An optical power meter measures the total optical power incident on a sensor, while illuminance quantifies the perceived light intensity on a surface; the two are related through the spectral sensitivity of the human eye and the area over which the light is distributed.

Optical Power Meter

An optical power meter (OPM) is a device used to measure the absolute optical power of a light source, typically in watts (W) or milliwatts (mW) for fiber optic systems or free-space light sources. It consists of a calibrated photodiode sensor, an amplifier, and a display, and its readings are often wavelength-dependent, requiring the user to set the correct test wavelength for accurate measurements (Wikipedia) . Optical power meters measure radiometric power, which represents the total energy per unit time incident on the sensor, without weighting for human visual perception.

Illuminance

Illuminance is a photometric quantity that measures the intensity of light falling on a surface, taking into account the wavelength-dependent sensitivity of the human eye (photopic vision). It is defined as the luminous flux per unit area, with the SI unit lux (lx), where 1 lux equals 1 lumen per square meter (lm/m²) . Illuminance is measured using a lux meter or photometer, which typically contains a photodiode with a filter that mimics the eye's spectral response.

Connecting Optical Power and Illuminance

The relationship between optical power and illuminance depends on the spectral distribution of the light source and the area over which the light is spread:

• Conversion principle: Illuminance $E_v$ can be calculated from optical power $P$ using the luminous efficacy function $V(\lambda )$, which weights the power at each wavelength according to human eye sensitivity: $$E_v=683\text{lm/W}\times \int P(\lambda )V(\lambda )d\lambda /A$$ where $A$ is the illuminated area, and 683 lm/W is the maximum luminous efficacy at 555 nm .
• Distance effect: For a point source radiating isotropically, illuminance decreases with the square of the distance from the source (inverse square law): $$E_v=\frac{{\Phi }_v}{4\pi d^2}$$ where ${\Phi }_v$ is the luminous flux and $d$ is the distance to the surface .
• Practical implication: While an optical power meter gives the total power, illuminance accounts for human perception and the area illuminated, making it possible to convert radiometric measurements to photometric ones if the spectral power distribution is known .

Summary

In essence, an optical power meter measures physical light energy, whereas illuminance quantifies perceived brightness on a surface. The two are linked through the eye's spectral sensitivity and the geometry of illumination, allowing engineers and lighting designers to translate radiometric measurements into photometric quantities for practical applications in lighting, display calibration, and optical system testing.