Hz, 1.9 kVA 208 V - 220 V, 60 Hz, 2.0 kVA |
| Cold head service interval | > 15 000 hours |
| Absorber service interval | 15 000 hours |
| Water cooling | 3 l/min minimum at 20°C |
| Radiometric Properties | |
| Receiver response | 1.2 K/mW approximately |
| Maximum power | 2 mW |
| Resolution at maximum power | 1 in 105 |
| Time constant | 70 seconds |
| Brewster window transmission | 400nm to 800nm 99.96% |
| Cavity Absorption | 200nm to 2000nm >99.99% |
| Options | |
| Vacuum pumping station |  |
| Detector comparator stage | |
| Laser stabilisation facility | |
| Brewster window assembly optimised for mid infrared use | |
| Brewster window transmission measurement system | |
NPL support includes the installation of the facility and training in its use
For further information, please contact: Malcolm White