Human vision is enabled by three primary modes:
  • Photopic vision: Vision under well-lit conditions, which provides for color perception, and which functions primarily due to cone cells in the eye.
  • Mesopic vision:  A combination of photopic vision and scotopic vision in low lighting, which functions due to a combination of rod and cone cells in the eye.
  • Scotopic vision: Monochromatic vision in very low light, which functions primarily due to rod cells in the eye.
Although all three modes of vision help us see under different conditions, nighttime vision is generally dominated by scotopic mechanisms (for very dark conditions with no ambient light) or mesopic mechanisms (for semi-dark conditions, such as a full moon and heavily lit commercial roadways). Unfortunately, virtually all photometric tests used to determine light output from street lighting sources are based on photopic vision, which is not representative of the human response to light under low light (nighttime) conditions. Photopic measurements favor “warmer” light, such as the orange light produced by common HID street light sources, including high pressure sodium lamps. Scotopic and mesopic measurements are more representative of a broader spectrum of light, including the “cooler” light generated by most LEDs used in street lighting applications. Because of these differences, many leading scientists and lighting experts believe that photopic measurements should be used for daytime and indoor lighting measurements, and scotopic or mesopic measurements should be used to evaluate nighttime lighting measurements.
The Illuminating Engineering Society of North America (IESNA) —which currently uses photopic measurement criteria for evaluating street lights—is currently reviewing the photopic versus scotopic/mesopic measurement issue, and revised street light standards are expected to be issued from the IESNA in the near future. In the meantime, many LED users are evaluating light output based using both photopic and scotopic measurements. Since most photometry is based on photopic measurements, a scientific conversion factor is used to create scotopic measurements from photopic measurement data. These conversions are described in groundbreaking research on this topic conducted by Drs. Sam Berman and Don Jewett.  Download a full copy of Drs. Sam Berman and Don Jewett’s research.
To convert from photopic to scotopic measurements, simply multiply the photopic measurement for the light source in question by the appropriate factor in the table below: