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An optical power meter makes the definition operational: collect a specified beam on a calibrated photodiode, measure its electric current, and infer incident radiant flux using spectral responsivity. In the manufacturer-authored Model 1830-R Series Optical Power Meters User's Manual by Newport Corporation, §5.5.1 specifies a concrete sequence: connect the detector and its calibration module, select watt mode and the source wavelength, block the beam and press ZERO, then unblock it and read the display. The stored background includes dark current and ambient light present during zeroing. For narrowband radiation,The electric current difference divided by spectral responsivity has units $\mathrm{A}/(\mathrm{A}{\mathrm{W}}^{-1})=\mathrm{W}$. For a stipulated $R=0.40\mathrm{A}{\mathrm{W}}^{-1}$ and difference $0.20\mathrm{mA}$, the inferred radiant flux is $0.50\mathrm{mW}$; these numbers illustrate the inference and are not reported measurements. The entire beam must hit the active area, and §5.6 calls for correct wavelength, background subtraction, and operation below detector saturation. If an optical attenuator is fitted, §5.4 requires the calibrated detector–attenuator pair and the matching meter setting.

The calibration itself must connect electric current to independently known incident power. NIST's Spectroradiometric Detector Measurements: Ultraviolet, Visible, and Near-Infrared Detectors for Spectral Power by Thomas C. Larason and others describes this chain. A cryogenic radiometer, an electrical substitution radiometer, compares temperature changes from absorbed light with those from known Joule heating (§5.2). Transfer and working detectors carry that watt scale to a spectral comparator (§5.1). A source, monochromator, focusing optics, shutter, beam splitter, and monitor photodiode illuminate a standard and test detector in turn; calibrated current-to-voltage electronics and voltmeters provide the readout (§§3.2.2–3.2.4, 4.3.1). At each wavelength, subtract blocked-beam signals and normalize each detector signal by the simultaneously sampled monitor signal. Comparison with the standard yields the test detector's spectral responsivity. NIST uses two working standards and alignment normal to the beam (§6.1.1).

Controls distinguish a smaller collected radiant flux from a faulty conversion: block and unblock the beam to check the baseline; check that its footprint fits the active area; verify the calibrated optical attenuator configuration; and test proportional response within the specified operating range. Source drift faster than monitor sampling is not removed. NIST discusses finite spectral bandwidth, stray light, temperature, and spatial response variations (§§3.1.1, 6.2, 7.2). A broadband optical spectrum requires the weighted integral in spectral radiant flux, rather than dividing by an arbitrary single spectral responsivity. These limitations explain why the meter's displayed watts need a stated collection boundary, spectral assumptions, and calibration conditions.

An electrical substitution radiometer measures radiant flux by replacing absorbed optical power with calibrated Joule heating. At fixed temperature, feedback control holds the receiver equally warm with the beam blocked or admitted: after corrections, absorbed optical power equals the reduction in supplied electrical heating. Incident radiant flux additionally requires radiometric absorptance and entrance-path corrections. This connects radiometry to electrical standards.^[ref]

Picture a small absorbing cavity connected to a colder reference block by a radiometer thermal link. A heater supplies Joule heating, a thermometer senses the receiver, and feedback control adjusts the heater. When light supplies some of the heating, the controller supplies less. The readout is the change in heater power, rather than the beam's brightness or its number of photons. NIST's institutional description gives a concrete receiver with a conical interior, absorbing coating and weak Kapton support, operated with a receiver heater and thermometer.^[ref]