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Use the triangular $w$ in equal radiant power under visual weighting and stipulate the detector's spectral responsivityThis model has nonnegative response: $w$ is zero outside $405$–$705\mathrm{nm}$. For a $1\mathrm{mW}$ ideal calibration line at $555\mathrm{nm}$, $i_A=0.5\mathrm{mA}$ and ${\tilde{\Phi }}_{v,A}=0.683\mathrm{lm}$, so $C_A=1366\mathrm{lm}{\mathrm{A}}^{-1}$. An equal-radiant flux test line at $630\mathrm{nm}$ gives $w=1/2$, $r=0.375\mathrm{A}{\mathrm{W}}^{-1}$ and $i_T=0.375\mathrm{mA}$. The unchanged conversion indicates $0.51225\mathrm{lm}$, whereas the model's intended weighted result is $0.3415\mathrm{lm}$. The required spectral mismatch correction is $F=2/3$: the uncorrected result is 50% too large. All these numbers are declared model outputs, not instrument measurements or standard photometry data.

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]

Photoresistor spectral calibration determines how a photoresistor's electrical reading depends on the illumination spectrum under specified geometry, background, temperature, bias and adaptation history. A resistance-versus-lux curve measured with one lamp is not automatically valid for another spectrum. The manufacturer's primary CdS datasheet specifies a $2854\mathrm{K}$ source for its light-resistance tests and preconditions cells with light before electrical measurements.^[ref]

The distinction is between irradiance and illuminance. With spectral irradiance $E_{\lambda }(\lambda )$, illuminance is approximatelywhere $V$ is the photopic luminous efficiency function. The detector weights wavelengths according to its own physical response, which need not be proportional to $V$. NIST's detector-based photometric realization measures that spectral response, rather than assuming an arbitrary detector reproduces the photometric weighting.^[ref]

For a locally linear small-signal model about a fixed operating state, writeHere $s_G$ is the differential spectral responsivity of electrical conductance, including the stated illumination geometry. It is not automatically a global, illumination-independent material constant. A background-dependent photoresistor spectral response requires rechecking this derivative when the operating point changes.

An original two-band example makes the mismatch quantitative. Stipulate relative photometric weights $V_a=0.8$, $V_b=0.2$ and equal detector generation weights $s_a=s_b$ for two narrow bands. Equal illuminance requires four times as much irradiance in band $b$, so its detector-weighted generation is four times greater. If, over this illustrative range, $R\propto g^{-0.7}$, the resistance ratio is $R_b/R_a=4^{-0.7}\approx 0.379$. These weights and exponent are stipulated, not measured specifications or tabulated values for named wavelengths. Equal lux can produce substantially unequal resistance.

A proposed spectral characterization uses narrow-band sources or a monochromator, a reference detector to measure incident irradiance, fixed beam geometry and controlled detector temperature. Record resistance or conductance after a defined settling time at several background levels. A separate calibrated photometer can establish illuminance; it cannot by itself establish the photoresistor's spectral weighting. Keep electrical loading and self-heating fixed while changing the source.

For a broadband transfer, characterize the reference and test spectra and test whether the measured spectral-response model predicts their relative readings. Existing spectral mismatch correction methods address the analogous transfer problem for photometers, but a nonlinear photoresistor first needs a justified operating-point model.^[ref] The result is a calibration valid for stated conditions, not a universal conversion from resistance to optical power.