= Photometric calibration
<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 $I_v=E_vd^2$, 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.<NIST Measurement Services: Photometric Calibrations by Yuqin Zong and others>[§§3.2–3.5.6, printed pp. 16–19, 22–28, 33–37: apparatus, dark subtraction, readout, corrections and check-lamp procedure]{ref}
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