Luminous flux Photometry

The various terms that are used for the description of the emission of electromagnetic radiation from a radiant source or for the receipt of electromagnetic radiation by a specified surface element are summarized in Tab. 3-9. The terminology of electromagnetic radiation measurement is divided into radiometry and the subset of photometry (Fig. 3-18). The former is the science that involves the energy measurement of electromagnetic radiation in general. The latter is applied for the same purpose when visible radiation is to be described or measured in relation to the human eye s response. Important photometric quantities are for example luminous flux, luminous intensity, illuminance and luminance (McCluney, 1994). Every photometric quantity has its counterpart in radiometry, and vice versa. 

The human eye responds to electromagnetic radiation in the wavelength range from about 360 nm (violet) to 820 nm (red), with a peak sensitivity near 555 nm (green). While the detailed shape of this response curve depends on the individual person, studies on representative samples of human subjects have led to adoption of a standard function relating the perceived brightness (luminous flux) to the actual power of the spectral radiation. This function is referred to as V(X), the photopic spectral luminous efficiency function, and it plays an important role in photometry. 

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In Sl-derived units, these are both measured in lux (lx) or lumens per square meter (cdsrm ). In the CGS system, the unit of illuminance is the phot. One phot is equal to 10,000 lx. Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. 

In addition to these two radiometric assessments of the beam intensity, for beams of visible light there is a third possibility, which is to quantify the intensity of the beam by the intensity of visual perception by the human eye. Physical quantities connected with this physiological type of assessment are called photometric quantities, in contrast to the two radiometric quantities described above. In photometry, the intensity of the beam is called the luminous intensity 7y. The subscript v stands for visual . The luminous intensity 7v is an ISO recommended base quantity the corresponding SI base unit is the candela (cd). The luminous flux is determined as the product of the luminous intensity and the solid angle. Its dimensions therefore are luminous intensity times solid angle, so that the SI unit of the luminous flux < v turns out to be candela times steradian (cd sr). A derived unit, the lumen (Im), such that 1 Im = 1 cd sr, has been introduced for this product. 

Flux O is a concept we are very familiar with in the energy regime radiant flux Og is more commonly referred to as power - the rate at which energy is transferred. The units are watts (W). One watt is one joule per second. If we are working with photons, the flux is the photon flux and the units are photons/second (ph/s). If we are doing photometry, this is called the luminous flux Oy, and the units are lumens/second (lum/s). 

Radiance and luminance L are the flux into a small projected soUd angle dQ. from a small source area dA, divided by the projected solid angle and source area, so that d = L dA dQ.. The nomenclature here does not follow the generic pattern of the first three For the unit of power this is called the radiance with units of W/(cm sr). For photometry it is called the luminance with units of Lylm/(cm sr). For photon units either the photon radiance or the photon luminance is accepted, both with symbol Lp and units ph/(s cm sr). 

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