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Illuminance is incident luminous flux per receiving area, measured in lux. It is the visually weighted counterpart of irradiance. Uniform illumination of an aperture of area $A$ gives collected luminous flux $A{E}_v$. An aperture measurement alone does not give a source's total emitted luminous flux.^[ref]

A photometer measures a visually weighted radiation quantity. A filtered photodiode approximates the photopic luminous efficiency function with its spectral responsivity; calibration establishes how its electric current maps to illuminance or collected luminous flux. NIST's detector-based realization traces the spectral responsivity to a cryogenic radiometer and the aperture area to length measurements. ^[ref]

NIST Measurement Services: Photometric Calibrations by Yuqin Zong and others describes a concrete experiment (§§3.2–3.3.2): a silicon photodiode, multilayer photometric matching filter, precision aperture, temperature sensor and current-to-voltage converter. A monochromator supplies a small beam; scanning wavelength yields spectral responsivity, and scanning position checks response across the aperture. Finite spectral bandwidth requires correction. The average aperture response converts uniform illumination to illuminance.

On the photometric bench (§§3.4.1, 3.5.3, 3.5.6), align lamp and aperture reference planes, set the lamp's electric current, stabilize, and compare a check lamp with standard photometers. The readout is a voltmeter signal. Subtract the shutter-closed reading (§3.4.4), divide by converter gain to infer electric current, and use calibrated illuminance responsivity $S_A$ to obtain $E_v=i/S_A$. For a distant source, luminous intensity follows from $I_v=E_v{d}^2$, with $d$ between reference planes.

Controls include repeat lamp operations, distance changes to test the inverse-square law, temperature corrections and gain linearity checks. Spectral mismatch correction is source dependent (§3.4.3). Unknown optical spectra, spatial nonuniformity and incorrect reference planes remain limitations.^[ref]