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# Radiometry and Photometry ## What is Radiometry? Radiometry is the measurement of electromagnetic radiation. This includes visible light, but also ultraviolet, infrared, and radio waves. Radiometric quantities are absolute and can be directly related to physical units. ## What is Photometry? P...

# Radiometry and Photometry ## What is Radiometry? Radiometry is the measurement of electromagnetic radiation. This includes visible light, but also ultraviolet, infrared, and radio waves. Radiometric quantities are absolute and can be directly related to physical units. ## What is Photometry? Photometry is the measurement of light as perceived by the human eye. It is based on the eye's sensitivity to different wavelengths of light. Photometric quantities are weighted by the luminous efficiency function, which describes the average human eye's response to light. ## Radiometric vs. Photometric Units | Quantity | Radiometric Unit | Symbol | Photometric Unit | Symbol | | ------------------ | ----------------------- | ------ | ---------------- | ------ | | Radiant Energy | Joule (J) | $Q_e$ | Luminous Energy | $Q_v$ | | Radiant Flux | Watt (W) | $\Phi_e$ | Luminous Flux | $\Phi_v$ | | Radiant Intensity | Watt per steradian (W/sr) | $I_e$ | Luminous Intensity | $I_v$ | | Radiance | W/sr m$^2$ | $L_e$ | Luminance | $L_v$ | | Irradiance | W/m$^2$ | $E_e$ | Illuminance | $E_v$ | | Radiant Exitance | W/m$^2$ | $M_e$ | Luminous Exitance| $M_v$ | ## Relationships ### Luminous Flux $\Phi_v = K \int \Phi_e(\lambda)V(\lambda)d\lambda$ Where: * $\Phi_v$ is the luminous flux (in lumens) * $\Phi_e(\lambda)$ is the spectral radiant flux (in watts per nanometer) * $V(\lambda)$ is the luminous efficiency function (dimensionless) * $K$ is a constant equal to 683 lm/W ### Luminous Efficacy $\eta = \frac{\Phi_v}{\Phi_e}$ Where: * $\eta$ is the luminous efficacy (in Lumens per Watt) * $\Phi_v$ is the luminous flux (in lumens) * $\Phi_e$ is the radiant flux (in watts) ## Summary Table | Term | Radiometry | Photometry | | ------------------- | -------------------------- | ------------------------------- | | Measures | Electromagnetic Radiation | Visible Light (as seen by the eye) | | Based on | Absolute Physical Units | Human Eye Sensitivity | | Key Consideration | Energy | Brightness | | Primary Application | Scientific, Engineering | Lighting, Display Technology | ## Additional Notes * **Steradian (sr):** A unit of solid angle. A sphere has $4\pi$ steradians. * **Lumen (lm):** The SI unit of luminous flux, equal to the amount of light emitted per second in a solid angle of one steradian by a point source of one candela. * **Lux (lx):** The SI unit of illuminance, equal to one lumen per square meter. * **Candela (cd):** The SI unit of luminous intensity, approximately the luminous intensity of a common candle. ## Spectral Irradiance Example Imagine a plot or graph of Spectral Irradiance against wavelength, where the x-axis is wavelength from 300nm to 800nm, and the y-axis is spectral irradiance in $W/m^2/nm$. There are several curves plotted, each representing a different light source such as "Sun", "Tungsten", and "LED". The "Sun" curve has a peak in the visible spectrum.