Photometry Luminance and Illuminance units vs. Radiometry Radiance and Irradiance units

Photometric Terms:

Light sources that illuminate test charts reflectively are typically described in terms of Photometric units that are comparable to the response of the human eye or an image sensor that has a spectral response similar to the human eye (such as a silicon sensor with a near-IR cutoff filter).

Why are lightboxes described in Lux?

Lux values for lightboxes represent the illuminance measured “point blank” at the diffuser surface.

Assuming Lambertian emittance from a lightbox: lux = nit * π

Although there is a precedent within the imaging industry to describe lightbox luminous flux in Lux, this is misguided because lightboxes are made for luminance, not illumination.

This may have originated because Luminance meters can be an order of magnitude more expensive than Illumination meters. See Imatest Light Measurement Equipment.

Imatest’s LED lightbox is described as 10,000 lux which is equivalent to about 3183 nits.

Imatest’s “High Lux” Lightbox is described as 100,000 lux which is equivalent to about 31,831 nits.

Radiometric Terms:

Alternatively, light sources can be described in Radiometric units that consider the wider electromagnetic spectrum or more specific spectral bands.

See Also:

• Radiometry and Photometry for Autonomous Vehicles and Machines - Fundamental Performance Limits (R. Jenkin, C. Zhao 2021)