A discussion on candelas
In a celebration held at the Faculty of Engineering, Auckland University on May 20, 2019, the Measurement Standards Laboratory of New Zealand (MSL) unveiled a video clip commemorating the redefinition of the International System of Units (SI). The video, produced by Jonathon Potton of Chillbox Creative, shared the fascinating story of metrology development, with a particular focus on the candela.
The candela, the constant unit used to define luminous intensity, has an interesting origin. It is defined to be as bright as a standard candle, providing a tangible and relatable reference point. However, the conversion factor K, which links the candela to the original or absolute power perceived by the eye, is more complex. K is equal to 683 cd sr W at 540 x 10 Hz, a value chosen because it corresponds to the peak sensitivity of the human eye to visible light in the yellow-green region of the spectrum.
The frequency of 540 x 10¹² Hz was selected for the definition of the candela because it is where the luminous efficiency function V(λ), representing the average spectral sensitivity of human visual perception under well-lit conditions, reaches its maximum. This frequency is associated with a wavelength of about 555 nm, well-known as the wavelength of maximum eye sensitivity under photopic (daylight) vision.
By fixing the luminous efficacy at this frequency to a value of 683 lumens per watt, the candela is tied to human visual perception, making it a unit of luminous intensity, rather than just radiant intensity. Radiation at other frequencies is measured in candelas by correlating their perceived brightness to this peak reference using the standard luminous efficiency curve V(λ).
This connection between physical radiometry and human photometric perception is crucial, ensuring that the candela remains a relevant and accurate unit in the field of metrology. For instance, in operating theatres, MSL keeps the candela to ensure the brightness is sufficient for a surgeon to see the patient clearly.
The redefinition of the candela is a significant milestone in the evolution of the SI, reflecting the ongoing efforts to align units of measurement with human perception and experience. The video produced by MSL provides an engaging and informative overview of this process, making complex scientific concepts accessible to a general audience.
The redefinition of the candela, a unit of luminous intensity, is linked to health-and-wellness and fitness-and-exercise as it ensures adequate lighting in operating theaters, enabling surgeons to clearly see their patients during procedures. By aligning the candela with human visual perception, science now has a more accurate and relatable unit of light, thereby impacting health and wellness, and fitness and exercise.
