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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).

Candelas measures light intensity in a specific direction

Luminance measures the luminous intensity per unit area of light traveling in a given direction

Illuminance measures the total light falling on a surface:

Candelas can't be used to describe illuminance because candelas measure light intensity in a specific direction.

Why are lightboxes often described in terms of their illuminance (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:

Photometry (Wikipedia)

Radiometry (Wikipedia)

Luminance (Wikipedia)

Nits (Wikipedia)

Illuminance (Wikipedia)

Lux (Wikipedia)

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