We are familiar with the idea that the adjective 'heavy' can refer to weight or density, which are fundamentally different things.

People sometimes ask why there need to be so many different units, or ask for conversions between units that can't be converted ( lumens and candelas, for example). Many different units of measure are used for photometric measurements. Study of the chemical effects of ultraviolet radiation led to characterization by the total dose or actinometric units expressed in photons per second. Use of the human eye as a detector led to photometric units, weighted by the eye's response characteristic. The total heating effect of infrared radiation as measured by thermometers led to development of radiometric units in terms of total energy and power. Measurement techniques varied depending on the effects under study and gave rise to different nomenclature. Measurement of the effects of electromagnetic radiation became a field of study as early as the end of 18th century. Mesopic vision occurs between these limits and is not well characterised for spectral response. Scotopic vision occurs below 2Р’В Р“вЂ”Р’В 10 РІв‚¬вЂ™5 cd/m 2. Photopic vision is characteristic of the eye's response at luminance levels over three candela per square metre. Photometry is typically based on the eye's photopic response, and so photometric measurements may not accurately indicate the perceived brightness of sources in dim lighting conditions where colors are not discernible, such as under just moonlight or starlight.

The eye has different responses as a function of wavelength when it is adapted to light conditions ( photopic vision) and dark conditions ( scotopic vision).

The standardized model of the eye's response to light as a function of wavelength is given by the luminosity function. Photometry attempts to account for this by weighing the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength. The human eye is not equally sensitive to all wavelengths of visible light.

2.1 Photometric versus radiometric quantities.

Visual lighting photopmetric