The candela is used to measure the visual intensity of light sources, like light bulbs or the bulbs in torches. It is the only SI base unit based on human perception.

In the past, the unit used to measure visual intensity was called the candle, based initially on a ‘standard’ wax candle and then on specially designed oil or gas flame standards. However, these standards varied from country to country, and even from one device to another, meaning they lacked sufficient accuracy and reproducibility. The new candle, or candela, was introduced in 1948 and replaced these earlier standards with a black furnace operating at a specified temperature, with a further refinement in 1979 to link visual intensity directly with the radiant intensity of light at a specified wavelength.

The human eye has different sensitivities to different frequencies of light. The peak sensitivity is at approximately 540 THz, which is in the green-yellow region of the spectrum. So we see this light more intensely than other colours of the same physical power.

The Candela and Optical Metrology course is part of the National Physical Laboratory’s (NPL) Fundamentals of Metrology training programme. The programme comprises of seven standalone modules, each corresponding to one of the seven SI base units. The module is delivered via a series of pre-recorded video lectures.

This course includes the following topics:

• Applications of the candela and optical radiation measurement.
• The definition of the candela.
• An introduction to radiometry.
• Common sources of uncertainty and best practice in optical radiation measurement.

Learners who successfully complete the course will receive an NPL Certificate of Completion.

Recommended Pre-learning
To lay the foundations for this module, it is recommended that all learners complete the two half-day NPL e-learning courses: Introduction to Measurement and Metrology and Introduction to Measurement Uncertainty.