A beacon or a source of light, known as "The Candela"
In a significant step towards standardising the measurement of visible light, the frequency of 540 × 10¹² Hz has been chosen as the basis for the definition of the candela. This decision was made due to its correspondence with monochromatic light near 555 nanometers wavelength, a region that lies at the peak sensitivity of the human eye's photopic (daylight) vision [1][2][3][5].
The candela, a unit of luminous intensity, is defined as the luminous intensity of a source emitting monochromatic radiation of this specific frequency, with a radiant intensity of 1/683 watts per steradian [3]. This choice links the unit of luminous intensity directly to a well-understood physical quantity (radiant intensity) at a wavelength where the human eye is most sensitive under typical lighting conditions, making the candela meaningful for human-centric measurements of light [1][3][5].
The conversion factor K, equal to 683 cd sr W at 540 x 10¹² Hz, provides a direct and consistent basis for quantifying visible light intensity aligned with human visual perception. Interestingly, a candela is also defined as being as bright as a standard candle.
The Measurement Standards Laboratory of New Zealand (MSL) played a crucial role in this redefinition, ensuring that operating theatres in New Zealand remain bright enough for a surgeon to see the patient. To celebrate this milestone, MSL presented a video clip on May 20, 2019, which was filmed and edited by Jonathon Potton of Chillbox Creative. The video, produced by MSL, aimed to share the story of metrology development and was filmed at the Faculty of Engineering, Auckland University.
This redefinition of the candela underscores the importance of aligning scientific measurements with human perception, ensuring that the units we use to measure the world around us are relevant and meaningful to us.
The candela, being defined as the luminous intensity of a source that emits monochromatic radiation at 540 x 10¹² Hz, also signifies a link between science and health-and-wellness, as it caters to the human eye's peak sensitivity in daylight vision for improved health-and-wellness conditions and safety in workplaces like operating theatres. Additionally, understanding and application of the candela can contribute to advancements in fitness-and-exercise environments, where precise light intensity is essential for maintaining optimal fitness routines and performance, and in nutrition, as appropriate lighting could have an impact on mood and appetite.
