Photometry

When measuring the brightness of light, there are two parallel systems of quantities that can be used:

• Photometric (Luminous) quantities – measures related to the perceived brightness of light
• Radiometric quantities – measures related to the absolute physical brightness of light

Photometry uses perceptual units like Lumen and Candela, whereas Radiometry uses absolute physical units like Watts and Joules. The relationship between them is fully defined by the Luminosity Function. However, the function is based on the Standard Observer (Color Normal), so the relationship for Colorblind individuals – namely Protans (or Monochromats especially) – can be very different.